PIARC factores humanos en diseño de carretera

www.piarc.org2012R36EN

HUMAN FACTORS IN ROAD DESIGN. REVIEW OF DESIGN STANDARDS IN NINE COUNTRIES

PIARC Technical Committee C1Safer Road Infrastructure

The World Road Association (PIARC) is a nonprofit organisation established in 1909 to improve international co-operation and to foster progress in the field of roads and road transport.

The study that is the subject of this report was defined in the PIARC Strategic Plan 2008 – 2011 approved by the Council of the World Road Association, whose members are representatives of the member national governments. The members of the Technical Committee responsible for this report were nominated by the member national governments for their special competences.

Any opinions, findings, conclusions and recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of their parent organizations or agencies.

This report is available from the internet site of the World Road Association (PIARC)http://www.piarc.org

Copyright by the World Road Association. All rights reserved.

World Road Association (PIARC)La Grande Arche, Paroi nord, Niveau 292055 La Défense cedex, FRANCE

International Standard Book Number 2-84060-306-1

STATEMENTS

HUMAN FACTORS IN ROAD DESIGN. REVIEW OF DESIGN STANDARDS IN NINE COUNTRIES2

2012R36EN


2012R36EN

This report was written by a working group dedicated to Human Factors within Technical Commitee C.1 “Safer road infrastructure”.

Contributors to this report are:

Dr. Sibylle Birth, (Germany); Babette Demgensky (Germany);Paul Schepers (Netherlands);Daniel Aubin (Canada-Quebec);Jiri Landa (Czech Republic);Eric Locquet (France);Deborah de Grasse (Canada);Marie-Line Gallenne (France);Carlos de Almeida Roque (Portugal);Lorenzo Dominichini (Italy).

Illustrations from on-site investigations:

Dr. Sibylle Birth, Babette Demgensky, Paul Schepers, Deborah de Grasse, Daniel Aubin, Marie-Line Gallenne, Emilio Majoral, Eric Locquet, Tchona Idossou, Jiri Landa, Ing. Jindrich Sachl, Jaroslav Král, Matt Petersson, Tomás Jilek, Jindrich Fric, Stanislava Jakesova, Tomas Hartl, Roman Turza, Vladimir Mensik, Pavel Skladany.

Editorial advice:

Marie-Line Gallenne (France);Emilio Francisco Mayoral Grajeda (Mexico); Tchona Idossou (Burkina Faso);John Douglas (Australia);Carlos de Almeida Roque, Lorenzo Dominichini, Deborah de Grasse – English version;Deborah de Grasse, Eric Locquet – French version.

The report was written by Intelligenz System Transfer Potsdam GmbH, Paul Schepers, Daniel Aubin, Jiri Landa, Deborah de Grasse, Eric Locquet, Marie-Line Gallenne, Carlos de Almeida Roque and Lorenzo Dominichini.


2012R36EN

English/French translations of the original version were produced by:Intelligenz System Transfer Potsdam GmbH (Germany), Deborah de Grasse for the English version, Eric Locquet, Kamil Daoud and Deborah de Grasse for the French version.

TC1.1 was chaired by Hans-Joachim Vollpracht (Germany). Lise Fournier (Canada-Quebec), John Douglas (Australia), Roberto Llamas Rubio (Spain) were respectively French-speaking, English-speaking, spanish-speaking secretaries.

The French version of this report is published under reference 2012R36FR; ISBN 2-840060-305-4


2012R36EN CONTENTS

GLOSSARY OF TERMS: ..........................................................................................................................7SUMMARY ................................................................................................................................................13

1. INTRODUCTION .................................................................................................................................151.1. STARTING POINT .......................................................................................................................151.2. HUMAN FACTORS IN SPATIAL PERCEPTION – A BASIC FACTOR

FOR SAFER ROAD INFRASTRUCTURE .................................................................................161.3. ROAD DESIGN – THE STANDARD WAY OF PLANNING AND BUILDING .......................181.4. PROCEDURE FOR RELATING HUMAN FACTORS DEMANDS

TO THE STAGES OF A DESIGN PROCESS .............................................................................191.5. FROM PSYCHOLOGICAL HUMAN FACTORS TO GUIDELINES AND STANDARDS ......20

2. COMPARISON OF CURRENT NATIONAL DESIGN ELEMENTS AS THEY RELATE TO HUMAN FACTORS OF SPATIAL PERCEPTION ..................................................................22

2.1. INTRODUCTION .........................................................................................................................222.2. HUMAN FACTORS SAFETY RULE NO.1: DRIVERS NEED ENOUGH TIME ....................24

2.2.1. Transition Zone .................................................................................................................242.2.2. Perception and Visibility ..................................................................................................31

2.3. HUMAN FACTORS SAFETY RULE NO.2: THE ROAD LEADS DRIVERS TO APPROPRIATE SPEED AND STABILIZES LANE TRACKING (FIELD OF VIEW RULE) ..............................39

2.3.1. Optical density of the field of view ....................................................................................402.3.2. Fixation objects in the lateral roadside environment support optimal lane tracking ......442.3.3. The depth of the Field of View supports optimal lane-tracking and reliable orientation

and anticipation of the road’s course ................................................................................532.4. HUMAN FACTORS SAFETY RULE NO.3: ROAD DESIGN SHOULD PRE-PROGRAMME

DRIVER’S ACTIONS CORRECTLY .........................................................................................592.4.1. Change of road function without corresponding change in design

and optical characteristics (e.g. town entrance)...............................................................602.4.2. Change of roads direction despite a dominant eye-catching orientation line

(e.g. city by-pass dilemma) ...............................................................................................622.4.3. Effect of pre-programmed habits and routines .................................................................642.4.4. Multiple critical points occur concurrently ......................................................................692.4.5. Deficiencies in traffic control devices ..............................................................................73

2.5. GENERAL CONCLUSIONS OF THE HUMAN FACTORS AUDIT .........................................773. BEST PRACTICES ...............................................................................................................................79

3.1. 6 SECONDS RULE .......................................................................................................................793.1.1. Transition Zone .................................................................................................................793.1.2. 6 seconds rule - I.2. Perception and Visibility .................................................................94

3.2. FIELD OF VIEW RULE .............................................................................................................1043.2.1. Optical density of the Field of View ................................................................................1043.2.2. Fixation objects in the lateral roadside environment support lane-tracking..................1123.2.3. Depth of Field of View ....................................................................................................123


2012R36EN

3.3. LOGIC RULE ..............................................................................................................................1423.3.1. Change of road function without corresponding change in design and optical characteristics (e.g. Town Entrance) .....................................................................1423.3.2. Effect of pre-programmed habits and routines ...............................................................1463.3.3. Multiple critical points occur concurrently ....................................................................1593.3.4. Deficiencies in traffic control devices .............................................................................163

4. PROPOSALS FOR MISSING LINKS .............................................................................................1734.1. OPTICAL DENSITY OF THE FIELD OF VIEW: MONOTONOUS SURROUNDINGS

OF ROADS AVOIDED ...............................................................................................................1734.1.1. Best practices for noise barriers .....................................................................................1744.1.2. Best practices for planting ..............................................................................................1764.1.3. Works of art along the road.............................................................................................178

4.2. TRANSITION ZONES ATTRACT THE ATTENTION OF THE DRIVER...............................1784.3. FIXATION OBJECTS IN THE LATERAL ROAD SIDE ENVIRONMENT SUPPORT

OPTIMAL LANE TRACKING ..................................................................................................1834.3.1. Structures over the road ..................................................................................................1854.3.2. Vertical elements along the road .....................................................................................1874.3.3. Risk of optical bottlenecks ..............................................................................................1904.3.4. Distance illusion .............................................................................................................1904.3.5. Eye-catching objects .......................................................................................................193

4.4. DEPTH OF THE FIELD OF VIEW - DOMINANT EYE-CATCHING OBJECTS SUPPORT DETECTION OF CRITICAL POINTS ....................................................................193

4.4.1. Dominant eye-catching objects support detection of critical points ..............................1944.4.2. Change of road’s direction despite a dominant eye-catching orientation line

(e.g. city by-pass dilemma) .............................................................................................1965. CONCLUSIONS ................................................................................................................................ 200

5.1. INTRODUCTION .......................................................................................................................2005.2. CONCLUSIONS .........................................................................................................................2015.3. DISCUSSION ..............................................................................................................................2025.4. RECOMMENDATIONS ............................................................................................................203

6. REFERENCES ................................................................................................................................... 205


2012R36EN GLOSSARY OF TERMS:

Safety Rule No.1: Give road users enough time! Average drivers need 4–6 seconds to completely change their driving programme. At a speed of 100 km/h, this means that the driver travels a distance of up to 300 m while the change is being made. A user-friendly road will allow appropriate adjustments of driving actions to a new situation. It is necessary to arrange transition zones, remove visibility restrictions, make junctions perceptible or use markings to indicate at least 6 seconds ahead of critical points such as junctions, curves, railway crossings, bus stops or bicycle paths.

Abutment: The end support of a bridge deck, usually retaining an embankment.

Advance warning section: Section 7-10 seconds ahead of vehicle’s manoeuvre section before a critical point. It ensures calm preparation of the needed change in driving activity.

Alignment: The optimal roadway layout given the topography of the land and other constraints.

Anti-phantom grid: Grid that prevents erroneous perception of a signal caused by an interfering light.

Arc length: Distance from the start point to the end point of a curve.

Carriageway width: Width of paved lanes (excluding shoulders and central reserve).

Clear zone: The roadside distance, starting at the edge of the carriageway, that is free of obstacles and available for safe use by errant vehicles. This area may consist of a shoulder, a recoverable slope, a non-recoverable slope, and/or a clear run-out area. Vehicle restraint systems should be located beyond the clear zone wherever possible.

Curvilinear: Roadway design joining curved lines.

Critical point: Any area where drivers have to adapt their driving program by changing speed, braking, steering or changing lanes. Normally ahead of junctions, intersections, stops of public transport, exits, driveways, curves, carriageway width reductions.

Decision Sight Distance: Distance that drivers need to identify an unexpected or unusual situation with more complex decision demands; normally 2.5 seconds.


2012R36EN

Delineator: A marker post with a retro-reflector.

Distance illusion: Over-/underestimation of the distance up to the next critical point by non-parallel optical guiding lines in relation to road’s edge.

Distributor road: A road providing access from estate roads to the primary road system.

Field of View: The visual area over which information can be extracted at a brief glance without additional eye or head movements. It decreases with age, most likely due to decreases in visual processing speed, reduced attentional resources and less ability to ignore distracting information. The Field of View performance is correlated with a number of real-world functions including the risk of an automobile crash.

Safety rule No.2: Ensure appropriate speed + lane tracking! Motorised driving changes the field of view much more than any other movement. Monotonous or high-contrast periphery, optical misguidance and illusions affect the quality of driving. The field of view can either stabilise or destabilise drivers, and can tire or stimulate them. Speed, lane keeping and reliability or directions are functions of the quality of the field of view.

Field-dependency: Degree to which human perception is dependent on the holistic perceptual field so that perception performance cannot be separated from the overall impression of the environment. Psychological tests concerning field-dependency are now used to select pilots that are able to separate the perception of their own position independently from the visual information they get.

Fixation: Humans and many animals do not look at a scene in fixed steadiness. The eyes move around, locating attracting parts of the scene. These parts are scanned frequently. This is the basis for building up a mental, three-dimensional “map” of the scene.

Full cube technology: New optical technology with the ability to double the brightness of typical roadway signs.

Gantry: A fixed structure comprised of two columns linked at their tops by a beam spanning the carriageway and permitting the installation of traffic signs above the lanes.

Grade: The slope of the roadway between two vertical points of intersection.


2012R36EN

Horizontal alignment: The projection of a road - especially its centre line - on a horizontal plane.

Horizontal curve: Bend in a straight line along a roadway.

Horizontal shield: A shield that goes across a viaduct that is not horizontal in order to prevent the illusion of the road below inclining left or right. The effect makes the viaduct look horizontal and thus the road horizontal.

Human Factors: Peoples’ contributions to damaging events. It is the generic term for those psychological and physiological patterns which are verified as contributing to operational mistakes in machine and vehicle handling. In the case of road safety, the Human Factors concept considers road features that influence drivers’ right or wrong driving actions. It sees the causes of driver operational mistake as the first step in a chain of actions which may proceed to an accident. Many often observed operational mistakes result from a direct interaction between road characteristics and the driver’s perception characteristics.

Safety rule No.3: Pre-Programme drivers behaviour correctly! Drivers follow the road with an expectation and orientation logic formed by their experience and recent perceptions. Unexpected abnormalities disturb a mostly automated chain of actions and may cause drivers to “stumble”. Several critical seconds pass before the disturbance can be processed.

Monotony: Wearisome uniformity or lack of variety of colours and brightness contrasts along a roadway.

Optical density of the field of view: Amount of color and brightness contrast that results in a sufficient or a poor optical flow. It influences the quality of driving and the driver’s speed and is a function of the number of objects/information that contrasts with the background.

Optical illusion: The word illusion comes from the Latin verb illudere meaning “to mock.” Illusions are the result of the complex information processing of the brain and the visual system that tricks us into perceiving something as different from what it actually is. Thus what we see does not correspond to physical reality.

Orthogonality: Intersecting or lying at right angles to the roadway.

Prismatic VMS (Variable Message Signs): Easily changeable roadway message signs based on three-sided prism technology.


2012R36EN

Reaction time: The delay between the presentation of a stimulus and the initiation of a response.

Retro-reflective material: Reflective material used for signage and markings.

Rotation of the pavement: The rate the pavement must rotate from curve to curve without destabilising vehicles.

Self-explaining road: Roads that are easy to use and navigate without any signs or signals. They are self-evident to road users and clearly show how drivers should behave.

Sight distance: Distance, measured along the road axis, from which drivers at a given location on a road may observe a point on the road or at a given height above the road, when their sight is not hindered by traffic or by lateral obstacles. Sight distance is generally a function of running speed, reaction time, type of event and distance necessary to manoeuvre (Modified from SETRA 1994).

Spatial perception: Process through which humans and other organisms become aware of the relative positions of their own bodies and objects around them in the environment. Spatial perception provides cues, such as depth and distance, which are important for movement and orientation. Space is perceived by humans as an oval. The eye scans this oval in subconscious small interior circles of 15° in a counter-clockwise direction. All of the information collected forms a three-dimensional space. This space ends at 8m. Beyond this point, people can only perceive two dimensions, but they can still judge distances and estimate which of two objects is nearer to them.

Stopping Sight Distance (SSD): Distance necessary for the driver of a vehicle (2-3 seconds) moving at a given design speed to stop his vehicle after observing an object on the road. It is the sum of the braking distance and the travelled distance during the perception-reaction time (modified from SETRA 2006).

Superelevation: The slope or “bank” of the roadway in a horizontal curve that helps offset the centrifugal force on the vehicle traversing the curve.

Transition curve: Transition path for entering and exiting a circular horizontal curve.

Transition zone: Passage from a higher-speed road section to a lower-speed road section such as the change from a rural road to a town or village streetscape. It allows the driver to make an appropriate adjustment of driving behaviour to a new situation. It should give enough time for anticipation, driver’s decision and braking


2012R36EN

manoeuvres. The length depends on the kind of change. The more complex the scene and demands are, the longer it has to be.

Vertical alignment: Projection of a road vertically over the topography of the land.

Viaduct: A road or railway bridge generally with a large number of spans and/or with piers of great height

LIST OF ABBREVIATIONSAASHTO American Association of State Highway and Transportation Officials

AS/NZS Standard created by Standards Australia, specifies performance and installation requirements for public lighting

ASD Approach sight distancecm CentimetreDSD Decision Sight DistanceEKL Entwurfsklasse [design class]H DistanceHF Human FactorsInIR Instituto de Infra-Estruturas RodoviáriasIRF International Road Federation ISD Intersection sight distanceIST Intelligenz System TransferITS Intelligent Transport Systemskm/h Kilometres per hour

K-values Plot of the rate of vertical curvature, or length per percent of difference in grade [distance required to effect a 1 percent change in gradient]

L Lettering legibility distanceLG Länge der Geraden [length of straightaway]LNEC Laboratório Nacional de Engenharia Civilm MeterMGSD Minimum gap sight distancemin. MinimumMUTCD Manual of Uniform Traffic and Control DeviceNCHRP National Cooperative Highway Research Program

NE Not, but elsewhere: Not in Standard, but in other technical conditions it is included (and identified by audit)

NPDM National Road Planning and Design ManualP Partly


2012R36EN

LIST OF ABBREVIATIONS (continued)

PIARC (Permanent International Association of Road Congresses) World Road Association

PSD Passing Sight DistanceRep. RepublicRSI Road Safety InspectionRSA Road Safety Audits Secondssec SecondsSSD Stopping Sight DistanceTAC Transportation Association of CanadaV Speed (Latin velocitas)VB Visual BasicVMS Variable Message SignY YesYD Yes, DirectlyYI Yes, Indirectly


2012R36EN SUMMARY

In the previous World Road Association cycle from 2008 to 2011, the “IST Checklist 2008” was used to identify the degree to which the Human Factors items are explicitly or implicitly addressed in 9 countries´ currently used national design standards and guidelines for rural distributor roads. The checklist contains about 100 validated Human Factors (HF) criteria especially for spatial perception. Guidelines from the following countries were examined: Portugal, Canada, Australia, Japan, China, Hungary, Czech Republic, France and the Netherlands.

The HF Checklist consists of three groups of criteria. How well these are addressed in current design guidelines is described in the following sections.

I. The need for the driver to anticipate any critical point in the driving environment and adequately respond to that is well described in the standards of developed countries. Of the HF demands, 49% were met, 28% were partly met and 23% were not met.

II. Management of the field of view to ensure appropriate speed and lane tracking is poorly addressed in the design standards. Of the HF demands, 9% were met, 32% were partly met and 59% were not met. The subject of spatial perception underlying the HF demands seems to be a blind spot in the field of road design. The value of parallel planting and guiding structures for the lane tracking task and to avoid optical illusions is fully described in only one design standard, partly described in two and not described in seven. The need for contrasts in the visual periphery to avoid monotony and manage the speed perception of drivers is not addressed in the standards or only addressed in a rudimentary way (50%). The possibility of using eye-catching objects to support lane tracking and the detection of critical points is not addressed in the standards (69%) or only partly addressed (18%). Only 13% of the standards mention the need to avoid misguiding and misperception. But even then, there is no clear specification for designers about how to do that.

III. The HF demands to pre-programme driver’s behaviour are addressed in guidelines differently. Of the HF demands, 34% were met, 27% were partly met and 39% were not met. In most guidelines, drivers’ expectations are not well covered. Changes of function or direction despite the dominant optical characteristics of the road are not mentioned (56%), or only in a rudimentary way (22%). There is broad agreement in saying “Never surprise the driver”. However, it seems to be difficult to include this along with practical requirements in standards, because the underlying principles of the subconscious regulation of driving actions, reasoning and learning are not well known.


2012R36EN

From the above we may conclude that management of the field of view in particular, as well as preprogramming driver’s behaviour, are not well incorporated in design standards and in RSIs and RSAs. This may hinder the application of such knowledge by road designers. The following recommendations help to spread the knowledge and improve road safety:

1. Train future road designers and auditors in subjects like spatial perception, management of the field of view and pre-programming of driver’s behaviour.

2. Train them also in the basics of self-explaining road design, design-for-all, and the basic principles of a hierarchical system of road categories.

3. Use best practices from other countries’ design standards/guidelines, as described in chapter 3, to supplement one’s own guidelines with HF items.

4. Develop proposals for missing links, as described in chapter 4. The design guidelines need systematic improvement concerning the Human Factors principles of spatial perception, the management of the field of view to ensure appropriate speed and lane tracking, correct pre-programming of driver behaviour and other more specific features (see chapter 4).

It is now up to the responsible governmental parties to use these results to avoid preventable accidents, save human lives and ensure that state-of-the-art science and technology will be used to achieve this goal.


2012R36EN

1. INTRODUCTION

1.1. STARTING POINT

Roads are made for drivers. Designers hope that their newly designed road will have an elegant alignment, provide good mobility and multi-modal connections and a high safety level with a low risk of accidents. But some newly opened roads unfortunately show collision/crash concentrations (black spots) in a short time. Along these stretches of road, a road safety inspection is needed to identify the deficiencies that are causing accidents. Sometimes the causes are obviously a breach of design and road safety rules. But sometimes design and construction are according to the rules, thus road designers are unable to detect any mistake and the reasons for the accidents.

The limitations and the physiological, sensorimotor and cognitive principles of spatial perception can reliably explain accidents. They provide a solid basis for developing effective remedies. In Germany over the past 10 years, about 1,500 technically “unexplainable” accidents on older as well as newly built rural roads have been investigated. About 70% of these accidents were caused by misperception, overloading of drivers’ ability to perceive and react, deficiencies in optical orientation and guidance, and inconsistencies between drivers’ expectations or habits on the one hand and design/signage on the other. All these causes could be clearly related to the laws of human spatial perception and orientation.

During the process of investigating the accidents and rehabilitating the black spots, it turned out that expensive full reconstruction was often not required. In many cases, it was sufficient to provide a gradual transition for the driver or to improve optical guidance, manage the field of view or remove the inconsistencies between drivers’ expectations and design/signage. A new kind of optical guidance and some creative low-cost remedies proved surprisingly effective as supports for drivers’ spatial perception, anticipation and orientation.

That said, it remains that most of the laws and limitations governing the spatial perception and orientation of humans are known in only a rudimentary way as they apply to road design. The existing Human Factors guidelines of Australia for example, of the TRB or in the AASHTO Highway Safety Manual, provide a great deal of theoretical knowledge and practical advice for designers wishing to take into account the physiological, neurological and cognitive limitations of drivers. However the subject of spatial perception is not clearly covered in the guidelines.

Do road designers know that estimation of distance depends on individual visual acuity and that results get worse the more (this is ok) and faster the driver moves? Or that the speed of oncoming and crossing vehicles is underestimated systematically?


2012R36EN

Or that a curve to the right is – due to the specifics of our balance organs in the ears, joints, muscles and soles of the feet – more difficult to negotiate than a curve to the left? Visual perception alone is an enormous area of knowledge and research. There are hundreds of critical values to consider just for the perception of curves/slip roads, for lane keeping or the choice of speed. Especially interesting is it, when the designer has a choice between haptic, acoustic and visual signals. In this case it is good to know that a rumble strip stimulates the driver much more and much faster than white markings.

There is little known about the laws of driving along curves – about the risk of right-hand curves, the oscillating trajectory, the function of an optical frame in the outer curve and its minimal length. Spatial perception, information processing at 70km/h of higher is completely different from what is generally assumed, tending to be unpredictable and not complying to common sense principles. The investigation and description of this varying human behaviour is a special task for Human Factors investigation. Its characteristics – of which the most peculiar is its curious resistance to all well-intended instructions and indoctrinations - should be understood and learned by road designers.

In this spirit, the Human Factors subgroup of Technical Committee C.1 “Safer Road Infrastructure” tried to take a closer look at homo automobilensis and evaluate the chances of survival in the field of road design. For this purpose 9 national design standards were audited to check whether Human Factors needs and the limitations of road users are incorporated in design standards. The result clearly shows that spatial perception needs, management of the field of view and management of drivers’ expectations should be more clearly addressed in the standards. The incorporation of these prior Human Factors in guidelines and standards is an indispensable step in reducing the likelihood of accidents on newly constructed roads. They can also be used as a kind of checklist in “on-the-spot” investigation of accident locations or in road safety inspections (RSIs). Road Safety Auditors are able to validate the planning and design processes by performing road safety audits (RSAs). This document will be helpful in initiating such change. Best practices and principles will lead road design practitioners towards the design and construction of a safer road network.

1.2. HUMAN FACTORS IN SPATIAL PERCEPTION – A BASIC FACTOR FOR SAFER ROAD INFRASTRUCTURE

The definition of Human Factors used in the field of road safety is a very comprehensive, contextual one. It allows inclusion of all kinds of human-related problems such as education, behaviour, situational motivation, performance and also temporary physical and mental conditions like intoxication, heart attack, ongoing use of medication, alcohol abuse, fatigue, aggression, anger and so on. This has led to an inflationary use of the term as a buzzword for all kinds of issues that arise when humans have to cope with any demand in a technical, operational or organizational aspect of road traffic.


2012R36EN

“Human Factors is an applied, scientific discipline that tries to enhance the relationship between devices and systems, and the people who are meant to use them. As a discipline, human factors approaches system design with the “user” as its focal point. Human factors practitioners bring expert knowledge concerning the capabilities and limitations of human beings that are important for the design of devices and systems of many kinds. There has been a number of elements within the field of transportation engineering that have benefited from human factors research, including sight distance requirements; work zone layouts; sign design, placement, and spacing criteria; dimensions for road markings; color specifications; sign letter fonts and icons; and signal timing, etc.” (Transportation Research Board of the National Academics 2008).

Human Factors expertise calls on the one hand for a knowledge of the technical and physical factors that contribute to the dysfunction of a technical system; on the other hand it also requires sufficient knowledge of cognitive and experimental psychology and its principles of learning, memory, knowledge representation, perception, information processing, logical reasoning and so on.

When examining and reconstructing extreme collisions there is evidence that the existing technical and psychological knowledge do not link adequately to provide realistic Human Factors doctrine and definition. Damaging events in a nuclear power station are not comparable with those in road traffic, firefighting or handling of missiles. They are normally accessible in only a very restricted way to cognitive and experimental psychology. Otherwise academic research on cognitive and experimental psychology is not really able to clarify these specific and demanding technical events.

Only a clear and unambiguous definition of the term will make it possible to include the human restrictions of spatial perception and information processing in road design and accident prevention. Hence the Human Factors subgroup of PIARC Technical Committee C.1 “Safer Road Infrastructure” has decided to use a definition which focuses on road features which influence drivers’ performance and which can cause misperception, optical illusion and other operational mistakes that result in accidents. This goes back to the origin of the use of the term in German psychotechnics, which in the early 1900s began to respond to all the questions that had arisen about the use of trains, cars and machines following the industrial revolution:

“Human Factors (HF) has been a technical term ever since the 1930s. It is defined as the contribution of human nature to the development of a technical dysfunction or failure in handling machines, technical systems or vehicles. Human Factors is not an equivalent to human behaviour or human performance. It is the term for those physiological, sensori-motor and cognitive principles of patterns which are verified as contributing to operational mistakes in machines, technical systems and vehicle handling.” (Sieber, 1986)


2012R36EN

In the case of road safety, the Human Factors concept covers road features that influence a driver’s right or wrong driving actions. It covers the causes of driver operational error as the first step in a chain of actions which may lead to an accident. Many often-observed operational mistakes result from direct interaction between road characteristics and the driver’s reaction characteristics. Since these characteristics cannot be changed, the road design has to be self-explanatory.

Drivers’ temporary mental or physical conditions or situational motivation are only moderator variables in operating cars safely. They are excluded to concentrate instead on those conditions which can be influenced by road design and construction.

This Report is focused on an audit of 9 national standards of road design, looking at the question of whether accident-triggering road features are already incorporated in national design standards. The detailed definitions of accident-triggering road features are taken from the “Human Factors Principles of Spatial Perception for a Safer Road Infrastructure” – an update of PIARC’s Human Factors Guideline of 2007. The result shows that driver’s needs related to spatial perception, optical guidance, management of expectations and field of view should be more clearly incorporated in standards.

1.3. ROAD DESIGN – THE STANDARD WAY OF PLANNING AND BUILDING

Road geometry is typically studied and designed in terms of the three components – horizontal alignment, vertical alignment and cross-section – that define a roadway as a three-dimensional ribbon.

Your comment is valid however when you consider a ribbon undergoing horizontal or virtual curves, it is in three dimensions.

The design of the three components is done sequentially (in a static or in a dynamic way) and it should be coordinated according to existing orientation from several design standards and guidelines. It is mainly the horizontal and vertical alignments that should be well coordinated in order to have a good result in the three dimensional alignment. Such coordination is better achieved using software tools that show the road project in three dimensions, covering the total land requirement and adjacent terrain – how the road fits into the landscape. The downside of this software, and the digital terrain models that come with it, is that a road design can be prepared in almost no time, and one can be quite happy with it, yet not have a proper knowledge of what has been done or of how the software did it (e.g. superelevation runoff).

When road geometry was studied using curve templates (circles, clothoids and cubic parabolas) on paper maps and terrain profiles, and only some of the cross-sections


2012R36EN

were drawn, at the most difficult points, to see how the road adapted to the terrain, it took much more time than it does now. Designers took advantage of this longer time for an in-depth analysis of multiple solutions, which were drawn on the map using different colours. Nowadays that longer time does not exist, and sometimes the road project has political or economic deadlines that are incompatible with a well thought out and well reviewed road design.

It is quite difficult, if not impossible, to comply with all the constraints imposed on road geometric design, and to achieve a solution that optimizes most of them. Topography, geology, geotechnics, land use, drainage, environment, preservation of historical and cultural values, traffic operations, road equipment, tunnels, bridges, viaducts and other engineering structures, median openings, emergency escape ramps, roadside design, intersections, rest area and toll booth locations, landscape treatment and cost- benefit analysis are some of the multiple constraints that the designer must keep in mind during the geometric design process.

Within this process, the designer creates a three-dimensional field of view for the driver. He should therefore have all the information about complementary designs (road equipment, engineering structures, landscape treatment, etc.) throughout the road design process, from the feasibility and preliminary design stages to the detailed design stage. He should also have basic knowledge and experience about how the three-dimensional field of view can be configured to ensure safe and reliable spatial perception and optical guidance.

It is essential in road design – seen as the final look of the road for the driver – that Human Factors guidelines should be respected in all design phases by all members of the design team, mainly by the geometric designer who has the very difficult task of modelling the three dimensional sculpture consisting of the road and its environment. That is why he should also have a thorough knowledge and understanding of spatial perception and the characteristics of the 70km/h motion of drivers.

1.4. PROCEDURE FOR RELATING HUMAN FACTORS DEMANDS TO THE STAGES OF A DESIGN PROCESS

This report is based on a specific research process that sought to find the link between the Human Factors demands of drivers and all aspects of a complete road design.

The first stage was to understand the customary process from the beginning to the end of a highway project. Because a project starts with an idea or a requirement, the planning of all aspects of the design needs to start well before the construction starts. Usually the entire project is described in small technical steps such as: feasibility study, conceptual design, preliminary design, permission design (specific step for Germany), detailed design and construction stage. For each study or document that


2012R36EN

has to be produced, a list of activities was prepared and linked with one or more Human Factors that could be applied to or could influence the activity in question.

This exercise was the first time that Human Factors, seen from the psychological point of view, were connected with specific technical activities. From this first step, the procedure was modified and a decision was made to start from the items on the Human Factors checklist “IST Checklist 2011” from Intelligenz System Transfer and link them with the technical activities of road design (Birth,et al., in publication 2011).

From that point, a complete table with all possible linkages between Human Factors items and design/technical activities was prepared. The table formed the basis for the analysis of standards from different countries, with a view to finding existing practices which take into account Human Factors needs in each standard.

A brief comparison table was produced and an evaluation for each Human Factors item was prepared to identify the extent of incorporation in the standards. The findings were transferred to another table, which is the basis of this present report. The compilation of best practices provides the designer, and principal decision makers from transportation agencies, with a good starting point for determining how to incorporate Human Factors demands in their own standards without having to study the enormous area of knowledge and research concerning visual perception, action regulation, learning and human reasoning.

1.5. FROM PSYCHOLOGICAL HUMAN FACTORS TO GUIDELINES AND STANDARDS

The incorporation of Human Factors in guidelines and standards is the next step, and this document will be of assistance in initiating that effort. Best practices and principles will provide road designers the tool necessary to accomplish this. The next part of the document will explain the limitations on the implementation of new practices in guidelines and standards.

Guidelines

The role of guidelines is to provide information and background to assist the designer in choosing the appropriate combination of features, dimensions and materials for a given design. However, it is important to understand that guidelines themselves do not state the dimensions for any given design. That is the designer’s responsibility. The final design is the sum of all decisions taken during the design process.

Road design guidelines are necessarily general, because they cannot cover all site-specific conditions. They are based on prevailing and forecast vehicle dimensions (yes, it refers to vehicle dimensions such as in size and geometry, so the expression


2012R36EN

is good) and performance, driver behaviour and current technologies. Human Factors are to be incorporated in guidelines to explain their importance and specifically define the influence of spatial environment on the user of the infrastructure. These guidelines will vary with time and location requiring periodic revision and updating. How guidelines are applied depends on agency policies, transportation characteristics and new implementation, for Human Factors will be easier if they are incorporated in official guidelines.

The most important point in the incorporation of Human Factors is to include the result or best practices directly in design elements. From past experience, even if a quite well organized discussion is presented in a guideline, as long as the detail is not specifically incorporated in design elements, few designers will be aware of it, because the direct application of a table and a formula found in geometry or roadside safety issue could prove to be easier.

Examples provided in this report as best practices could be incorporated directly in existing guidelines with some basic adjustments. For the principles proposed as solutions to respond to specific influences of the spatial environment, it will be the responsibility of agencies to study in more detail the proposal provided in the chapter in this report before incorporating them in their guidelines.

Even if guidelines are the basis of most of the existing standards, human factors should be included directly in standards.

Standards

Over the past several decades, design standards, usually based on laws of physics or empirical data, have been used by designers. Increasingly, designers have come under pressure to reduce construction costs by using lower standards, on the assumption that even minimum standards are always acceptable.

Design dimensions that do not meet standards do not necessarily result in an unacceptable design – and dimensions that meet standards do not guarantee an acceptable and safe design. In assessing the quality of a design, it is not appropriate simply to consider a checklist of standards. The design has to be reviewed with judgment; standards merely assist the reviewer in making judgements. Every detail which has a relationship with any Human Factors element should be included in standards and properly explained.

Most of the incidents on highways which were correctly designed following the standards, with proper inclusion of Human Factors best practices found in different countries; most of these incidents will be solved.


2012R36EN

Agencies must change their own standards to integrate Human Factors and specifically the influence of spatial perception and optical guidance on users. This report provides a good starting point to improve agencies’ knowledge and facilitate translation of subconscious psychological influences into highway design elements. Since part of this report is based on principles which have not been verified on site, multiple studies are needed to transform these principles into empirically well documented new standards.

Best practices from foreign countries should be evaluated and adapted to each standard, but agencies should keep in mind the origin and the reason of any changes related to Human Factors, to help the designer judge the situation in relation to safety.

Finally, the spatial perception of the environment influences the driver, and the situation should always be analysed with a view to having a safer project. Changes in standards to incorporate Human Factors do not necessarily mean an increase in construction cost. Agencies should inform and train their designers to implement design elements where the Human Factors are included.

2. COMPARISON OF CURRENT NATIONAL DESIGN ELEMENTS AS THEY RELATE TO HUMAN FACTORS OF SPATIAL PERCEPTION

2.1. INTRODUCTION

Engineers build roads for the use of ordinary people. The design must of course be influenced by human traits and limitations of perception and the way drivers regulate their activity. Human Factors (HF) takes into consideration various traits that are critical to creating a safe and efficient driving environment. Scientific values have been attributed to these human traits, such as how our visual organs search and process information, how fast we can detect critical incidents, how quickly we can react after information is processed, and so forth. For an effective and safe road environment, these values must be considered in the design process and used as design controls. The evaluation criteria for this audit were taken from the latest update of the “IST Checklist 2008” which is a part of the PIARC Guideline “Human Factors Guiding Principles of Spatial Perception” and is based on the Human Factors Guideline of Brandenburg, Germany (Birth, Sieber, & Staadt, 2004). The “IST Checklist 2008” is a complex diagnostic instrument for the prediction of the accident probability of road sections. It contains some 100 reliable items with a validity of 0.65 (Birth & Pflaumbaum, 2006).

This report compares current national road design standards and guidelines for rural distributor roads. The Human Factors and the examples are generally valid for other road categories like access roads, motorways and through roads. The report includes the results of the audited guidelines as well as the best practices of the following countries:


2012R36EN

• Portugal, Canada, Australia;• Japan, China;• Hungary, Czech Republic;• France, The Netherlands (NL).

The audit focused on how well HF safety rules were incorporated in the design, standards for such critical design elements as sight distance, perception and reaction times and design consistency. A review and audit of standards, guidelines and best practices (from here on referred to as “standards”), as they relate to HF, is the essential first step in choosing the most effective standards for the subsequent development of international guidelines and best practices.

While all the countries’ design standards for rural distributor roads were audited using the same auditing template, the auditing style varied somewhat, producing a range of results. With regard to voting terminology:

• “Yes, Directly” (YD) was used where HF were explicitly included in the design. • “Yes, Indirectly” (YI) was used where HF were included in the design but not

explicitly explained. • “Partly” (P) was used where there was partial inclusion of HF but the rationale was

not explicitly discussed. • “No” was used where no HF were considered or were considered elsewhere.

The term selected was not consistent from auditor to auditor, which shows that they had differing perceptions of the level of HF-inclusion.

It was stated at times that various standards incorporate specific HF rules to address a given design issue, consistent with HF principles. However that information was not always evident. At times it was necessary to go back to the raw data (technical design information), where available, to provide a consistent comparison.

While most standards provided guidance for design using HF principles, the majority did not provide explicit information regarding HF, but set values and design criteria that implicitly included them. In general, some of the most comprehensive and inclusive examples were found in the Portuguese and Dutch standards.


2012R36EN

2.2. HUMAN FACTORS SAFETY RULE NO.1: DRIVERS NEED ENOUGH TIME

HUMAN FACTORS SAFETY RULE NO.1: GIVE ROAD USERS ENOUGH TIME!

The time it takes an average driver to adapt from one traffic situation to the next, or to adjust to new requirements, is much longer than what is stated in many current guidelines. Because humans are not constantly alert and searching for new information, they need more time – especially when information is difficult to find or is unusual, when they are confronted with complex decisions or when unusual manoeuvres are required. Instead of one or two seconds (simple “stimulus–reaction time”), it takes the average driver at minimum 4-6 seconds to adapt to a new driving requirement (“anticipation–response time”, [1], [2]).

A user-friendly road will give drivers the necessary time to adapt to new and unexpected situations. It will give them the time they need to safely re-organize their driving program. That is why it is not enough to provide the driver with a sufficient reaction time of 2-3 seconds (Stopping Sight Distance, SSD, with manoeuvre section and response section). The design should also provide an anticipation section with a minimum of 2-3 seconds to identify an unexpected or unusual situation with more complex decision demands (Decision Sight Distance, DSD). In situations that are more complex or involve higher speeds, it is recommended that there also be an advance warning section with proper signing and instructions.

2.2.1. Transition Zone

2.2.1.1 Manœuvre, response and anticipation sections

Minimal adapting time = 4-6 secDriving action

Vehicle response technical time to break/slow down

drivers responsedetection + decision time, 2-3 sec

Anticipationtime for identification of unexpected situations, 2-3 sec

Preparationof driver with signing and warning, 3-4 sec

Manoeuvre section

Response section

Anticipation section

Advance warning section

Trasition zone

Section

FIGURE 1 - SKETCH OF 6 SECONDS SAFETY RULE(Source: Birth, Sieber & Staadt, 2004)


2012R36EN

Human Factors evaluation criterionIn the driver’s response section, the driver has to slow down or stop before an intersection, a curve or an exit. He has to re-organize his driving behaviour. He has to detect the driving requirement and then he has to react. Most countries allow 2.5 seconds for this task in their design requirement and refer to the distance in question as the Stopping Sight Distance. The Stopping Sight Distance includes the time for braking / slowing down and the time for the driver to respond.

In case of a sudden change in the situation (e.g. changes in the course of the road, traffic density, speed or function) drivers need more detection-decision and anticipation time to cope precisely and reliably. Analysis of accidents shows that drivers indeed recognize the type of road but are often surprised by an unpredictable or complex situation. A safe road design provides time to reanalyze the situation and to arrive at a suitable solution without risk. The design should provide a response section with enough detection-decision time as well as an anticipation section. Each should offer a minimum of 2-3 seconds to identify an unexpected or unusual situation with more complex decision demands (Decision Sight Distance, DSD and Riding Distance). In this report, changing lanes was regarded as a special case and not considered.

TABLE 1 - THERE IS A MANOEUVRE SECTION, A DRIVER’S RESPONSE SECTION, AN ANTICIPATION SECTION

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Japa

n

Hun

gary

Cze

ch R

ep.

Chi

naManoeuvre section exists? YD YI YI YD YD YD YD YD YIDriver’s response section exists (2-3 sec.)? YD YI YI YD YD YI P P YI

Anticipation section exists (2-3 sec.)? YD P YI No YD P P No No

In Canada’s guidelines, for instance, the following values for Stopping Sight Distances and Decision Sight Distances are provided:


2012R36EN

TABLE 2 - STOPPING SIGHT DISTANCE FOR AUTOMOBILES AND TRUCKS WITH ANTILOCK BRAKING SYSTEM FOR ROADS WITHOUT GRADE

Design Speed

Assumed Operating

Speed*

Perception + Reaction(“driver’s

response”) Time Distance

Coefficient of

Friction

Braking Distance(vehicle’s response)

Stopping Sight Distance (rounded)

(km/h) (km/h) (s) (m) (m) (m)40 40 2.5 27.8 0.38 16.6 4550 47 - 50 2.5 32.7 - 34.7 0.35 24.8 - 28.1 60 - 6560 55 - 60 2.5 38.2 - 41.7 0.33 36.1 - 42.9 75 - 8570 63 - 70 2.5 43.7 - 48.6 0.31 50.4 - 62.2 95 - 11080 70 - 80 2.5 48.6 - 55.5 0.30 64.2 - 83.9 115 - 14090 77 - 90 2.5 53.5 - 62.5 0.30 77.7 - 106.2 130 - 170100 85 - 100 2.5 59.0 - 69.4 0.29 98.0 - 135.6 160 - 210110 91 - 110 2.5 63.2 - 76.4 0.28 116.3 - 170.0 180 - 250120 98 - 120 2.5 68.0 - 83.3 0.28 134.9 - 202.3 200 - 290130 105 - 130 2.5 72.9 - 90.3 0.28 155.0 - 237-6 230 - 330

(*Range of assumed operating speed is from average running speed for low-volume conditions to design speed)

TABLE 3 - DECISION SIGHT DISTANCES Design speed (km/h) Decision sight distance for avoidance manœuvre (m)

A B C D E50 75 160 145 160 20060 95 205 175 205 23570 125 250 200 240 27580 155 300 230 275 31590 185 360 275 320 360100 225 415 315 365 405110 265 455 335 390 435

120+ 305 505 375 415 470Avoidance manœuvre A: stop on rural roadwayAvoidance manœuvre B: stop on urban roadwayAvoidance manœuvre C: speed/path/direction change on rural roadwayAvoidance manœuvre D: speed/path/direction change on suburban roadwayAvoidance manœuvre E: speed/path/direction change on urban roadwaySource: Transportation Association of Canada (TAC), 1999, updated December 2007

According to Dutch standards, road markings and critical points such as crossings should be visible within a “Riding Distance” (see the part of the text below about


2012R36EN

riding distance at the anticipation of curves). This special “Riding Distance” is used to ensure visibility over the course of the road and decision points between 8 and 10 seconds ahead of time. To anticipate a curve, a perception-reaction distance and a certain visible length of the curve are provided. The latter refers to that part of the curve which the driver has to be able to perceive to detect the curve.

That is why no Intersection Sight Distance is needed to provide a view of oncoming intersections. This is appropriate to the anticipation section which drivers need in case of unusual or complex information processing demands and contains additionally an advance warning section (see next point).

TABLE 4 - NECESSARY SIGHT DISTANCES AT THE APPROACH TO CURVES

Speed Sight distance preceding the curve Visible part of the curve

Total sight distance on the curve

km/h s m s m s m

60 2 55 3 85 5 14080 1.75 40 3 65 4.75 105100 1 20 3 35 3 50

Source: CROW, 2002, Table 7-2.

It is recommended that a parameter be chosen for the transition curve that is as short as possible in order to help make the curve visible. This is not to see that no transition curve is needed because it supports smooth steering.

In the transition zones, French standards recommend a design that allows for a deceleration distance, an acceleration distance, and sight distances (linked to the design speed). In exit lanes, the required sight distance is linked to the 85th percentile. The total stopping distance consists of the braking distance under normal conditions plus the distance travelled during the perception-reaction time, which in France is 2 seconds.

Australian standards outline road design philosophies and principles that lead to the concept of the “self-explaining road”. This is a road whose features “tell” the driver what type of road it is and therefore what design can be expected. This provides a confidence in expectations for the driver, who then operates the vehicle in accordance with those expectations, which are in tune with the nature of the road. The standards use Stopping Sight Distances for emergency braking, and Intersection Sight Distances to ensure an unobstructed view of the entire section, including traffic control devices. The driver should be able to anticipate the course and driving situations to avoid collisions with other cars.

The Portuguese standards state that the Decision Sight Distance should be provided in all places where driver expectancy might be violated or where it is probable that


2012R36EN

the driver will have doubts about the information received. This includes intersections, interchanges, changes in cross-section (reduction in the number of lanes), service areas and similar facilities. Lighting guidance is provided to reduce night-time collision risks at critical points. There is also encouragement to plant shrub screens in the outer part of curves, to mark the alignment and facilitate night driving.

Czech road design standards contain standard values for the design speed of highways for the important parameters like recommended or minimal. Anyway, there is Passing Sight Distance and Stopping Sight Distance related to speed and grade, but not in the values recommended from the point of view of Human Factor needs. The Czech standards, like the Canadian, include variable friction coefficients and slopes when calculating Stopping Sight Distances.

The Hungarian standards seem logical from the HF perspective when addressing transition zones, but the zones do not seem to be explicitly defined.

Conclusion Design speeds, Stopping Sight Distances and Decision Sight Distances are well defined in 7 of 9 standards. Difference in speeds when transitioning from one segment to another is also addressed, and speeds are suggested for each classification of road. Canadian and Portuguese standards specifically explain operating speed consistency and violation of driver expectancy. The perception and reaction time for the Stopping Sight Distance is typically defined as 2.5 seconds in most countries.

The Sight Distance for the anticipation of complex or unusual situations is included directly in only 3 of 9 standards; in another 3 it is partly included, and in 3 it is not mentioned. It goes up to 8-10 seconds ahead the critical point in the Canadian, Dutch and Australian standards.

2.2.1.2 Advance warning section

FIGURE 2 - CRITICAL POINT IS NOT VISIBLE(Source: Birth, 2008)


2012R36EN

Human Factors evaluation criterionThe critical point is not always visible early enough. For instance, if there are work zones or hidden exits after a parking space or a curve, then the advance warning section allows for calm preparation and a safe adaptation of alertness and the needed change in driving activity. While the manoeuvre, response and anticipation sections are necessary for safe driving, the advance warning section offers more comfort and safety to the driver. The desired time for this section is at least 4 seconds, depending on the permitted speed.

TABLE 5 - ADVANCE WARNING SECTION EXISTSN

L

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Advance warning section exists (about 4 sec.)? YI YI P P NO NO NO NO NO

The Portuguese pavement marking guidance states that white transverse bar markings provide a warning to drivers to slow down at certain locations. They consist of a sequence of pairs of transverse white lines that move closer together as they approach the critical location. This seems to have elements of HF. Portuguese standards also define a number of rules to be respected, following HF principles such as the selection of the best horizontal and vertical alignment combinations.

Dutch standards discuss the coordination of horizontal and vertical alignment where sharp horizontal curves are treated with standard advance warning signs and beaconing. The pavement marking standard states that in situations such as approaching a priority road, arrows and striping and a triangle painted on the pavement should be used to warn drivers of the change. They also state that enough space must be provided between advance signposts and the decision point (intersection) to avoid confusing drivers. ‘Riding’ or ‘anticipation’ sight distance is defined as the distance of roadway over which a driver has to be able to see for safe and comfortable driving. The driver has to be able to see along the road in order to perceive, anticipate and react to information such as downstream traffic, marking and signing. A continuous view along the road is needed to:

• determine lateral position (for the tracking task);• anticipate and react to downstream events.

Information in the periphery of the field of view and edge-lines over a short distance are necessary for tracking. A Riding Sight Distance of 8 to 10 seconds is shown in table 6 for three speeds.


2012R36EN

TABLE 6 - RIDING DISTANCES Speed Riding distance (m)

(km/h) 10 s 9 s 8 s60 - - 13580 - 200 -100 280 - -

Source: CROW, 2002a, Table 7-1.

Before discontinuities like intersections, (tight) curves and changes in the cross-section where changes in driving behaviour are needed, sufficient Riding Sight Distance is a necessity. This has consequences for the alignment preceding the discontinuity and for sight-restricting objects on the shoulder.

The Portuguese and Dutch standards have the most specific requirements about providing guidance to drivers on advance warning sections.

Indian design standards provide general points that should be considered in designing a highway, such as the importance of horizontal curvature and the vertical profile being made as flat as feasible at highway intersections where sight distance is critical and vehicles may have to slow down or stop.

French standards ensure that horizontal and vertical alignment is properly integrated into the surroundings, and that there is compliance with visibility rules as far as possible and a degree of visual comfort.

Czech road sign manuals contain rules for placement of advance traffic signs for interchanges and intersections, railway crossings and similar situations. These are related to the design speed and size of the crossing roads.

In Canada, the design of private or public access to roads is generally treated in the same way as roadway intersection design. The standards define all details of signals, but do not directly relate them to HF. Location and size of signs along the road is related to the design speed. The Ontario Traffic Manual does recommend oversized signage for work zone warnings as well as advance warning signs up to one kilometre ahead of any new facility that may surprise the driver.

Conclusion Signing and marking are well defined in the various standards. For an advance warning section 2 of 9 standards (NL, Portuguese) already use Human Factors principles by increasing the number of bright and colour contrasts to increase drivers’ attention and slow them down. In addition, advance warning and instructions for the driver are provided for when there are work zones or hidden exits after a parking space or hidden curves – especially in the design of motorways.


2012R36EN

However in 7 of 9 standards, there is a failure to cover user needs on rural distributer roads. Because the advance warning section allows calm preparation, a safe adaptation of the alertness and the needed change in driving activity, it should be incorporated in standards for rural distributer roads in order to minimize the number and severity of accidents.

2.2.2. Perception and Visibility

2.2.2.1 Critical point is visible and clearly identifiable

FIGURE 3 - INVISIBLE CRITICAL POINT – COVERED BY PLANTING

(Source: Birth, Sieber, & Staadt,2004)

Human Factors evaluation criterion On self-explaining roads, critical points should be visible early enough without signals. A critical point is any requirement to adapt the driving program to a new situation like crossings (with/without traffic lights), public access, lane losses/merges, public transportation stops, motorway entrances/exits, changes of function or entrances to towns/villages etc.

TABLE 7 - CRITICAL POINT VISIBLE AND CLEARLY IDENTIFIABLE

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Critical point visible and clearly identifiable (at least 4-6 sec. prior to manoeuvre section)?

YD P P P P NO P P NO

Chinese guidelines ask that coordination of horizontal and vertical alignment be such as to ensure good visuals. Regarding signage, the general rule is that traffic signs and markings should be visible for drivers.


2012R36EN

The Dutch design guidelines ensure the visibility of critical points by requiring sufficient riding distances near discontinuities such as intersections, (tight) curves and changes in the cross-section where changes in driving behaviour are needed. Road signs should be visible (not hidden by planting or engineering structures) and legible at sufficient distances under all circumstances. Extra (vertical) beaconing in addition to the markings on the road, for instance sight posts, may be required, for example with tight curves. Intersections and pedestrian crossings often need more specific lighting to provide visibility.

French, Portuguese and Canadian standards cover HF by considering co-ordination of the horizontal and vertical alignment. French standards also address the fact that motorists need to behave in a manner compatible with the road design. This is linked with the logic axiom which requires continuity in design and avoidance of any sudden changes of characteristics.

Czech road design standards contain rules for the sight distance and recommendations for the location and visibility of intersections, pedestrian crossings etc., but there is no general definition of “critical points”.

Japanese standards state that visibility of traffic signs, traffic lightss, markings and safety barriers is required, using a reaction time of 2.5 seconds for sight distance of traffic lights.

The Portuguese standards state that with roundabouts, the landscaping should allow for distant legibility of the intersection and it should facilitate orientation. It should be used for modelling the natural terrain as well as the use of visual arts. Any visual barriers, including shrubs, should be placed 2.0 m or more from the inner edge of the roundabout ring. Areas of the central island aligned with entrances should be given special attention, as collisions while entering a roundabout are very serious. These areas should only contain massive shrubs, which will considerably decrease the severity of collision. In alignment redesign, special attention should be given to the geometry of the transition zones, in order to ensure traffic safety and comfort. Information on sight distance and signals is also provided.

Conclusion Visibility of critical points is sufficiently described in 1 of 9 standards, partly described in 6 and not mentioned in 2. The Dutch standard emphasizes the use of riding distances to ensure early visibility of critical points. The Portuguese standard additionally emphasises the use of landscaping for modelling the natural terrain. However 8 of 9 standards fail to state clearly that critical points should have visibility unobstructed by plantings, ongoing curves, buildings or traffic facilities. These points should be directly incorporated in design standards.


2012R36EN

2.2.2.2 Curves are visible

FIGURE 4 - VISIBILITY OF CURVES(Source: Birth, Sieber & Staadt, 2004)

Human Factors evaluation criterionDriving reliably through a curve is critically dependent on the quality of the field of view. Best driving results are achieved when the driver has an unobstructed view on the course through the inside of the curve, and the outside of the curve has a continuous optical framing parallel to the roadway. On self-explaining roads, therefore, the presence and course of a curve should be visible sufficiently early without traffic lights.

TABLE 8 - CURVES ARE VISIBLE

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Curves are visible (at least 4-6 sec. prior to the manoeuvre section)

YD NO P YD YD P NO P NO

Canadian standards state that sharp horizontal curvature should not be introduced at or near the top of a pronounced crest. Also, design values well above the minimums for the design speed are desirable. Pavement markings are defined by type of road and follow the precise geometry of the roadway, though this is not specifically related to HF.


2012R36EN

The Portuguese landscape guidelines state that shrub screens should be planted along the outer part of curves to mark the alignment and facilitate night driving. Pavement marking plans for the edge of roadway markings are to be used on rural road links, and on entrance and exit ramps. They should be used to outline the directional islands.

French standards address sight distance as an HF consideration regarding curve perception and timely adaptation of behaviour. This is consistent with the notion of perception and activity adaptation corresponding to the anticipation section and the response section. Clear zone which contributes to the lateral vision, should exclude any aggressive devices such as trees, unless protected. Stopping Sight Distance must be considered at every point of the road.

Dutch standards state that guiding lines adjacent to the road support the visibility of the road’s course, as do items such as plantings, safety barriers and lighting posts. In addition the standard marking the super elevation is important for a clear view on the road surface. The standard also gives advice about making the inside and outside of the curve visible (see also “Optical framing of curves”). It also provides a detection-decision distance and a certain length of curve that has to be visible to the driver to enable him to notice it.

The Czech road design standard contains rules for combining horizontal and vertical alignment so that a horizontal curve will be visible before a crest. There is a set of graphics showing good and bad alignments.

Conclusion Visibility of curves is described directly in 3 of 9 standards, partly in 3 and not at all in 3. The Dutch standard has the most comprehensive approach to ensuring visibility, through detection-decision distances and optical framing of curves. The Portuguese standard also emphasises the use of plantings to mark the alignment, and other standards give advice on making the inner curve visible.

In sum there is a shortfall in 6 of 9 standards concerning continuity and parallelism of an optical guiding frame for the outside curve. The user’s need for anticipation and reliable guidance should be directly incorporated in the standards. Co-ordination of horizontal and vertical alignment alone is not sufficient to ensure reliable optical guidance for a curve.


2012R36EN

2.2.2.3 Intersection – visibility triangle from minor roads is not obstructed

major road

planted divisional island (60 cm)

transition

minor road

sight triangles

advanced warning section with planting

optical crossbar

FIGURE 5 - VISIBILITY TRIANGLE FROM MINOR ROAD IS NOT OBSTRUCTED(Source: Bielenberg, et al., 2002)

Human Factors evaluation criterionDriving reliably through an intersection or any crossing depends critically on the quality of the field of view. Best driving results are achieved when the driver has enough time to decide whether he has to slow down, stop, or drive through.

Plants, buildings or traffic facilities often hide other traffic or provide a too large view of the oncoming traffic. The visibility triangles at accident spots are often not guaranteed. Often they are too small, so that drivers cannot reliably anticipate the situation. Or they are too large, so that drivers speed up so as to have an unbroken drive through the intersection if other cars are approaching from the crossing road.

TABLE 9 - INTERSECTION - VISIBILITY TRIANGLE FROM MINOR ROAD IS NOT OBSTRUCTED

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Intersection - visibility triangle from minor road is not obstructed?

YD YI P YD YD NO YI YI P


2012R36EN

The Canadian guidelines provide a clear description of a sight triangle and explicit design requirements for approach sight triangles and departure sight triangles (Decision Sight Distance). Sight distance requirements for specific traffic control devices take into account requirements based on the perception and reaction time of drivers, vehicle acceleration etc. Inattentive drivers should be alerted to the fact that an intersection is ahead and should have enough time to react accordingly through provision of adequate deceleration and acceleration lengths, etc., given the design speed. Sound judgment is required in selecting the design elements that meet the expectations of the driver.

Distances for intersection spacing are provided, as is information on horizontal and vertical alignments to promote appropriate sight distances, which is important for collision-free operation. All types, patterns and geometries of intersections are defined, from minor road to roundabouts.

The Portuguese standards state that the location of intersections is dependent on the Decision Sight Distance. Intersections should, whenever possible, be located on straightaways with gradients not greater than 3%. They also should never be located near curves or crests, because these reduce the efficiency of the brakes, and crests and gradients affect the possibility of acceleration and limit visibility.

In China, the minimal Stopping Sight Distance has to be observed in all cases, and intersections should be on a straight section or a curve with a long radius. Coordination of horizontal and vertical alignment has to ensure the stopping sight distance.

Dutch standards recommend not designing too large a sight triangle at minor road approaches to major roadways, because drivers who have to give way may approach the intersections at high speed. However, 5m before the stop line, drivers need to be able to see the major road over a long distance. In addition, approaching drivers also need to have a good view of people on bicycles and mopeds if there is a cycling lane along a priority road.

French standards state that at the approach to any road access point, whether an interchange or an ancillary area, the driver must be able to decide to change direction and make the necessary manoeuvres. The notion of choice corresponds to the 6 second axiom (anticipation + decision). The necessary time to change a lane is conventionally taken as 2.5 seconds. Choice of the interchange type is based on the required traffic manoeuvres, traffic intensity, operating system and site configuration.

Australian standards provide comprehensive coverage of sight distance requirements for vehicles at intersections. The types of sight distances that must be provided in the design of all intersections include the approach sight distance (ASD), the safe intersection sight distance (ISD) and the minimum gap sight distance (MGSD).


2012R36EN

Intersections should be designed to provide the more conservative of these distances for all vehicle movements that involve giving way to other vehicles at an intersection. The design speed for traffic on a priority road at intersections will generally depend on the alignment standard adopted for the road in general.

The Czech standards state that the preferable location of an intersection is on a straightaway or flat curve. The visibility triangle has to be approved on the basis of the speed, the time for manoeuvre and the length of trucks.

Conclusion The visibility of intersections from minor roads is well defined in 6 of 9 standards; this factor is missing in only 3 standards. The sight distance requirements are described mostly in a way that ensures minimal sight distances for driver’s response. The Dutch standard also specifies avoiding too large a sight triangle, which can result in drivers speeding up. The Australian standard has the most comprehensive approach. It sets an example of how to incorporate this Human Factor in design standards.

2.2.2.4 Intersection – minor road has an unmistakable right of way

major road minor road

sight trianglesoptical crossbar

divisional island with planting > 60 cm

planted embankment

horizontal swing of minor road

new coursecovered old course

FIGURE 6 - MINOR ROAD HAS AN UNMISTAKABLE RIGHT OF WAY(Source: Bielenberg, et al., 2002)

Human Factors evaluation criterionAn unmistakable right of way is not guaranteed by regular signing alone. For the driver, the overall optical impression of the situation causes an expectation concerning the right of way. Drivers regularly expect to have the right of way if:

• their road is wider, • a line of trees or buildings suggest an optically unbroken continuous course,


2012R36EN

• the pavement on their road is optically not interrupted or distinguishable from the crossing road,

• or the road of their own direction has a more comfortable pavement than the crossing road.

The optically unbroken course of minor roads after an intersection should be covered by landscaping, road design measures (such as a horizontal swing of the course of minor roads before intersections) or special pavement and markings. The fixation of the driver should be guided reliably to the intersection with sufficient time for anticipation.

TABLE 10 - INTERSECTION - MINOR ROAD: UNMISTAKABLE RIGHT OF WAY

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Intersection - minor road: unmistakable right of way? YD YD P NO NO YD P YI NO

Canadian standards address the type, pattern and geometry of intersections from minor road to roundabout, and information provided for intersections with private access. Detailed information is also provided on cross-sections for all type of roadways.

The Chinese guidelines state that minor road geometry and cross-section design have to adapt to the main road.

The Portuguese guidelines cover the subject most comprehensively by addressing DSD and road width differences. It should be clear, from the lay-out of the intersections, which is the main road (always wider than the secondary road) and which is the secondary road (always narrower). Pavement design and markings are provided in the Portuguese guidelines, but they are not explicitly related to HF.

Dutch standards state that the design of the intersection should support the right of way. The appearance of the two roads has to be different. The cross-section, which is the width of the pavement and obstacle free zone, adds to the recognisability of the road type.

Additionally, the choice of pavement material is based on other considerations, and pavement markings are needed to increase the ability to recognize the road category and speed limit.

Japanese standards simply state that the right of way should be clear and unmistakable.


2012R36EN

Depending on the traffic volume, the minor road ought to be narrower than the main road, and traffic islands or road markings should be designed adequately.

Czech standards state that main roads are to be more comfortable and wider than minor roads, to have optically and physically uninterrupted flow, a higher level of pavement structures and different colours from the minor roads. The lanes of minor roads should narrow before intersections in order to psychologically influence the driver to slow down.

ConclusionThe design differences in road width and pavement between minor and major roads are described very well in 4 of 9 standards; in the other 5, the subject is not described sufficiently. In order to avoid severe accidents as a result of an optically misleading design, this factor should be incorporated directly into the standards.

2.3. HUMAN FACTORS SAFETY RULE NO.2: THE ROAD LEADS DRIVERS TO APPROPRIATE SPEED AND STABILIZES LANE TRACKING (FIELD OF VIEW RULE)

HUMAN FACTORS SAFETY RULE NO2: ENSURE APPROPRIATE SPEED AND LANE TRACKING!

Monotonous, clouded, deceptive or distracting impressions affect the quality of driving. The road, together with its surrounding field, offers an integrated field of view. This can either stabilise or destabilise the driver; it can tire or stimulate them. It can also result in either increased or reduced speed. Speed, lane-keeping and reliability of direction are functions of the quality of the field of view.

A user-friendly road will give drivers a well designed field of view with sufficient contrasts to increase alertness. It will provide good optical guiding and orienting facilities and with symmetrical and orthogonal impression.

A good-quality field of view safeguards the driver and keeps him from drifting to the edge of the lane or even leaving it. Misleading eye-catching objects in the periphery of the field of view activate subconcious changes in direction. The most serious consequences arise from eye-catching objects that differ from the road axis. These lead in extreme cases to a horizontal swing of the complete field of view: The driver has the feeling that the road and its surroundings are moving while he is in an unmoved position. Such objects lead to gross mistakes in steering. At minimum they lead to disturbances in lane-keeping, though these can mostly be corrected.

An experienced and HF-trained designer will avoid monotony in curvature and visual appearance. He will avoid optical illusions or misleading objects that destabilise drivers and negatively impact their driving.


2012R36EN

2.3.1. Optical density of the field of view

2.3.1.1 Monotonous approaching section or road surroundings are avoided

High monotony Good contrast

FIGURE 7 - AMOUNT OF INFORMATION INFLUENCES DRIVER’S SPEED(Source left picture: Birth,2009a; Source right picture: Forschungsgesellschaft für Straßenwesen Arbeitsgruppe

Strassenentwurf.1985)

Human Factors evaluation criterionDrivers adapt their speed to the given road situation. It is well known that the amount of information that has to be processed influences the quality of driving (Yerkes-Dodson Law). The amount of information also influences a driver’s speed. The term used for this is optical density of the field of view. It is a function of the number of objects that contrast with the background.

The presence of very few contrasting objects leads to monotony as well as reduced performance and reactivity. To avoid monotony, the driver subconsciously changes his driving activities in order to increase information input: he swerves, brakes or – in most cases – increases speed.

Consequently, it is desirable to achieve an optimal level of brightness and colour-contrast (optical density) to support the correct choice of speed. That is why efficient speed management relies on changing brightness and colour contrasts to avoid subconcious speeding up.


2012R36EN

TABLE 11 - MONOTONOUS APPROACHING SECTION OR ROAD SURROUNDINGS ARE AVOIDED

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Monotonous approaching section / road surroundings avoided?

P P NO NO NO NO P NO NO

The Canadian guidelines provide only general information on alignment and lane configuration combined with DSD and SSD. The horizontal and vertical alignments should be designed to enhance attractive scenic views of the natural and man-made environment. Otherwise, attention to the roadside to avoid monotony is not provided.

Dutch standards mention that drivers need changes in the field of view to avoid monotony. Long straight sections should be avoided, and requirements are provided for the radii of subsequent curves. The standards state the importance of the lateral field of view. However there is no direct conclusion referred to design.

Hungarian standards simply state that monotony and long straight lines should be avoided. Diversified planting is recommended.

Czech standards state that vegetation is to be designed for optical guidance of drivers, for sunshine protection and for the identification of the direction of horizontal alignment, while also demanding that sight distance always be considered.

French standards state that circular curves of moderate radius only may be used. Architectural and landscape treatment is not specifically required to alleviate monotony, but it is stated that this type of treatment has an aesthetic objective in relation to the area crossed, as well as a functional objective (such as slope stabilization, cultural and tourist information, and livening up of the road)1.

Regarding lighting, public lighting is only necessary at toll booths and in tunnels, but may also be advisable where a road crosses or borders an area with extra lighting that is liable to hamper traffic on the road (activity area, airport, etc.).

1 The approach of SETRA: was inspired by urban analysis. It bases on perception processes and itinerary interpretation. The reading of landscape is done by different conditions of visibility (by day, by night), consistency, homogeneity, comfort and constraint. Constraint and comfort are also used successively to break monotony and to maintain driver’s attention (SETRA, 2003)


2012R36EN

The Portuguese standards address monotony. Tourist signage aims to break the monotony that drivers may feel when travelling long distances along a road, and helps them stay on course.

Conclusion3 of 9 design standards include partial mention of the need to avoid monotony to provide a good field of view. They emphasize diversified planting as a countermeasure. 6 of 9 standards do not mention this Human Factor.

No standard refers to this subject in relation to the alertness and activity level of the driver. In addition, the fact that speed depends on the number of contrasts in the periphery of the field of view is not mentioned. Although this road feature is one of the important factors in subconscious speeding up as opposed to efficient speed management, it is not included in design standards. This Human Factor should be incorporated directly in the standards.

2.3.1.2 Lengthy visible approaching sections before critical points are avoided (problem of subconscious speeding)

wrong eliminated

FIGURE 8 - AVOIDANCE OF LENGTHY VISIBLE APPROACHING SECTIONS BEFORE CRITICAL POINTS THROUGH SWINGING ROAD ALIGNMENTS (SUFFICIENT CURVATURE)

(Source: Birth, Sieber & Staadt, 2004)

Human Factors evaluation criterionOne factor that is correlated to the optical density of the field of view is the curvature of the road. A regular swinging alignment with a good curvature leads to an increase in attentiveness and a decrease in speed. This can be explained by the increased workload involved in steering the vehicle and the increase in information from the organs of balance, which react to the change of position and curve acceleration.


2012R36EN

TABLE 12 - LENGTHY VISIBLE APPROACHING SECTIONS BEFORE CRITICAL POINTS ARE AVOIDED

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Lengthy visible approaching sections before critical points are avoided

P P NO P P NO P P NO

Dutch standards provide values for the maximum length of tangents for various speed limits. Specifically, the maximum length of a tangent for a posted speed limit of 100km/h should be 2000 m, for 80km/h the maximum is 1600 m and for 60 km/h the maximum is 1200 m. Variation in curvature is provided in the Canadian standards but not specifically in relation to HF.

Hungarian standards simply state that monotony and long straight lines should be avoided.

Czech road design standards cover limitation of straightaway sections and modification of alignments close to railway crossings and intersections from minor road side to ensure no opportunity to speed up and miss critical points.

The Portuguese standards have the most comprehensive information to address monotony, providing guidance on lengths of straightaway sections, landscape design, pavement markings, lighting and signalling. Long straightaways should not be on constant longitudinal gradients. In order to reduce glare at night and driving monotony, the maximum length of a straightaway, with constant gradient, should be 20 VB (m), where VB is the design speed in km/h. The straightaway should also provide optical comfort: with two-lane roads, the minimum length of a straightaway must be 6 VB, which ensures good optical orientation. A curve with a small radius should never follow a long straight section.

The Australian standard AS/NZS 1158 provides guidance on the location and intensity of lighting required and the transition zone between non-lighted and lighted areas. There are proposals to prevent drivers being subjected to excessive lighting contrast between lengthy dark stretches and upcoming well-lit zones in rural areas. The problem of subconcious speeding up in case of long straightaways is not mentioned.


2012R36EN

Conclusion6 of 9 design standards partly mention the need to avoid lengthy visible approaching sections to ensure a proper speed near the critical point. They emphasize diversified planting as a countermeasure.

3 of 9 standards do not mention this Human Factor.

No standard refers to this subject in relation to the alertness and activity level of the driver. In addition, the fact that speed also depends on the curvature of the alignment is not explicitly mentioned. Although this road feature is one of the important factors in subconscious speeding up as opposed to efficient speed management, it is not included in design standards. This Human Factor should be incorporated directly in the standards.

2.3.2. Fixation objects in the lateral roadside environment support optimal lane tracking

2.3.2.1 Structures over the road support optimal lane-tracking

wrong reduced

reducedwrong

FIGURE 9 - STRUCTURES OVER THE ROAD SUPPORT OPTIMAL LANE TRACKI(Source: Birth, Sieber, & Staadt, 2004)


2012R36EN

Human Factors evaluation criterionTracking, braking and accelerating are largely performed subconsciously. The perception of position and speed are derived from the view on the road and its surroundings. In addition to the central part of the field of vision, people subconsciously process peripheral information in the lateral part of the field in order to orient themselves. This is what enables people to walk through a forest in the twilight without hitting trees – a feat which is almost impossible by trying to consciously fixate on all the trees.

If designers fail to take this fact into account, they may not make the right prediction about how the finished design will influence lane-keeping by drivers. Structures over the road like bridges, advertising, signalling and toll facilities should be symmetrical and of equal height, and the angle of skew should be less than 15° from perpendicular.

It was found at accident black spots that asymmetrical posts of a bridge or pitched bridges/ advertisements confuse and disorientate drivers with regard to lane tracking and result in run-off-road accidents.

TABLE 13 - STRUCTURES OVER THE ROAD SUPPORT OPTIMAL LANE-TRACKING

NL

Port

ugal

Can

ada

Ger

man

y

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Structures over the road support optimal lane tracking?

NO P NO NO NO NO NO NO NO NO

The Portuguese standards talk about phased construction of cross-sections. It is recommended that landscaping and planting design facilitate the correct perception of the type of road that hide / cover superstructures. Delineators are always to be placed on each side of the road. Variable Message Signs should be centred above the roadway.

Conclusion1 of 9 standards mentions partly the visual and balance disturbances that can result from irritating superstructures. 8 of 9 design standards do not mention this subject, although this road feature can cause at field-dependent drivers subconcious irritations, significant disturbances of lane- keeping and severe crashes. Run-off-road accidents in particular, for which commonly no technical explanation is available, can be explained by these deficits in the management of the field of view.


2012R36EN

2.3.2.2 Eye-catching objects ARE NOT disturbing lane-tracking

FIGURE 10 - EXAMPLE OF GOOD AND BAD PRACTICE FOR EYE-CATCHING OBJECTS

(Source: Birth, S. (2009). “Human Factors Design Features Supporting Spatial perception”)

Human Factors evaluation criterionEye-catching objects like trees, buildings, technical facilities or other dominant single objects should not disrupt the continuity of lateral orientation cues. They should be symmetrical to the road’s centre-line. The view axis to an eye-catching object should not differ with the road axis.

It was found that at black spots, misleading eye-catching objects confuse and disorientate drivers with respect to lane-keeping and cause severe run-off-road accidents.

TABLE 14 - EYE-CATCHING OBJECTS ARE NOT DISTURBING LANE-TRACKING

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Eye-catching objects are not disturbing lane-tracking? NO NO NO NO NO NO NO NO NO

The Dutch and Portuguese standards state that if beaconing is necessary in a tight curve, then the first three chevrons should be placed directly within the central field of view for approaching drivers.

Czech road design standards indirectly provide a general recommendation for optimal lane- tracking, but in practice there are thousands of misleading inputs like minor road continuations in the original direction of the road, strong beamers attracting to adjacent areas, and thousands of advertisements and billboards disorienting the driver.


2012R36EN

ConclusionFor optimal lane-keeping, consideration of the lateral roadside features in the field of view is critical. Dominant eye-catching objects that may attract driver’s attention away from the road and make him fixate on objects other than the road axis will interfere with safe operation of the vehicle. Special features, advertisements or billboards should either be avoided or placed in the narrow lateral field of view around the road axis.

This Human Factor is not adequately addressed in any of the design standards and should be incorporated directly.

2.3.2.3 Illusion-free optical guidance supports optimal lane- tracking

accident tree non-accidenttree

FIGURE 11 - OPTICAL DISTANCE ILLUSION CREATED BY A CONVERGING PLANTING LINE AND A DIVERGING CRASH BARRIER AT LOCATIONS WITH SEVERE RUN-OFF-ROAD ACCIDENTS

(Source Picture above: Birth, 2003a; Picture below: Birth, 2008)

Human Factors evaluation criterionAll lateral orientation cues should be parallel to the road edge, regularly spaced and equally sized to stabilise lane tracking. This is important for markings, hard shoulders/side strips, safety barriers, snow and wildlife fences, plantings, bicycle-riding and rescue paths and also for public maintenance roads.

It was found that at black spots, non-parallel orientation lines lead to the impression of prolonged (converging lines) or shortened (diverging lines) distances up to critical points. They cause subconscious swerving, sudden driving manoeuvres and “unexplainable” run-off road accidents because field-dependent drivers try to correct their wrong distance estimate at the last moment.


2012R36EN

TABLE 15 - ILLUSION-FREE OPTICAL GUIDANCE SUPPORTS OPTIMAL LANE- TRACKING

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Illusion-free optical guidance supports optimal lane-tracking? NO P NO NO NO NO NO NO NO

Canadian guidelines state that lane and shoulder widths, special purpose lanes, other fundamental elements of the design and related features such as general grading and drainage should be consistent on any given road and on similar roads, i.e. those of constant width. The use of pavement of a contrasting colour and/or texture on paved shoulders or the use of rumble strips is encouraged, but it is not mentioned that they should be parallel to the road edge, regularly spaced or equally sized.

The Portuguese standards state that lanes are to be of constant width, with longitudinal and guiding lines. Any change in the number of lanes is carefully transitioned and is signed and marked accordingly. Additional lanes begin and end tapered. The median width is constant on dual roadways. Safety barriers are not designed as guidance equipment. Most of the time they are parallel to the edge line, but at their extremities they have special end treatments. Depending on the safety barrier type and the location of the extremity, end treatments may not be parallel to road guidelines. Noise screens usually follow the top of a slope.

Dutch standards mention parallel guiding lines with respect to edge line and centre line marking. They also suggest a standard cross-section, but it can be interrupted under bridges and viaducts, resulting in non-parallel safety barriers for example. Guiding information, for instance through safety barriers, is not considered.

Czech road design standards give a general recommendation for visibility of alignment, including parameters, but there is no direct focus on illusion-free roads.

ConclusionFor optimal lane-keeping, consideration of the lateral roadside features in the field of view is critical. Non-parallel safety barriers, plantings, buildings, posts etc. cause severe and technically “unexplainable” run-off-road accidents with field-dependent drivers.

This Human Factor is not adequately addressed in any of the design standards and should be incorporated directly.


2012R36EN

2.3.2.4 Carriageway width reductions are well delineated

FIGURE 12 - CARRIAGEWAY WIDTH REDUCTIONS ARE WELL DELINEATED(Source: TC 1.1)

Human Factors evaluation criterionCarriageway width reductions should be made plain as early as possible. They should also be visible under day and night conditions. Especially on 2+1 roads, lane management has to be logical and expected by the driver.

TABLE 16 - CARRIAGEWAY WIDTH REDUCTIONS ARE WELL DELINEATED?

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Carriageway width reductions are well delineated? Y YI YI YD YI YI P YI P

Canadian standards define every detail for pavement markings but not directly in relation to HF. Location and size of pavement markings along the road are related to the design speed, and warning lines and warning arrows are used when there is a width reduction.

Portuguese pavement marking plans discuss the use of markings, pavement colour, rumble strips, warning lines and arrows. Rumble strips are used in central longitudinal and borderlines. According to the Vienna Convention, danger warning signs should be “sited to give warning of possible road hazards which are difficult for a driver proceeding with due caution to perceive in time”.


2012R36EN

Dutch standards describe the use of arrows and striping but recommend always using physical measures as well, not just striping. Signs are also required for lane narrowing.

Australian standards discuss the width for each type of roadway and give distances that must be complied with when the width is reduced as well as details of signals to inform the driver.

Czech road sign manuals contain rules for placement of warning signs before carriageway width reduction. For traffic slowing purposes, there are manuals for traffic calming, “gates” to the city limits, “Tempo 30 zones” etc.

ConclusionDesign measures to delineate road width reductions are well described in 7 of 9 standards and partly in the other 2.

In order to avoid severe accidents through misleading design, this factor should be incorporated directly in the standards.

2.3.2.5 Roadside objects appear to be vertical

FIGURE 13 - ROADSIDE OBJECTS APPEAR TO BE VERTICAL(Source: Birth,2009a)

Human Factors evaluation criterionThe orthogonality of the surrounding environment can affect precision of lane-keeping. Trees or buildings can create a non-orthogonal impression that especially affects field-dependent drivers who tend to swerve and make spontaneous steering manoeuvres to compensate for the optical misperception.

Roadside objects like trees and delineators, as well as architectural constructions like posts of bridges or traffic control devices should provide an orthogonal appearance to stabilise lane-tracking.


2012R36EN

TABLE 17 - ROADSIDE OBJECTS APPEAR TO BE VERTICAL

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Roadside objects appear to be vertical? NO NO NO NO NO NO NO NO NO

Minimal or no information is provided regarding this issue. Most standards specify that all road signs should be vertical. Most of the light poles are vertical along high-speed roads. In urban areas, more flexibility is possible and different designs of non-vertical light poles are included.

ConclusionMinimal or no information is provided in 9 of 9 standards regarding this issue. In order to avoid severe accidents through misleading features in the field of view, this factor should be incorporated directly in the standards.

2.3.2.6 Optical framing of curves

wrong

wrong reduced

reduced

FIGURE 14 - ILLUSION-FREE OPTICAL FRAMING OF CURVES(Source: Birth, Sieber & Staadt, 2004)


2012R36EN

Human Factors evaluation criterionDriving reliably through a curve critically depends on the quality of the field of view. Best driving results are achieved when the driver has an unobstructed view through the inside of the curve, and the outside of the curve has closed optical framing. A driver’s response to inner curves which are obstructed, and especially to a fragmentary or even non-existent framing of outside curves, will be spontaneous speed changes and steering manoeuvres. If the frame of an outer curve is not parallel to the road’s edge, a curve illusion is created that will misinform the driver about the sharpness of the curve.

It was found that at black spots, an absence of these three features (unobstructed inner curve, parallel outer curve framing and closed framing without optical gaps) causes subconscious swerving to the outer curve and “unexplainable” run off-road accidents.

TABLE 18 - OPTICAL FRAMING OF CURVES

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Optical framing of curves? P P NO P NO P P P NO

Canadian roadside design documents state that safety barriers are not intended to function as guidance equipment and are not to be used as a replacement for lane-keeping in curves. Guidance is provided for the coordination of horizontal and vertical design and there is discussion of the need for the designer to avoid situations where visibility is lost. Signalling required to support sharp curves is addressed as well as the need to add chevrons and delineators to help with tracking.

Landscape design is covered quite comprehensively in the Portuguese standards. Specifically mentioned are the need for roads to have a pleasant appearance and the need for safety to be explicitly considered in the design process, for example by planting a shrub screen along the outer curve to mark the alignment and facilitate night driving.

Dutch standards state that the visibility of the inner curve is important for estimating road curvature. The edge marking of the inner curve is therefore chosen as the axis of rotation for the super-elevated section. Gas stations and rest areas should not be located on the inner curve because they will impede visibility, but instead on the outer curve or near a sign indicating an intersection or exit.

Czech road design standards contain rules for sight distance and visibility triangles, including rules for clearance in curves with respect to safety barriers, terrain and shrubs. Optical framing is mentioned like verbal recommendation.


2012R36EN

ConclusionOptimal lane-keeping requires an optical frame for the outer curve while the inner curve must be fully visible. Non-parallel safety barriers, plantings, buildings, posts etc. in the outer curve can cause severe and technically “unexplainable” run-off-road accidents. A hidden inner curve will result in wrong anticipation of the curve’s course as well as the wrong speed.

This Human Factor is not adequately addressed in any of the design standards and should be directly incorporated.

2.3.3. The depth of the Field of View supports optimal lane-tracking and reliable orientation and anticipation of the road’s course

Human Factors Information: Principles of spatial perceptionSpace is perceived by humans as an oval. The eye scans this oval in subconscious small interior circles of 15° in a counter-clockwise direction. The information thus collected forms a three-dimensional space. It ends after 8 m and beyond this point humans can only perceive two dimensions. People can still judge distances and estimate which of two objects is nearer to the observer, but this mental process is highly mistake-prone and susceptible to optical illusions and there is thus a need for clear and unambiguous spatial information.

The driver orients himself on the basis of his environment. To estimate his position relative to the road, the surroundings and other drivers, he depends on changes of position, the changing view axis and the changing reference points and lines in the environment. All this information has to be related to the drivers’ coordinate system, which operates subconsciously. While a moderate change of position keeps attentiveness high and reduces mistakes, an opposite effect is caused by sudden, unexpected, inconsistent or too frequent changes in the drivers’ surroundings. The overload or misleading information cannot be processed accurately. As a result, drivers are susceptible to optical illusions or perceptual traps.

An experienced and HF-trained designer will avoid dominant eye-catching objects that differ significantly from the view axis. The designer avoids any optical illusions and ensures that the course of the road is clearly visible. He tries to stabilise drivers and provides support for driver spatial perception.


2012R36EN

2.3.3.1 Dominant eye-catching objects support lane-tracking and detection of critical points

FIGURE 15 - WRONG AND CORRECTED SITUATION FOR EYE-CATCHING OBJECTS(Source: Birth, Pflaumbaum & Sieber,2006)

Human Factors evaluation criterionTracking, braking and accelerating are largely performed subconsciously. The perception of position and speed are derived from the view along the road and its surroundings. Eye-catching objects should therefore guide the view to critical points and should not distract the attention of the driver. This is particularly important when there is an intersection with an eye-catching object that guides the eye in the wrong direction, against the road’s course. The probability of wrong anticipation and steering is then high.

It has been found that at black spots, such wrong optical guidance, leading to wrong anticipation, creates a need for sudden, sharp corrections of driving manoeuvres that have already been initiated, and is frequently followed by accidents.


2012R36EN

TABLE 19 - DOMINANT EYE-CATCHING OBJECTS SUPPORT LANE-TRACKING AND DETECTION OF CRITICAL POINTS

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Dominant eye-catching objects support lane-tracking and detection of critical points?

P P NO NO NO P NO NO NO

Canadian standards define signals to inform the driver but no special attention is paid to the detection of critical points.

Portuguese standards define the conceptual landscape design but not the types, patterns and geometry of intersections for this situation. Advance direction signs and priority traffic signs road at intersections and interchange entrance ramps provide guidance regarding the design of the intersection and exit ramp. There is not mention of signs or landscape design to support lane-tracking and detection of critical points.

Dutch standards discuss the need for task-relevant elements to be at least as conspicuous as other elements in the surroundings. Size, colour, contrast, movement and (flashing) lights all contribute to the conspicuousness of an element. It depends on the driving task whether objects such as billboards become a problem. Extra caution is needed near decision points such as interchanges. Both advance warning signs and traffic signs are to be put near critical points.

ConclusionNone of the 9 national standards addresses the issues that may arise when a dominant eye-catching object is in competition with other road features that may be critical for driving. Correction of this problem, through use of perspective-forming objects, is discussed to some extent in the Dutch standards but not specifically in relation to HF principles.


2012R36EN

2.3.3.2 Optical illusion avoided

accident tree non-accidenttree

FIGURE 16 - EXAMPLE OF AN DISTANCE ILLUSION AFTER REPLACEMENT OF A ROAD WITHIN AN OLD LINE OF TREES

(Source: Birth, S. (2003). Report “Human Factors accident analysis” B109/140”)

Human Factors evaluation criterionRoad edge and plantings, safety barriers or other technical facilities should be parallel to the road edge, to prevent a distance illusion. Drivers overestimate or underestimate the distance up to the next critical point. Also, a curve in combination with a sag or in combination with a crest leads to underestimation / overestimation of curve radius.

It has been found that at black spots, non-parallel converging or diverging plantings, buildings, safety barriers etc. lead to “unexplainable” accidents.

TABLE 20 - OPTICAL ILLUSION AVOIDED

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Optical illusion avoided? NO NO NO P NO NO NO NO NO

This issue is only partly covered in the design standards; only minimal, general information is provided. Canadian standards only consider the coordination of horizontal and vertical alignment.

A Portuguese practice is to facilitate the coordination of horizontal and vertical alignment and noise protection so that noise screens usually follow the top of slope.


2012R36EN

This is not specifically related to optical illusions.

Dutch standards state that the combination of horizontal and vertical curves can cause difficulties with the estimation of curve radii. For instance, the radius of a horizontal curve in a sag curve may be underestimated, leading to a risk of high speeds. Several recommendations are made for the combination of horizontal and vertical curvature. To give the road a fluent appearance, the tangent point of horizontal and vertical curves should coincide as much as possible.

ConclusionNone of the 9 national standards addresses the issue of optical illusions arising from non-parallel optics. Correction of this issue, through strict use of parallel safety barriers or other linear structures, should be included in the guidelines.

2.3.3.3 Course of the road clearly visible

FIGURE 17 - COURSE OF THE ROAD IS NOT VISIBLE, THE MISGUIDING OPTICS ARE IRRITATING

(Source: http://norwegen.kaifler.net/bilder/kurioses.shtml)

Human Factors evaluation criterionThe depth of the field of view is crucial if drivers are to be able to detect and prepare for changes and potential hazards like crossings, (tight) curves, and other critical points. This is a basic condition for reliable spatial perception.

Drivers should have an unobstructed view along the course of the road, especially in situations such as driving around a curve, overtaking, or driving at high speed. On national roads and motorways in particular, the sight distances before the manoeuvre section of crests or bridges should be 4-6 seconds. The road should


2012R36EN

provide good optical guidance that is consistent with expectations. In addition, sufficient overtaking opportunities with good visibility are necessary to avoid the extreme tiring that result from driving in a line-up of vehicles for a long time.

TABLE 21 - COURSE OF THE ROAD CLEARLY VISIBLE

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Course of the road clearly visible? YD P NO YI YD NO P YI P

Canadian standards discuss the relationship of design speed, alignment and straightaway sections. Stopping Sight Distances are provided, including where structures such as bridges, viaducts, overpasses, etc. are present. Passing Sight Distance guidance is also provided in the Geometric Design Guide for Canadian roads.

The Portuguese standards require provision of Stopping Sight Distance at all times. Passing Sight Distance on single two-lane roads must be provided with frequent intervals so as to meet the desired level of service and allow for safe passing. As a general rule, it is recommended that Passing Sight Distance should be provided on least 40% of road length, to facilitate overtaking. On mountain roads, it is assumed that there are singular points which will not provide Stopping Sight Distance, and therefore Passing Sight Distance will be provided only over a small percentage of the length of road. Stretches with additional overtaking lanes should be provided in places where that is affordable.

Czech road design standards contain the practical rule that the relationship between horizontal and vertical alignment be understandable, and the sketches are similar to the German standard. Stopping Sight Distance has to be secured in new design sections; in practice, drivers have to accommodate driving speed to real sight distance.

Dutch standards cover sight distances for sufficient visibility of the course of the road. Information is provided for overtaking opportunities.

Australian standards cover all the elements using HF but they are not specifically provided within the audit tables.

Conclusion4 of 9 standards address the issue of visibility of the course of the road, 3 of 9 only partly. But this is not always specifically related to HF. The visibility of bridges and


2012R36EN

crests 6 seconds prior to the manoeuvre section should be included more clearly in the standards. Also, the rule of sufficient coordination of horizontal and vertical alignment should be incorporated where lacking.

2.4. HUMAN FACTORS SAFETY RULE NO.3: ROAD DESIGN SHOULD PRE-PROGRAMME DRIVER’S ACTIONS CORRECTLY

HUMAN FACTORS SAFETY RULE NO3: PRE-PROGRAMME DRIVER’S BEHAVIOUR CORRECTLY.

Drivers follow the road with an expectation and orientation logic formed by their experience and recent perceptions. These affect their actual perception and reactions.

The same principle applies when climbing stairs. After only a few steps, motion balance adjusts to the sequence of steps just perceived. In most cases, this is a subconscious process. However, if one step is of a different height, the motion balance will become considerably disordered, possibly resulting in a stumble or fall. Adjustment of driving behavior on the road is similarly subconscious.

The perception of the lane, the edge and the lateral periphery produces a general overall impression. Drivers react to these road elements with their actions, in the same way as the person climbing stairs reacts intuitively to the height, depth and width of the steps. Unexpected objects disturb the automatic sequence of operations, possibly causing the driver to “stumble”. After several critical seconds, the disturbance can be handled.

Planners and designers therefore try to keep road characteristics flowing in a logical sequence. They should introduce inevitable changes as early and clearly as possible, and exclude any sudden changes that would confuse the driver.


2012R36EN

2.4.1. Change of road function without corresponding change in design and optical characteristics (e.g. town entrance)

2.4.1.1 Visual cues reinforce the changed road function and Eye-catching objects are used to reinforce the change

FIGURE 18 - VISUAL CUES REINFORCE THE CHANGED ROAD FUNCTION AND EYE-CATCHING OBJECTS REINFORCE THE CHANGE


Human Factors evaluation criterionDrivers need to adapt their driving behaviour when entering a built-up urban area or when the road functions changes significantly. They need to decrease speed and be more attentive as more decisions and reactions are required.

Generally there should be unambiguous visual cues to recognize the change of function, for instance by a horizontal swing of the road’s course, optical sight barriers, planted central islands, special speed-reducing markings or a combination of these.


2012R36EN

TABLE 22 - VISUAL CUES AND EYE-CATCHING OBJECTS ARE USED TO REINFORCE THE CHANGE?

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Visual cues reinforce the changed road function? YD P NO YD NO NO P P NO

Eye-catching objects are used to reinforce the change (signs, flags, objects, etc.)?

YD NO NO YD NO NO P NO NO

Transition zone adequate? YD NO YI P NO NO P NO NO

Dutch standards state that a town entrance should be located near the border between a rural and a built-up area, where a change of driving behaviour is needed. There are specific and comprehensive design, marking and signing requirements depending on the characteristics of the situation (see chapter 3).

Portuguese standards provide lighting plans and conceptual landscape design. Plans cover signing, signing and all above-ground tools for traffic management, including ITS and traffic lights. Criteria are provided for use of road signs, and defining an entry in a location to indicate the beginning of a built-up area, which implies a change in the road environment. Pavement marking plans address specific markings, pavement colour, rumble strips, etc., including longitudinal markings, arrows, symbols, inscriptions, oblique stripes, etc. for different road classifications.

Czech standards discuss the design categories of roads and expressways and provide sufficient technical measures to guarantee perception of the changing environment and traffic conditions, through step-by-step speed reduction. Reference is made to urban guidelines, which include traffic calming measures.

The “Canadian Guide to Neighbourhood Traffic Calming” publication by the Transportation Association of Canada and the Canadian Institute of Transportation Engineers addresses traffic calming measures but only for local and collector residential streets; it is not meant for traffic calming on arterial or rural roads. Design for traffic calming measures such as speed humps, sidewalk extensions and curb extensions can also be considered for gateway applications on rural roads.

ConclusionOnly 2 of 9 standards directly address the issue of change of road function without corresponding change in design and optical characteristics. In 7 of 9 standards, this Human Factor is mentioned partly (3) or not at all (4). This subject should be more clearly included in the standards.


2012R36EN

2.4.2. Change of road direction despite a dominant eye-catching orientation line (e.g. city by-pass dilemma)

2.4.2.1 Change of road direction is visible and clearly perceived

FIGURE 19 - CHANGING ROAD DIRECTION IS NOT UNAMBIGUOUSLY VISIBLE, ANTICIPATION IS DISTURBED(Source: Birth, S. (2004a). Report “Human Factors Accident Profiling Ortsumgehung Nauen”

Human Factors evaluation criterionDrivers need eye-catching objects to realize that there is a change in the road direction despite other dominant eye-catching orientation lines. The change of direction has to be supported by covering misguiding view / orientation axis.

It has been found that at black spots, dominant eye-catching objects such as a line of trees, buildings or straight road sections impede the correct anticipation of a road’s course even though signing is present. Road features that mislead spatial perception cause technically “unexplainable” accidents.

TABLE 23 - NEW ROAD LAYOUT IS VISIBLE AND CLEARLY PERCEIVED

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

New road layout is visible and clearly perceived? P NO NO P NO NO P P NO

There is no specific information for avoiding the problem of bypass problems, i.e. the old road seems to be a continuation of the new road while the new road actually turns. It is recognized that all elements along the road should support drivers’ expectations to prevent misjudgment. The road should be designed in a consistent fashion so that the driver is clear as to the upcoming road environment.


2012R36EN

In Canada, the Ontario Traffic Manual does recommend oversized signage for warnings in work zones, which can be similar in nature to new road layouts, as well as advance warning signs up to one kilometre before any new installation (or a road change) that may surprise the driver.

The Czech road design standard does not explicitly include this issue and as a result new bypasses were built that had a high risk of confusing the driver under adverse conditions.

Please replace this figure to another part of the report (see commentary)

ConclusionNone of the 9 national standards explicitly considers the situation of a changed course despite a dominant misguiding view / orientation axis (4 of 9 partly, 5 of 9 not). This subject should be more clearly included in the standards.

2.4.2.2 Eye-catching objects (traffic signs, objects, plants, etc.) are used to focus attention on the changed alignment of the road and there are No misleading visual cues along the old road alignment

new roadold road

FIGURE 20 - EYE-CATCHING OBJECTS ARE USED TO FOCUS ATTENTION ON THE CHANGED ALIGNMENT OF THE ROAD

(Source: Bielenberg,et al., 2002)

Human Factors evaluation criterionDrivers need eye-catching objects to realize that there is a change in the road direction despite other dominant eye-catching orientation lines. The change of direction has to be supported by covering misguiding view / orientation axis and by a clear enhancement of a changing course through embankments, planting lines or other optical guiding measures.


2012R36EN

The goal is to guide drivers’ fixation unmistakably on the critical points. A misleading feature can be easily covered or corrected with planted embankments or – if that is not possible - with a safety barrier that provides additional optical guidance through glare protection on top (min. 120 cm).

TABLE 24 - EYE-CATCHING OBJECTS AND VISUAL CUES ARE USED TO FOCUS ATTENTION

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Eye-catching objects are used to focus attention on the changed alignment of the road (traffic signs, objects, plants, etc.)?

P NO NO P NO NO P NO NO

There are no misleading visual cues along the old road alignment

P NO NO P NO NO P NO NO

The NL is the only standard that partially considers this situation. It states that an unexpected tight curve should be lighted, and that discontinuities such as roundabouts or zebra crossings often need lighting. Lighting is especially important in cases where sub-optimal road designs exist. Most of the standards recognise the importance of consistent and unambiguous road and roadside design. However, none of them provided specific guidance in situations where the new alignment must have more prominent visibility than the old, so as to provide a clear understanding of the new course of the road.

ConclusionNone of the 9 national standards explicitly considers the use of eye-catching objects or guiding measures to focus attention on a changed alignment (3 of 9 partly, 6 of 9 not). This subject should be more clearly included in the standards.Effect of pre-programmed habits and routines

2.4.3. Effect of pre-programmed habits and routines

Human Factors InformationVisual perception is not a simple, mechanistic process. Drivers’ ability to react to a hazardous object or event depends on their experience and previous conditions. For example, drivers travelling on a roadway with few access points and wide curves may be unprepared to stop for a slow-moving vehicle ahead. The time they need to react safely will substantially exceed the time they would have needed in a busy urban traffic environment. Driving is a smooth and continuous activity and drivers’


2012R36EN

attentiveness and expectations will depend upon the preprogramming they derived from previous impressions.

Drivers need a road environment that is in accordance with their expectations programmed by the road from previous impressions. Conditions that represent a fundamental change from the preprogramming of the preceding stretches of road need to be introduced in time and with enough visual information.

2.4.3.1. Requirement for a new driving program recognized and changes are introduced to “re-program” driving habits and routines




planted embankment



FIGURE 22 - EXAMPLE OF A REQUIREMENT FOR A NEW DRIVING PROGRAM (Source: Bielenberg, et al., 2002)

Human Factors evaluation criterionChanging the right-of-way or altering the course of the road, such as a new alignment, will challenge drivers’ habits and expectancy. Appropriate and timely signals or visual clues are required to inform the driver and provide an adequate time for correct anticipation. The required reaction could be significantly different from the habitual one. In order to avoid surprises, various design principles need to be considered.

It has been found that at black spots, newly built intersections that are not introduced properly lead to incorrect anticipation of the situation even though signing is present.


2012R36EN

TABLE 25 - REQUIREMENT FOR A NEW DRIVING PROGRAM RECOGNIZED AND CHANGES INTRODUCED TO ‘REPROGRAM’ DRIVING HABITS AND ROUTINES

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Requirement for a new driving program recognised? YD P NO YD NO P NO NO NO

Changes introduced to “re-program” (relearn) driving habits and routines?

YD P NO P NO NO P NO NO

Canadian standards only address the type and geometry of intersections and lighting, such as the contrast between lengthy dark stretches and upcoming well-lit zones. The Ontario Traffic Manual does however recommend oversized signage for work zone warnings and advanced warning signs up to one kilometre before any new installation that may surprise the driver.

There are strict rules concerning right-of-way in relation to Dutch road categories on the structural level. Right-of-way is always arranged on distributor roads, whereas within urban areas there are only junctions, without any designated priorities. The speed limit is also determined by the road category, i.e. 80 km/h on rural distributor roads and 60 km/h on rural access roads. Like the town entrance described above, the Netherlands has guidance on changes between speed zones, i.e. between categories, e.g. marking, signs and speed humps.

The Portuguese standards partially cover this issue while the Dutch standards take all aspects into consideration except the landscape design (see details above).

ConclusionOnly the Dutch standards explicitly consider the subject of reprogramming drivers’ habits and routines. 3 of 9 standards mention this Human Factor partially, while 5 of 9 do not consider it at all. This subject should be more clearly included in the standards.


2012R36EN

2.4.3.2 Road alignment conforms to driver expectations

Continuous bends in a roadR min

R min

Discontinuous bends in a road

FIGURE 22 - CONTINUOUS AND DISCONTINOUS BENDS IN A ROAD(Source: Forschungsgesellschaft für das Straßen- und Verkehrswesen e. V. (FGSV), 1995)

Human Factors evaluation criterionDrivers follow the road with an expectation and orientation logic formed by their experience and recent perceptions. These affect their perception and reactions.

As a result, planners and designers try to keep road characteristics flowing in a logical sequence. They should introduce inevitable changes as early and clearly as possible, and exclude any sudden changes that would confuse the driver.

TABLE 26 - ROAD ALIGNMENT CONFORMS WITH DRIVER EXPECTATION

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Road alignment conforms with driver expectations? YD YI YD P YD YI P YI NO

Transition zones and critical points: visible, understandable and progressively introduced?

YD YD P YI P P YI NO YI

The Portuguese standards address horizontal and vertical alignment in combination to ensure that sight distances are sufficient. They also state that the design speed should be constant along the road. The design speed of adjacent sections should not differ by more than 10 km/h, to achieve a gradual and comfortable change in speed. When it is essential to change the geometric characteristics of a stretch due to markedly difficult topography, transition zones should be provided so that change takes place gradually. In order to obtain a homogeneous route, which is recommended for safety as well as economic and environmental reasons, there must be a balanced relationship between the radii of successive curves.


2012R36EN

Australian standards state that the design must ensure that where drivers have to slow down for a horizontal curve, the speed reduction must be considered on the basis of normal driver capability and expectations. They point out the importance of a consistent road design. Consistency needs to be addressed in three areas – cross-section, operating speed and driver workload. There are some situations, however, where a change in topography requires a substantial reduction in the design speed. In these circumstances, it is preferable for the horizontal curve radii to be gradually reduced through a series of curves, with appropriate warning signs in place. Additionally, the information on vertical alignment provides details for correct alignment design, but there is no specification for designers to introduce an appropriate change in the roadway geometry so as to re-program driving habits and routines.

According to Canadian standards, when a sharp curve follows a flat curve, the radius of the flat curve should generally not be more than 50 percent greater than the radius of the sharp curve. A ratio of 1:1.25 is more desirable on high-speed roads where the speeds are near the maximum for the curvature. For horizontal alignment, design speed applies only to curves, not to the straight sections that connect these curves. As a result, the maximum operating speed on a long straightaway can often significantly exceed the design speed of the horizontal curves.

Conclusion6 of 9 standards explicitly state that the road alignment should meet drivers’ expectations and be consistent; 2 mention this need partly, and 1 does not. However, the goal of informing the driver sufficiently is not completely addressed, due to a lack of coordination between alignments and the holistic overall impression of spatial perception. This subject should be more clearly included in the standards.


2012R36EN

2.4.4. Multiple critical points occur concurrently

2.4.4.1 Multiple critical points are avoidedAccumulation of critical points without sufficient anticipation time at an accident spot

Invisible town entrance

covered intersection

invisible railway crossing

2 km straight section followed by an inconstistent sharp curve

inside bend covered, no anticipation of course of the curve

FIGURE 23 - BAD EXAMPLE FOR MULTIPLE CRITICAL POINTS(Source: Birth, Sieber, & Staadt, 2004)

Human Factors evaluation criterionDrivers’ attention and ability to process information is limited. Driving requires multiple tasks, such as control, guidance and navigation. Drivers can focus on one piece of information at a time and multiple distractions or critical points may result in overload.

Too many decision points in too short a time can overload the capacity for information processing and result in safety risks.

TABLE 27 - MULTIPLE CRITICAL POINTS AVOIDED

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Multiple critical points avoided? YD YD YD P NO P NO NO NOAll critical points visible? YD YI YI YD P NO NO P YI


2012R36EN

Dutch standards state that private road accesses should be avoided along distributor roads. Access roads or roads that run parallel to a distributor road are recommended. There is also a recommendation on the minimal distance between intersections on distributor roads.

French standards do not allow private road access along main roads except for maintenance and emergency services. The total Stopping Sight Distance must be considered at every point on the roadway. Between two interchanges, the recommendations take into consideration the flow rates and the directional signing, which is based on the speed limit. Between two successive exit lanes, for example, it is necessary to have a minimum distance greater than the “read distance”, which is a function of the speed limit and advanced sight distance.

Minimum Sight Distance for intersections is defined in the Japanese standards. Reaction times of 6s in urban areas and 10s in suburban areas are considered, but longer is better. Signalled intersections require larger sight distances than unsignalled intersections.

The Canadian standard states that the efficiency of a road network depends largely on the quality of the operation and design of the intersections. In order to provide a desired level of service through an intersection, HF also have to be considered. A minimum distance between intersections is provided for each class of road.

The Portuguese standards define the minimum distance between at-grade intersections and interchanges for rural roads. Type, pattern and geometry of intersections should be considered, as well as the following HF: usual behaviour and decision capabilities of drivers, surprise effect, reaction and decision times. Characteristics of existing intersections should be taken into consideration so that consistency may be ensured along a road.

In the Chinese standards, the distance between intersections is based on vehicle operating speed, traffic capacity and safety. The type, pattern and geometry of intersections are determined by highway function and volume and should give consideration to sight distances.

Austroads encourages a balance between traffic movement and the access functions of various classes of road. Access management and the concept of access categories are discussed. Detailed guidance on the type, pattern and geometry of intersections is also included.

Czech road design standards contain minimal distances between intersections, pedestrian crossings etc., but this is not guaranteed for elimination of multiple critical points.


2012R36EN

Conclusion3 of 9 standards state explicitly, and 2 partly, that the road alignment should provide enough distance and time between consecutive intersections. 4 standards do not mention this need. Additionally, there is no explicit statement that the combination of sequences of design elements like curves, changes of function (e.g. town entrance), crossings, public traffic stops or similar critical points has to be coordinated to ensure reliable and correct driving actions given the limited capacity of drivers. This subject should be more clearly included in the standards.

2.4.4.2 Driver progressively informed of multiple critical pointsleft lane with 3 critical points: end of acceleration + of climbing lane (in a curve!) + end of intersection’s median

FIGURE 24 - EXAMPLE FOR MULTIPLE CRITICAL POINTS(Source: C. de Almeida Roque.)

Human Factors evaluation criterionIt has been found that at black spots, a sudden increase in drivers’ workload due to of a flood of information (“ forest of sign posts”) or driving requirements (simultaneous occurrence of multiple decision points) leads to accidents. This is especially true in situations when the driver also has to adapt physically to new conditions, e.g. after leaving a tunnel or in difficult right- hand curves with additional driving requirements. In such cases, drivers should have a clear view of the oncoming requirements. Additionally they should be informed by advance warning signs.

TABLE 28 - DRIVER PROGRESSIVELY INFORMED OF MULTIPLE CRITICAL POINTS

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Driver progressively informed of multiple critical points? YI YI P YI NO NO NO P P


2012R36EN

Most standards address consistency of design and the need to ensure minimal Stopping Sight Distance, and placement of warning signs and warning markings.

Canadian standards provide information for the designer to ensure that signals help the driver make correct decisions and process information. Different sight distances are given which the designer will use under various circumstances, with a perception and reaction time of 2.5 seconds. Designers should consider whether a longer or shorter time is appropriate for an atypical driver population. Driving Sight Distance should be provided in all places where driver expectancy should be significantly altered or where drivers will probably have doubts about the information received.

Portuguese standards state that signs should be located at sites that allow good visibility and legibility of the messages. To ensure road safety, it is essential that users are warned in good time of the intersection’s existence. Pedestrian or cycle crossings should be located so that their visibility is compatible with the operating speed (V85). Otherwise, physical mechanisms should be provided that cause a reduction of speed to the permissible values for the prevailing visibility.

Dutch standards state that enough space between advance signposts and the decision point (intersection) is needed to prevent confusion among drivers. There is no recommendation for traffic sign placement to prevent accumulation. For situations such as approaching a priority road, arrows, striping and a warning triangle should be painted on the pavement to warn drivers.

Czech road sign manuals contain rules for placement of advanced traffic signs for interchanges and intersections, railway crossings and similar situations. These are related to the design speed and the importance of the crossing roads. There are also warning sign rules, which are related to the Vienna Convention.

Conclusion3 of 9 standards explicitly state that signals and designs should support information processing and correct decisions by drivers. 3 standards mention this partly while 3 do not consider this need. There is no explicit statement that methods other than signing can be used to meet this need. The possibilities of landscaping and the use of special optical signals should be more clearly included in the standards.


2012R36EN

2.4.5. Deficiencies in traffic control devices

2.4.5.1 Traffic control devices visible against background, and Signs and letters are unambiguous and readable

3 invisible chevron signs

FIGURE 25 - THREE OVERSIZED CHEVRON SIGNS ARE NOT VISIBLE AGAINST THE BACKGROUND (MIMESIS)

(Source: Birth,2003b)

Human Factors evaluation criterionDue to higher traffic volumes, roads need to be equipped with traffic control devices for safety. Along with rules, traffic control devices organise the behaviour of road users. Under all light conditions and before all optical backgrounds, traffic signs should be visible, legible and detectable, and they should never be covered by plantings or structures. That is because the effect of mimesis may make even bright, oversized signs invisible to the driver.

Signs should be in accordance with driver’s expectations, and the road alignment should be consistent and in accordance with the traffic control devices. Otherwise there will be a risk of misleading and confusing drivers, leading to accidents.

TABLE 29 - TRAFFIC CONTROL DEVICES VISIBLE AGAINST BACKGROUND AND SIGNS / LETTERS ARE UNAMBIGUOUS AND READABLE

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Traffic control devices visible against background (size, contrast, brightness)

P P YD P YD P P P NO

Signs and letters unambiguous and readable? YD YD YI YD YD YD P YI YI


2012R36EN

Canadian standards state that most signs are designed to respect the visibility of drivers at specific speeds. Colour, retro-reflectivity and contrast have been studied to provide the most effective outcome. In the Illumination guide, instructions are given on lighting location and intensity and the transition zone between unlighted and lighted areas. Correct and adequate signals need to be provided, clearly visible day and night, to inform drivers of any critical point ahead of them. Signs are posted at a distance from critical points related to design speed. Information is provided about avoiding too many signs in an area (sign/information overload). For advance visibility, specific locations are defined for all different styles of signs, and references are given for the requirement of day- and night-time reflectivity.

In Quebec’s “Traffic Control Devices”, the size of lettering is given as an equation based on “H” - the distance at which the sign must be legible - and “L”- the lettering legibility distance (m/cm). L is the maximum distance at which the letters may be recognisable with various visual acuities.

Australian standards include information which ensures that colour, reflectivity and contrast provide the most effective outcome. They provide guidance with respect to alignment. Lighting is discussed with respect to intensity requirements and transition zones between unlighted and lighted areas, where drivers may be subject to too much light contrast between lengthy dark stretches and upcoming well-lit zones.

Portuguese guidelines state that signs are either retro-reflective or illuminated. Any materials used in their production should not cause glare or diminish the visibility of symbols or inscriptions. Signs should be located at sites that ensure good legibility of the messages. Road signs should not obstruct visibility at the intersection. When site conditions do not allow signals to be seen on the right side of the roadway, or when there are 2 or more lanes in the same direction, they should be repeated on the left side or above the roadway. Signs, including traffic guidance equipment on dangerous curves, should be located at sites that ensure good legibility of the messages. Portuguese standards state that all road signs should be readable with proper dimensioning.

The Chinese standards require that signing and all above-ground tools for traffic management, including Variable Message Signs and traffic lights, should not disturb drivers’ perception.

Dutch standards state that road signs should be visible (not hidden by plantings or engineering structures) and legible at a sufficient distance. The distance for visibility is about 1.5 times the distance for legibility. Along 80 km/h roads, traffic lights need to hang above the road for straight-ahead traffic with a visibility distance of at least 200 m. The visibility of traffic signs must be guaranteed in daylight and in the dark using retro-reflective material where necessary, and they must not restrict the visibility of the traffic itself.


2012R36EN

Czech standards provide detailed information for signs, letters on signs and VMS. Sometimes bigger, more visible signs are required for certain traffic conditions, which may be contrary to the current requirements within the standard.

Conclusion8 of 9 standards directly or indirectly state that traffic signs and markings should be visible and legible to drivers at all times and in all weather conditions (1 mentions this partly). They state that visibility and legibility of signs is a basic demand.

But there is only in 2 of 9 standards an explicit description that dictates how contrast and visibility work if the signs are embedded in their visual background (effect of mimesis) have to be checked. This element should be more clearly included in the standards.

2.4.5.2 Traffic control devices appropriate for the road characteristics and road alignment consistent with the traffic control devices and driver expectations

wrong

corrected

FIGURE 26 - EXAMPLE FOR ILLOGICAL (ABOVE) AND CORRECTED SIGNING (BELOW)(Source: Birth, 2004c)

Human Factors evaluation criterionVery often direction signs are crammed with information, the sketch differs from the real road situation, or it is not visible or legible.

Drivers cannot process more than 5 +/-2 destinations per approach to an intersection or more than 3 destinations per direction.

Drivers can also become confused if the direction of a sign differs significantly from the perceived road axis. It has been found at intersection black spot that such illogical signing causes irritations and accidents.


2012R36EN

TABLE 30 - TRAFFIC CONTROL DEVICES APPROPRIATE FOR THE ROAD CHARACTERISTICS AND ROAD ALIGNMENT, CONSISTENT WITH TRAFFIC

CONTROL DEVICES AND EXPECTATIONS

NL

Port

ugal

Can

ada

Fra

nce

Aus

tral

ia

Jap

an

Hun

gary

Cze

ch R

ep.

Chi

na

Traffic control devices appropriate for the road characteristics?

YI YI YI YD YD YI P YI YI

Road alignment consistent with traffic control devices? YI NO YD YD YD YD P YI NO

Traffic control devices in accordance with driver expectations?

YI YI YI YD YD YD P YI NO

Most standards address the horizontal and vertical alignment in combination, although this is not explicitly linked to the appropriate placing of traffic control devices. Several standards address the need for visibility and for ensuring driver expectancy. However, specific guidelines are not provided to help the designer (using HF principles) effectively combine alignment and traffic control devices.

This is something that would be considered in the design audit phase, if the road agency decided to perform this step, which is still not a standard practice for many agencies. Signs must respect the capacity of drivers to read and understand the messages and perceive correct information about the upcoming roadway and roadside.

Portuguese and Chinese standards include ITS (incl. VMS). Chinese standards generally state that traffic signs and markings should be appropriate for particular road characteristics, network, traffic and the social environment.

French standards consider the horizontal alignment and curve configuration, ensuring that any high risk sections (such as after a long downhill slope, at the approach to an interchange/ancillary area, or in areas subject to black ice) are clearly identifiable and do not surprise the driver.

The Dutch guidelines advise additional warning signs in case of sight restrictions due to alignment, e.g. ahead of a railway crossing located after a curve.

Conclusion8 of 9 standards directly or indirectly state that traffic signs should be consistent with road characteristics and alignment; 1 standard mentions this partly, and 1 do not consider this subject. Only a few national standards need additional advice on traffic control devices.


2012R36EN

2.5. GENERAL CONCLUSIONS OF THE HUMAN FACTORS AUDIT

The conclusions for all items in this chapter are summarized in table 31. The main conclusions are:

• The need of the driver to anticipate a transition and respond is the best described Human Factor in the standards. The Dutch standard recommends an anticipation sight distance (“Riding Distance”) for comfort and safety.

– The transition is addressed (58%). – However the visibility of critical points like curves, intersections and the visual impression of right-of-way is described partly or not in the majority of the cases (61%).

The design guidelines need additional advice on perception and visibility of critical points (especially the optical framing of curves), needed by human limitations of anticipation and orientation.

• The management of the Field of View is not well addressed in the design standards.

– Only about 9% of the user’s needs are described directly. – 32% are described only partly. – 59% are not described.

The subject of spatial perception underlying the HF demands seems to be a blind spot in the field of road design. Civil engineers, designers, auditors and road authorities should be systematically educated and trained to take these aspects into account in their work. Without a comprehensive understanding of the Human Factors principles – especially that of spatial perception -a self-explaining, safe road design with low accident risk cannot be achieved.

The design guidelines and standards need systematic improvement concerning the principles of spatial perception and optical guidance. This Human Factor also needs to be included directly in the education of designers and road authorities.

• The quality of how HF demands for managing drivers’ expectations, habits, visual orientation and information processing are addressed in standards varies.

The changes of road features in relation to driver expectations are only partly described in standards: 34% are described directly, 66% only partly or not.

The good explained aspect concerns traffic control devices. They are described in 69% of the standards directly and in 31% of the standards only partly or not. But the


2012R36EN

effect of mimesis in particular (invisibility of objects against a low contrast background) should be incorporated directly in the standards.

There is broad agreement in saying “Never surprise the driver”. However, it seems to be difficult to include this along with practical requirements in standards, because the underlying principles of the subconscious regulation of driving actions are not well known.

The management of drivers’ expectations, habits and visual orientation is well described in only about 39% of the standards. In about 28%, they are taken into account only partly and in 33% not at all.

It is therefore necessary to educate and train designers and road authorities in these matters. Without a comprehensive understanding of the Human Factors principles of spatial perception and of the subconscious regulation of driving actions, self-explaining, safe roads with low accident risk cannot be achieved.

In summary, 31% of human factors demands are already described and incorporated in standards; 29% are only partly incorporated and 41% are not incorporated.

TABLE 31 - SUMMARY OF ANSWERSHuman Factor demand Yes (%) P (%) No (%)

I.1. Transition zone long enough for perception, orientation and Decision Sight Distance 58% 20% 22%

I.2. Perception and visibility of intersections, curves and right-of-way is provided for 39% 36% 25%

∑safetyruleNo.1:Giveroadusersenoughtime! 49% 28% 23%II.1 Field of View with sufficient brightness and colour length visible

approaching sections avoided 0% 50% 50%

II.2 Fixation objects and visual cues on lateral roadside give optical guidance and support optimal lane tracking 13% 18% 69%

II.3 Eye-catching objects support lane tracking and detection of critical points, optical illusions avoided 15% 26% 59%

∑safetyruleNo.2:Ensureappropriatespeed+lanetracking! 09% 32% 59%III.1 Change of function is signalled by a change in the road’s optical

characteristics 22% 22% 56%

III.2 Change of direction is visible despite dominant eye-catching orientation 0% 37% 63%

III.3 Changes in road features that require re-learning of pre-programmed habits/routines are signalled early and clearly 39% 28% 33%

III.4 Multiple critical points do not occur concurrently 41% 26% 34%III.5 Traffic control devices are legible, in accordance with driver

expectation 69% 22% 09%

∑safetyruleNo.3:Pre-programmedriver’sbehaviourcorrectly! 34% 27% 39%∑∑of3Safetyrules 31% 29% 40%


2012R36EN

Based on these findings, chapter 3 will offer good design practices set out in design guidelines about aspects of the 6-Seconds Rule and certain HF matters such as traffic control devices that are part of the Logic Rule. Chapter 4 will focus on HF elements that are missing in design guidelines. It will describe best practices for aspects of Field of View management and several HF matters, related to principles of spatial perception and subconscious regulation of driving actions, that are part of the Logic Rule.

3. BEST PRACTICES

3.1. 6 SECONDS RULE

3.1.1. Transition Zone

The need to offer drivers a sufficiently long transition zone are fairly well described in the guidelines (e.g. stopping and decision sight distances given a certain design speed). However many standards for rural distributor roads are lacking with respect to the need for advance warning sections for critical points (e.g. exits after a parking place or hidden curves) in order to raise drivers’ alertness in timely fashion and avoid sudden steering and braking manoeuvres. It is recommended to include an advance warning section with introductory curves and signs in all guidelines for rural distributor roads.

3.1.1.1 Problem

Safe roads are those which are self-explaining: users know how to behave solely because of the design and control of the road. Drivers need to have enough time to accommodate their driving behaviour, especially awareness and speed, to new situations ahead. Apart from the standard reaction time for a sudden need to stop, average drivers need 4 – 6 seconds to adapt their normal driving programme in case of unusual or complex information processing demands.

3.1.1.2 Concept

Providing adequate sight distances at all points on the road which gives the driver 6 seconds before he reaches a new situation on the road is sometimes not achievable. Consequently provisions are needed for advance warnings.

It is necessary to check and confirm that a satisfactory transition zone is available between the original driving speed and a safe, desirable speed under new conditions.


2012R36EN

A transition zone consists of several specific parts:

• Driver’s preparation with signing and warning (advance warning section, e.g. Riding Distance, 3-4 sec),

• Driver’s anticipation with time for identification of unexpected situations (Decision Sight Distance, 2-3 sec),

• Driver’s response section and vehicle’s response (Stopping Sight Distance, 2-3 sec, plus braking distance for vehicle response).

Stopping Sight Distance and intersection sight distance are determined using the following formula:

GfVd

254

2

fV

fV

gfVd

25436001000

)81.9(22

2222

where G = the percent grade divided by 100 (up is positive, down is negative) and all other terms are defined in the next equation.

On a level roadway this distance can be determined using the following formula: GfVd

254

2

fV

fV

gfVd

25436001000

)81.9(22

2222

where d = breaking distance (m) V = initial speed (km/h) f = coefficient of friction between tires and the

roadwayThen SSD = dtV 278.0 where SSD = Stopping Sight Distance (m) t = perception and reaction time (s)

There are many critical points, where the driver has to adapt his driving programme:

• Approaching a city (village);• Approaching a horizontal/vertical curve of small radius;• Approaching a junction (yield, traffic lights);• Approaching a railway or pedestrian crossing;• Approaching a tunnel entrance, a change of luminance, a change in visibility

(smog, fog, heavy rain);• Work zones;• Worsening of pavement conditions (potholes, gravel, ice, snow);• Narrowing of the road pavement;• Obstacles on the road; toll plazas;


2012R36EN

• Slow vehicle ahead;• Herds, wild animals, falling rocks etc.;• Entering/exiting vehicles, parking vehicles;• Pedestrians/cyclists on the road;• Safety alert, fire or medical priority vehicles;• Other dangerous obstacles along the road, which call for slowing down.

3.1.2.3 Best practice

The best practice available is that there should always be a good, well signed and marked road with homogenous parameters, and no need to change the permitted, safe speed too often or suddenly. Or else warning signs or other markers well in advance, which provide the driver with enough time and distance for transition to a lower speed are needed.

In addition, curves, intersections, railway crossings, bus stops, private accesses, lane losses/merges, change of function or entrances to towns or villages should be clearly visible and identifiable about 6 seconds before these critical points. They should not be hidden by plantings, buildings, bridge posts or traffic facilities.

All advance warnings or information should be delivered to the driver in a way that gives him time to:

• Locate the information;• Identify the information;• Read the information;• Understand it;• Respond!

There are different rules for change of speed based on the category of road and speed limits, which correspond to the desired technical and safety standard on roads:

• Transition zones for motorways with speed limit of 120, 130 km/h or more;• Transition zones for interurban roads with speed limit of 90 or 100;• Transition zones for urban motorways with speed limit of 80• Transition zones for urban streets with speed limit of (70) or 50 or (30).

Advance warning should be based on standards adopted for signing on different classes of roads.Example: on motorways, never change the speed limit in one step – always change by 20 km/h steps: (130) → 110 → 90 → 70 → 50, or at most by 30 km/h steps.


2012R36EN

Decision Sight Distance review and evaluation of Shy Bassan (2011)The paper reviews Decision Sight Distance (DSD) highway design practices in several countries and proposes a model that includes a braking component after the pre-manoeuvre process and prior to the manoeuvre operation.

In Australia: the manoeuvre sight distance = distance during reaction time (2s) + distance during the evasive action (70m / 90km/h) = 120 m / 90 km/h.

In Canada and for PIARC, DSD is given as a function of design speed:

• Manoeuvre type A: stop on rural road: 185 m /90 km/h• Manoeuvre type C: speed, path, direction change on rural roadway: 275 m/90 km/h,• Manoeuvre type D: speed, path, direction change on suburban roadway: 320 m/90 km/h

In AASHTO 2004, DSD type C is 270 m for 90 km/h with the following components:

• Detection and recognition times: 2s• Decision and reaction times: 4.2 to 5.2 s• Manoeuvre time: 4 to 4.5 s• Total time: 10.2 to 11.2s.

The results of Shy Bassan’s model are:

• pre-manoeuvre time: 5.5 s (2 s (detection/recognition) +3.5 s (decision))• time to decelerate from design speed to manoeuvre speed: 4.2 s for deceleration:

3.36 m/s²• manoeuvre time (between 3.5 and 4.5s depending on the design speed).

FIGURE 27 - COMPARISON OF VISIBILITY DISTANCES (M) AS A FUNCTION OF DESIGN SPEED (KM/H) FOR DIFFERENT COUNTRIES


2012R36EN

(The model proposed in the article is at the bottom of the chart, except that of Australia, which does not include braking distance.)

Expectancy effectsDrivers’ expectations about design consistency are very important in their judgments about corresponding speed. Direct cues such as lane width, experience with previous geometry and standard significantly influence speed selection. On a good reconstructed road, a driver would never expect a sudden change of alignment without advance warning or direct visibility of the critical points 6 seconds ahead.

There is a Golden Rule which expresses the target for safe road design: “NEVER SURPRISE THE DRIVER!” The road and the environment should be so understandable that the driver is never suddenly surprised.

This rule is also important for driving reliably through a curve. Best driving results are achieved when the driver has an unobstructed view through the inside of the curve and the outside of the curve has continuous, parallel optical framing. On a self-explaining road, curves and their course will be visible early enough without traffic signs.

FIGURE 28 - ALIGNMENT HORIZONTAL LOSS (AT THE LEFT SIDE), LEGIBLE HORIZONTAL ALIGNMENT THANKS TO BANK (ON THE RIGHT SIDE)

(Source: Setra 2006a)

General acquisition of accident data keep records the higher accident density on curves, which have sharper curvature, greater central angle, lack of transition curve, more hazardous roadway conditions including width, steep grade at curve, long distance from last curve, lower pavement friction, wrong superelevation and lack of delineation and warning signs. Consistency in curve features eliminates the need to modify the speed from curve to curve, inappropriate entry speed and speed correction or steering correction within the curve. Additionally, the use of speed reduction signs and “curve ahead” warning signs, and improved visibility of edge and centre lines, are helpful low-cost engineering measures (Retting and Farmer, 1998).


2012R36EN

The key to effective warning is to notify drivers of the upcoming curve so that they can change their speed.

There are many possible improvements, including:

• Fluorescent yellow microprismatic chevron;• Fluorescent yellow chevron signs;• Fluorescent microprismatic curve-warning signs;• Red LED lights framing border on retroreflective warning signs;• Additional flags or flashers on warning signs;• Dynamic Advance curve-warning signs (VMS);• Different pavement markings.

However there are also problems with using chevrons, with or without yellow frame or underground:

• On most accident-prone curves, it is not only the physical features of the curve that are wrong.

• Also, the optical framing of the outer curve is interrupted or not parallel to the road’s edge and therefore disturbs the balance and stability of the driver.

That is why optical framing of the outer curve has to be continuous and not interrupted. The chevrons have to be placed close to each other without any gaps. This practice is not yet fully set out in the standards.

FIGURE 29 - INCOMPLETE INDICATION OF SHARP CURVE ON TRIESTE – TARVISIO EXPRESSWAY (LEFT) AND SIMULATED IMPROVEMENT GIVING THE DRIVER COMPLETE OPTICAL GUIDANCE AND BALANCE

SUPPORT (RIGHT)(Source: Roman Turza)

Under dark conditions in particular, differing intervals between chevrons often cause perspective illusions in field-dependent drivers, with a wrong expectation of the curve’s course.


2012R36EN

FIGURE 30 - PERSPECTIVE ILLUSION: AT NIGHT CHEVRONS MAY GIVE WRONG EXPECTATION OF CURVE DIRECTION. First impression is a left-hand turn (left); later it looks like a right-hand turn (right)

(Source: Birth, 2004b).

That is why single chevrons should be avoided and be replaced by a line of chevrons that are close to each other and without any gaps. The reader should take this into account when looking at the examples below.

For the process of driving through curves, there is detailed driving task analysis, distributed among the driving tasks, perceptual requirements, cognitive requirements and psychomotor requirements, including obtaining available speed information, making initial speed adjustment, making additional speed adjustment after assessing road condition, adjusting speed on the actual curvature and lateral acceleration to the acceleration on appropriate speed at the exit. There should be permanent adoption of new information about driving conditions by delineators, centre lines/edge lines, advance warning, advisory speed signs or chevrons (see also section 3.2.2. Fixation objects).

List of possible measures or problems to be checked to improve human understanding of “what’s going on” to accommodate the speed along the transition zone:

• Retroreflective sheeting for use on traffic control devices;• Apparent radii and visual distortion;• Influence of vertical alignment on horizontal curve perception;• Countermeasures to reduce red light running;• Curve speed management;• Effect of lane width on speed of cars in work zones;• Night-time legibility of traffic signs;• Variable message signs for advance warning;• Application of arrow boards in work zones;• Changeable message signs for lane closures (VMS);• Effects of sight distance on passing behaviour;• Driver braking reaction time;• Horizontal alignment design consistency;


2012R36EN

• Older driver perception-reaction time for intersection sight distance and object detection;

• Low-cost treatments for horizontal curve safety;• Decision Sight Distance for traffic control;• Delineation enhancements of curves for driving at night and under adverse

atmospheric conditions;• Dynamic advanced curve warning system;• Surprise braking;• Speed perception in road curves;• Pavement markings to reduce excessive traffic speeds before hazardous curves;• Driver eye fixation and safe driving behaviour;• Subjective road categorization and speed choice;• Self-explaining roads;• Use of warning signs and markings to reduce speeds on curves;• Effects of chevrons, post-mounted delineators and markers on driver behaviour on

curves;• Improper signing and geometrics.

Some examples of good and bad practice, illustrating how to make critical points more visible or understandable:

FIGURE 31 - GOOD PRACTICE FOR DECELERATION AT PEDESTRIAN CROSSING – SOUTH SWEDEN (Source: Jiri Landa)


2012R36EN

FIGURE 32 - GOOD PRACTICE FOR TOWN ENTRANCE – NYBRO, SOUTH SWEDEN(Source: Jiri Landa)

FIGURE 33 - ADDITIONAL SIGNING AND MARKING TO EMPHASISE THE CRITICAL POINT ON THE INTERSECTION

(Source: Jiri Landa)

FIGURE 34 - GOOD SAMPLE FOR LIGHTING OF PEDESTRIAN CROSSING (Source: Vladimir Mensik)


2012R36EN

FIGURE 35 - GOOD SAMPLE FOR TRAFFIC LIGHT (Source: Jindrich Sachl)

Note on figure 35: Repetition of Yield sign above the left lane minimizes the accident potential when traffic lights are out of order and the sign in the right lane may be hidden behind a truck. The “End of Priority” sign should be positioned in advance of the intersection and advertisements in the background should be removed to avoid confusion.

FIGURE 36 - EXEMPLE DE BONS FEUX DE SIGNALISATION

(Source: Jindrich Sachl)

Note on figure 36: Again, repetition of the Yield sign above the left lane can guide the attention of drivers when traffic lights are out of order and another Yield sign is hidden behind a truck (normal practice in Germany).


2012R36EN

FIGURE 37 - GOOD PRACTICE FOR SIGNALS AND MARKINGS (Source: Jindrich Sachl)

Note on figure 37: Repetition and retroreflective sheeting of pedestrian crossing sign in high pedestrian traffic area between bus and tram stops is hidden behind the thick poles for supplying power to trams. Horizontal markings at the crossing have significant imperfections, e.g. the transverse markings. They should end about 50 m before the point of stopping (pedestrian crossing) to ensure no interference with the braking manoeuvre (differences in friction coefficient and height of the markings that alter the dynamics of the vehicle). Additionally, there is no continuous line before the pedestrian crossing that would prohibit lane changing before the crossing. Drivers might therefore focus on lane changing and on traffic in the other lane instead of on the pedestrian crossing.

FIGURE 38 - BAD EXAMPLE: PRIORITY ON THE RIGHT SHOULD BE HIGHLIGHTED BY HORIZONTAL AND VERTICAL SIGNING(Source: Jindrich Sachl)


2012R36EN

FIGURE 39 - BAD EXAMPLE: THIRD CLASS ROAD – THE ALIGNMENT IS UNCLEAR TO THE LAST MOMENT (Source: Jindrich Sachl)

FIGURE 40 - BAD EXAMPLE: RAILWAY CROSSING IN URBAN AREA IS NEARLY INVISIBLE AND THERE IS NO RED LIGHT



2012R36EN

FIGURE 41 - GOOD EXAMPLE: LOW COST MEASURES POINT TO RAILWAY CROSSING, WHICH IS NEARLY INVISIBLE

(Source: Pavel Skladany)

Commentary by engineer Carlos de Almeida Roque on visible shortcomings of the above solution: The sign on the road is not legible, because it is not elongated as it should be ( figure 40). Figure 41 illustrates a normal sign (left) and an elongated one (right).

FIGURE 42 - EXAMPLE OF MARKINGS

(Source: Carlos de Almeida Roque)

The transverse markings in figure 42 should not continue on the curve because of the dynamics introduced on the vehicle such as super elevation and centrifugal force.


2012R36EN

Quoting Retting and Farmer (1998): “Because traffic speeds on curves are closely correlated to approach speeds, efforts to reduce speeds should concentrate on deployment of effective devices on tangent sections preceding curvature points.”

FIGURE 43 - EXAMPLE OF THE USE OF CHEVRONS AND A SIGN ON THE ROAD (COMPLEMENTING AN EQUIVALENT VERTICAL SIGN) IN A PREVIOUSLY VERY DANGEROUS CURVE ON IP 5

(TRUNK ROAD), PORTUGAL (Source: Carlos de Almeida Roque)

FIGURE 44 - ADVANCE WARNING SIGNS ON MOTORWAY WARNS OF UPCOMING WORK ZONE (Source: Jaroslav Král)


2012R36EN

FIGURE 45 - SIGNS FOR TRUCKS WITH SPECIFIC LOAD HAZARDS FOR WATER RESOURCES, LOCATED IN ADVANCE OF WATER

(Source: Jaroslav Král)

FIGURE 46 - ADVANCE WARNING IN CASE OF EMERGENCY – FLOOD DAMAGE (Source: Stanislava Jakesova)


2012R36EN

FIGURE 47 - ADVANCE WARNING OF CHANGE IN NUMBER OF LANES – 2+1 ROAD VIENNA. ZNOJMO(Source: Tomas Hartl)

3.1.1.4 Recommendations

Demands to offer drivers a sufficiently long transition zone are fairly well described in guidelines, for instance by Stopping and Decision Sight Distances given a certain design speed. Many standards for rural distributor roads are however lacking in an advanced warning section for critical points (e.g. exits after a parking place or hidden curves) to timely raise drivers’ alertness and avoid sudden steering and braking manoeuvres. It is recommendable to include an advance warning section with introductory curves and signs in all guidelines for rural distributor roads.

3.1.2. 6 seconds rule - I.2. Perception and Visibility

3.1.2.1 Problem

Drivers may not have adequate Decision Sight Distance (DSD) to perceive, react and potentially stop in places along the road where their expectancy might be significantly altered or where they would probably have doubts about the information received. This includes upcoming intersections, interchanges, change in cross-section, service areas, curves, public transport stops, public access, entrances to towns/villages, similar facilities and other critical points.

3.1.2.2 Concept

Provide adequate sight distances at all points on the road to ensure proper visibility, perception and reaction time when approaching critical points such as sharp curves, intersections, and other critical points.


2012R36EN


From the AASHTO Policy on Geometric Design of Highways and Streets (2004, page 115):

“Decision Sight Distance is the distance needed for a driver (1) to detect an unexpected or otherwise difficult-to-perceive information source or condition in a roadway environment that may be visually cluttered, recognize the condition or its potential threat, (2) select an appropriate speed and path, and (3) initiate and complete the manoeuvre safely and efficiently.”

Other sight distances that the designer will use are Stopping Sight Distance and Passing Sight Distance. Stopping Sight Distance should be used when there is no need to provide a longer sight distance (DSD or PSD).

A perception and reaction time of 2.5 seconds is typically used as representative of the 90th percentile of drivers and situations. Designers should consider whether a longer or shorter time is appropriate for an atypical driver population.

Critical point visible and clearly identifiable Correct and adequate placement of the critical point is the most important factor in providing drivers with safe orientation and anticipation. The critical points should be visible 6 seconds ahead of time, without signals and visual barriers.

Where this is not possible, traffic signs must be provided to inform drivers of any critical point ahead. Provision should be made for the use of signs where features of the road would not allow for the appropriate sight distance in the prevailing situation (stop or Decision Sight Distance). Adequate space between advance signposts and any decision point, such as an intersection or a change in road priority, is needed in order to prevent confusion among drivers. Traffic signs, signals, markings and VMS (variable message signs) should be located in such a way as to make the safety-related message visible and legible both day and night, and they should not restrict the visibility of the critical point or the traffic itself.

The distance of signs from the critical point is related to design speed. Road signs should be clearly visible and legible at a sufficient distance (the distance for visibility is about 1.5 times the distance for legibility). Along 80-km/h roads, traffic lights should hang above the road for straight-ahead traffic, with a visibility distance of at least 200 m. Along distributor roads, advance sign posting should be 200 m before the intersection, and if necessary also 400 m and 600 m before it.

A road feature that notifies drivers of critical points like intersections and signposting is a minimal clear zone without obstacles. This creates an unobstructed view of the


2012R36EN

intersection. The Dutch guideline for distributor roads recommends a clear zone of 6 metres or more (CROW, 2002a).

Several road features help drivers to detect critical points like intersections and roundabouts. Medians, especially those with bright colours or curb stones and vertical elements, attract driver attention. Figures 99 and 100 show examples of the application of raised medians and lane separators in the standard design of priority intersections and roundabouts in the Netherlands.

Pavement treatment can also be useful for identifying critical points, such as transverse rumble strips or white transverse bar markings. Both provide a warning to drivers to slow down at certain locations. White transverse bar markings consist of a sequence of pairs of transverse white lines that become closer together as they approach the critical location. Arrows, striping, and a warning triangle can be painted on the pavement to warn drivers of upcoming changes, such as an approaching priority road. Sharp horizontal curves should be treated with advance warning signs and chevrons. See figure 48 following page.

Portuguese “InIR’s Technical Disposition Criteria for location of road signs: Traffic guidance equipment on dangerous curves (chevrons - which need is determined, for a two-way two-lane road, by a LNEC software - PERVEL), are located so that the driver can see three chevrons at any moment (before the curve and all along the curve until its end), evaluate its curvature and have continuous guidance along the curve”.

However designers should be aware that the gaps between the chevrons may cause perspective illusions in field-dependent drivers under dark conditions. It would therefore be better to place the chevrons close to each other or to place optical guiding elements between them so that there is a continuous frame along the outside curve without any gaps.


2012R36EN

FIGURE 48 - CLASSES B AND C - MULTIPLE SUCCESSION LOCATION

OF CHEVRONS

FIGURE 49 - CLASS D - INDIVIDUAL LOCATION

(Source: Almeida Roque, 2009d)

Simulation of an continuous optical frame along an outer curve on the basis of the above figure from InIR’s Technical Disposition:

Information regarding the course of the road is especially important under circumstances of darkness, rain and glare. Under adverse light conditions, road users may need extra (vertical) beaconing in addition to the markings on the road. For instance, critical points or discontinuities that can be unexpected, like a roundabout, zebra crossing or a tight curve, often need lighting or special optical guiding elements

FIGURE 50 - CLASSES B AND C - MULTIPLE SUCCESSION LOCATION -

SUPPLEMENTED BY OPTICAL GUIDING ELEMENTS TO CLOSE THE GAPS

FIGURE 51 - CLASS D - INDIVIDUAL LOCATION


2012R36EN

as shown in figures 50 and 51. They should be neon lights of a yellow-green or orange shade to make them visible in the dark, reflecting optimum spectral sensitivity of the human eye.

FIGURE 52 - SPECTRAL SENSITIVITY OF THE EYE IN DARK CONDITIONS (BLUE) AND DAY CONDITIONS (RED)(Source: http://www.osram. ch/osram_ch/DE/Tools_%26_Services/Training_%26_ Wissen/

lichtlexikon_popups/images/pop_35_1_de.gif)

FIGURE 53 - ADDITIONAL SIGNING OF INVISIBLE INTERSECTION – LATER RECONSTRUCTED (Source: Jiri Landa)

In specific situations the standard may be insufficient because of the lack of a contrasting background or because another lane in the field of view may be visually misleading. All signs should be either retroreflective or illuminated: trans-illuminated or externally illuminated. Any materials used in their production should not cause glare or diminish the visibility of symbols or inscriptions. Intensity must be taken into consideration at transition zones between unlighted and lighted areas, such as in


2012R36EN

rural areas when drivers are subject to too much light contrast between lengthy dark stretches and upcoming well-lit zones, for example when they exit tunnels.

Landscaping can provide a variety of benefits and should be considered in conjunction with safety and other factors in the design process. Most importantly, plantations should not interfere with sight distances or impede the visibility of any critical points.

Curves are visibleHorizontal and vertical alignment is critical to providing a consistent and safe road environment. A combination of horizontal and vertical curves can cause difficulties estimating curve radii. For instance, the radius of a horizontal curve in a sag curve may be underestimated, with the risk of high speed. In crest curves, it is the other way around. The following are some points to consider when designing a new right-of-way.

• If a horizontal curve coincides with a sag curve, it is recommended that the radius of the vertical curve be at least 5 to 10 times as large as the radius of the horizontal curve.

• It is recommended not to choose a minimum radius (given the design speed) for both the horizontal and vertical curve and to increase at least one with 20%.

• One should not adjust vertical alignment to the landscape too much. This may cause the road view to fall into separate inconsistent parts.

• To give the road a fluent appearance, the tangent points of horizontal and vertical curves should coincide as much as possible.

Positive guidance measures such as advanced warning signs or chevrons have been proven effective where the design has not provided for adequate radii in curves. Chevrons may be used to provide additional guidance to drivers where there is a change in the horizontal alignment of the road; they should be installed on the outside of a curve or sharp turn, and should be located at right angles to oncoming traffic. Since site conditions will vary, the size and spacing of these signs should be determined by a field investigation to ensure maximum visibility and legibility to the driver given the road conditions, including the operating speed. For consistency, all chevron signs used at a location should be of the same size. They should be spaced so that the driver always has at least three in view until the change in alignment eliminates the need for the signs. The signs should be installed at a height of 1.5 m above the near edge of the nearest traffic lane to the bottom of the sign. Shrub screens can also be planted in the outer part of curves to mark the alignment of the curve and facilitate night driving.


2012R36EN

Intersection – visibility triangle from minor roads is not obstructedWhen designing an intersection, the following human factors should be considered:

• Usual behaviour and decision capabilities of drivers; • Surprise effect;• Reaction and decision times.

The sight triangle (see figure 53) – the area of visibility on a corner to allow for safe operations for all road users – should never be obstructed. Decision Sight Distance (DSD) should be provided at all intersections. Intersections should never be located near horizontal or vertical curves (figure 54), because in addition to limiting the visibility within the sight triangle, curves reduce efficiency of the brakes, and vertical curves and gradients have an influence on acceleration possibilities. Whenever possible, intersections should be located in straightaways with gradients not greater than 3%. Landscaping should in no way hider visibility within the sight triangle.

FIGURE 54 - CORRECT SIGHT TRIANGLE INTERSECTION CONFIGURATION (Source: Transportation Association of Canada (TAC), 1999, updated 2007)

If an intersection is on a skew, the following reconfiguration is advised to provide optimal visibility.


2012R36EN

FIGURE 55 - INTERSECTION BEHIND THE CREST (Source: Jiri Landa)

FIGURE 56 - SIGHT DISTANCE CRITERION AT THE APPROACH TO A ROUNDABOUT (Source: Bastos Silva, 2009)

FIGURE 57 - SIGHT DISTANCE CRITERION AT THE ENTRANCE TO A ROUNDABOUT (Source: Bastos Silva, 2009)


2012R36EN

FIGURE 58 - EXAMPLES OF INTERSECTION RE-ALIGNMENTS (Source: Transportation Association of Canada (TAC), 1999)

Intersection – minor road: unmistakable right of way Ensuring that the road is designed in a consistent manner that is recognizable to the driver, and familiar with respect to operating expectations for that road category and speed limit, will decrease the driver workload. Road features (elements of design) tell the driver what type of road it is and therefore what driving habits can be expected. This also gives the driver an unmistakable understanding of traffic priorities. This is addressed in the “self explaining” roads concept, where the driver is encouraged to alter his driving behaviour to make it consistent with the road design and function. The aim is to clearly identify each class of road that has specific features, so that when the driver encounters a road type, he/she will instinctively know how to behave (speed, overtaking, etc.). Simple and consistent design on self-explaining roads has been proved to reduce driver stress and mistake.

Consistency needs to be addressed in three areas – cross-section, operating speed and driver workload. The cross-section (width of the pavement and obstacle-free / clear zone) makes the road more self-explaining. Pavement markings also need to increase the recognisability of the road category and speed limit.

The design of an intersection should also support the right-of-way. The appearance of the two roads has to differ. When a driver approaches an intersection from a minor road, there should be a clear understanding as to which direction of traffic has the right-of-way. Characteristics of existing intersections should be taken into consideration so that consistency can be established along a road. A change in road category should not surprise the driver and should be recognizable by the physical characteristics of the category. A big change in speed limit (20 km/h or more) is preferably located at a crossing.

Transition zones should be considered so that any change that takes place happens slowly and gradually without surprising the driver. The design speed of adjacent sections should not differ by more than 10 km/h, so as to achieve gradual and


2012R36EN

comfortable transition of speeds. When altering the cross-section, special attention should be given to the geometry of the transition zones in order to ensure traffic safety and comfort. The standard cross-section should be continued under viaducts and tunnels, but the clear zone can be interrupted if necessary compensatory measures are taken, for instance safety barriers. To decrease costs, the emergency lane and medians can be narrowed in a tunnel.

Pedestrian or cycle crossings should be located so that their visibility is compatible with the operating speed (V85). Otherwise, physical mechanisms should be provided that cause a reduction of speed to the permissible values for the prevailing visibility. As a recommendation, it is noted that locations where traffic speed exceeds 50 km/h should have pedestrian crossings with signal control.

FIGURE 59 - VIEW OF PRIORITY TRAFFIC FROM A MINOR ROAD (Source: CROW, 2002a )

FIGURE 60 - EYE-CATCHING OBJECT – NEW DELHI, INDIA (Source: Jiri Landa)


2012R36EN


Demands are addressed in guidelines, for instance by sight distances, visibility triangles for intersections, road signing, advance warning signs, chevrons (and visibility of these types of road side furniture), raised medians and roundabout middle islands with bright curbs, etc. Several aspects regarding contrast to draw attention to critical points are not yet addressed. Recommendations to use planting to draw attention to curves and intersections are rare and guidelines are lacking in guidance on contrasts of for instance chevron signs against their background. So even without design exceptions (e.g. sight distances are in accordance with guidelines, the view is unobstructed, and chevron signs are put in the outside of the curve), curve detection may be hindered because features such as chevron signs do not draw drivers’ attention. Therefore, we recommend more advanced guidance on the conspicuity of critical points.

3.2. FIELD OF VIEW RULE

3.2.1. Optical density of the Field of View

3.2.1.1 Problem

Drivers adapt their speed to the given road situation. It is well known that the amount of information that has to be processed influences the quality of driving (Yerkes-Dodson Law). The amount of information also influences drivers’ speed. The term used for this is optical density of the field of view. It is a function of the number of objects that contrast with the background.

A very small number of contrasting objects leads to monotony and both reduced performance and reactivity. To avoid monotony, the driver subconsciously changes his driving activities in order to increase information input: he swerves, brakes or – in most cases – increases speed.

3.2.1.2 Concept

Spatial perception—and in particular the number of contrasts in brightness and colour, which influence driver speed—has to be considered in order to support the driver and stabilise his orientation. It is desirable to achieve an optimal level of optical density to support the correct choice of speed. That is why efficient speed management uses changing brightness and colour contrasts to avoid subconcious speeding up.


2012R36EN

The road environment should be well structured through:

• fixation objects that attract drivers’ attention, but do not distract;• a sinuous “rhythmic” alignment (sufficient curvature).

FIGURE 61 - GOOD PRACTICE – A WELL STRUCTURED OPTICAL FIELD OF VIEW WITH A SUFFICIENT NUMBER OF COLOUR CONTRASTS

(Source: Birth, Pflaumbaum & Sieber, 2006).


There is no reference in the guidelines to the possibility of structuring the visual field of the road environment using fixation objects. This will be considered in chapter 5.1.

We will therefore refer to best national practice for structuring the visual field through a curvilinear alignment.

Avoiding lengthy visible approaching sections before critical points

FIGURE 62 - BAD PRACTICE: LONG STRAIGHT SECTION BEFORE A CURVE – QUEBEC, CANADA(Source: Daniel Aubin, Google Earth)


2012R36EN

CanadaThe “Guide for the Design of Roadway Lighting” (2006) and “Illumination of Isolated Rural Intersections” (2001) both give instructions about lighting location, intensity required and the transition zone between unlighted and lighted areas. A specific guide was produced to propose solutions for rural areas to avoid being flashed with too much contrast between dark zones, which are usually considerably longer than the lighted zone.

The “Manual of Uniform Traffic Control Devices” provides information to implement correct and adequate signs that are clearly visible day and night and inform drivers of any critical points ahead. Signs are posted at a distance from critical point related to design speed.

AustraliaDesigners are required to make an approaching curve sufficiently visible that drivers can make a proper judgement of the radius and appropriate speed for the curve.

The length of arc that needs to be perceived must be seen from a point that allows the driver to react and then decelerate. The deceleration should only require comfortable braking; the maximum deceleration rate should be 2.5m/s/s. Typically, this means a distance of 25m will accommodate a 10km/h speed reduction from 90km/h, and 40m will accommodate a 15km/h reduction. Deceleration distances should be adjusted for the effects of grade.

The sight distance is the sum of the reaction distance, the arc length for perception and the deceleration distance. If the curve is transitioned, it is possible for the deceleration distance to coincide with up to the first half of the transition. If the curve is untransitioned, deceleration up to the curve tangent point can be assumed.

Provision of horizontal curve perception distance may require a larger crest than is required for Stopping Sight Distance.

• Tangents

The tangent or straight section is the most common element of the horizontal alignment. It provides clear orientation, but at the same time is visually uninteresting unless aimed at some landmark. If the focus goes straight ahead for a long distance, the driver’s field of view is stretched and narrowed, and subconscious speeding up results.

Tangents of suitable length are desirable on two-lane, two-way roads to facilitate overtaking manoeuvres and they should be provided as frequently as the terrain permits. Straight sections are too long (e.g. 1,000 m) if they encourage drivers to travel well in excess of the design speed and they should therefore be avoided if


2012R36EN

practicable. Tangents that are too short to provide a separation between adjoining curves that enables a desirable rate of rotation of the pavement should also be avoided because they have the potential to destabilise vehicles.

In flat topography, long straightaways on roads may have to be accepted. But designers then need to be aware that drivers will then subconsciously speed up. If curves are deliberately introduced into the design to break the monotony, they should have long arc lengths; otherwise they will look like kinks. Unless the change in alignment is considerable, oncoming headlights will be a nuisance. Further information about curvilinear alignment design can be found in the next section.

FIGURE 63 - CURVILINEAR ALIGNMENT- EXAMPLE FROM GERMANY(Source: Daniel Aubin, Google Earth)

Curvilinear Alignment Design on Flat Terrain

The traditional approach to the design of road alignment on flat terrain has been to use long tangents with relatively short curves between them (figure 64). In some cases, the straight section is exceptionally long, resulting in monotonous driving conditions and speeding up, or fatigue and reduced concentration (Yerkes-Dodson Law, figure 64).


2012R36EN

Driving quality depends on the degree of strain (Yerkes-Dodson law)

Driving quality

Strain

Understrained Overstrained

FIGURE 64 - YERKES-DODSON LAW(Source: Yerkes R.M., Dodson J.D. (1908))

The problems of long tangent/short curve alignment have been recognised for some time. A general conclusion has been that the ideal alignment is a continuous curve with constant, gradual and smooth changes of direction.

However if curves are too long – especially right-hand curves – they can disturb the equilibrium organs in the inner ear and cause nausea or feelings of dizziness. That is why curves should ideally not take longer than 30s to drive through. This has to be taken into account in the concept of curvilinear alignment, which has been defined as consisting of long, flat circular curves, simple and compound, connected by fairly long plan transitions, about two-thirds of the alignment being on the circular arcs and one-third on transition curves. Inherent in this definition is the premise that the alignment is made up of a range of curves varying in radius from about 6,000 m to a maximum of 30,000 m and accordingly the need for plan transitions is essentially removed. But designers should be aware that radii used in excess (driving trough time of more than 30 seconds) cause nausea or a feeling of dizziness. This is a fact that comes out of accident research.

• Advantages of Curvilinear Alignment

A road with curvilinear alignment is much more pleasant to drive on than one with


2012R36EN

long straight tangents, since it unfolds itself smoothly with no unexpected checks. The driver is more able to judge the distance to an approaching vehicle, and to assess its rate of approach since the driver sees it to one side, the lateral component of its movement providing the necessary information for driver assessment. Judgments on the safety of overtaking manoeuvres are also easier to make under these circumstances.

Because of the continuously curving alignment, the view ahead is constantly changing and it is also possible to direct the road towards interesting features of the countryside for short periods. This removes much of the monotony of long straight alignments and can create a sense of anticipation in the driver for what lies beyond.

At night, curvilinear alignment removes much of the approaching headlight glare problems common to long straight roads in flat country. On long straight sections, headlights become visible from a very long distance away; they can be annoying and distracting from a distance of over 3 kilometres. When vehicles approach each other on curvilinear alignments, the glow of the approaching vehicle headlamps can be seen well before the lamps become visible, and the rate of approach of the vehicle can be assessed.

In the daytime, when driving in the direction of the sun, curvilinear alignment removes much of the approaching glare problem caused by the sun’s rays common to long straight roads running in a west-east direction in flat country.

Conditions for both day and night driving are therefore much more comfortable on a road with curvilinear alignment.

On treeless plains, some of the effect of the curvilinear alignment is lost. It may be that in such circumstances, smaller (optimum) radii would be more effective in that they will increase the driver’s perception of relative change.

The principles of curvilinear alignment can be applied in a wide range of conditions using a wide range of curve radii together with transition curves. Considerable improvements in the quality of the road system can be achieved at no extra cost by the application of these principles. It may be noted that in flat country, curves with superelevation can be quite a problem for drainage. That is because of very low longitudinal grades superimposed to runoff lengths that make quite a long extent of the outside lane, or carriageway, horizontal, with the consequent problems of aquaplaning probabilities.

Tourist signs – France and PortugalTourist signs break the monotony that drivers may feel during long trips and they offer landmarks which drivers can use to find out about the geographical area they are traversing. On motorways, the landscape offered to the driver is, by design, often


2012R36EN

marked by the uniform characteristics of the infrastructure itself. In the long run, this may reduce drivers’ vigilance and jeopardize safety (Source: SETRA, June 2001).

FIGURE 65 - EXAMPLES OF TOURIST SIGNS FROM PORTUGAL (LEFT) AND FRANCE (RIGHT)(Source: JAE, 1999 and EGIS, A. Thomas)

Designers are required to limit the length of straight-ahead sections to 1 km. Standards also recommend a range of between 150 and 250 m for the length of a tangent before a roundabout.

Netherlands Dutch standards provide a table which sets out a maximum horizontal tangent length for different design speeds.

TABLE 32 - MAXIMUM LENGTHS OF STRAIGHT-AHEAD SECTIONSSpeed (km/h) Maximum length of tangent (m)

100 < 2,00080 < 1,60060 < 1.200

(Source: CROW, 2002a.)

Additionally, the the guideline offers a graph to estimate driving speed using the average curvature of a road section.


2012R36EN

Bh = Bendiness (gradian/km); γi = total change of angle; ti = change of angle of the transition curve; ai = change of angle of the curve between transition curve; L= total of the described section; B = width of the carriageway

FIGURE 66 - RELATIONSHIP BETWEEN BENDINESS AND SPEED (V85) (Source:CROW, 2002a)

GermanyThe German RAS-L advises limiting the maximum length of straight sections to 1,500 m. If there is a need for adaptation to the environment or other constraints, longer straight sections are possible. To ensure safe transitions, the radii of curves after straight sections should have a balanced relation to the length of the straight section. Both directions of traffic should then be considered. Figure 64 shows the allowed radii after straight sections.


2012R36EN

Cur

ve ra

dius

(m)

Length of straight section (m)

GOOD

AVOID

USABLE

FIGURE 67 - RELATIONSHIP BETWEEN ALLOWED RADII AND LENGTH OF STRAIGHT SECTIONS(Source: RAL (Draft 2012))

Straight sections between curves of the same direction should be avoided. If that is not possible, the minimal length of the straight section should be 1.5 times the radius of the smaller curve.


Except for the recommendation of a curvilinear alignment (i.e. try to avoid long straight road segments), guidelines mostly lack guidance on optical density, for instance about how to avoid monotonous road surroundings using colour contrasts, plantings and art work along the road and how to use contrast to attract attention to critical points. Another issue that needs more attention in guidelines is drivers’ need for structures to run parallel to the road to support lane-tracking (e.g. safety barriers, noise walls, plantings and building lines).

3.2.2. Fixation objects in the lateral roadside environment support lane-tracking

3.2.2.1 Problem

Tracking, braking and accelerating are largely performed subconsciously. The perception of speed and position are derived from the view of the road and its surroundings. Besides the view in the central part of the field of view, people use information in the peripheral part. Psychologists speak of ambient-near vision that


2012R36EN

people use to orient themselves, which is partly dependent on subconsciously processed peripheral information. This what enables people to walk straight through a forest at twilight without hitting trees, a feat which is impossible by consciously fixating on every tree. Drivers use information in both the central and peripheral part of the field of view, and unless designers take this into account, they may not appreciate how the finished design will appear to road users.

Tracking mistakes caused by deceptive or distracting cues in the surroundings of the road, or a lack of guiding information, contribute to rare events like run-off-road crashes. Aside from edge lines and other markings, the following elements play a role in the tracking task:

• Linear structures like safety barriers, planting lines, or a row of lighting posts that run parallel to the road’s axis support tracking (they offer a frame to hold on to) while structures that deviate from the road’s axis may distort the frame. A frame is especially important in the outside of horizontal curves; see figure 68 showing a curve on a road in Sweden that is largely lacking a frame along the outside of the curve.

• Vertical structures like buildings, posts and trees that appear perpendicular support the tracking task while structures that are out-of-true worsen tracking, as in figure 69.

• Structures over the road like bridges and crossovers support tracking if they appear horizontal and symmetrical. The picture in figure 69 shows an example where the abutment is not parallel and not symmetrical to the road’s axis (the lack of a frame along the outside of the curve behind the viaduct adds to the problem).

• Dominant objects that attract the attention and provoke fixations too far away from the course of the road (depending on the position relative to the road) worsen the tracking task, e.g. buildings, special facilities and billboards.

Several optical illusions may worsen tracking and speed behaviour, e.g. the impression of a bottle neck caused by converging line structures along the road (i.e. a ‘distance illusion’).

FIGURE 68 - BAD PRACTICE IN SWEDEN FIGURE 69 - BAD PRACTICE IN CANADA(Source: Paul Schepers) (Source: Daniel Aubin)


2012R36EN

3.2.2.2 Concept

A high quality field of view offers cues that prevent misperceptions of position relative to the road’s course for all drivers under all lighting conditions.


The guidelines in most countries offer little by way of best practices for achieving a field of view allowing optimal tracking. For instance, the importance of parallel guiding lines (see figure 70 with parallel lines of plantings that support tracking) is hardly described in guidelines. Only some of the items mentioned in the introduction are addressed. Items that are not yet part of current standards will be covered in E. chapter 4.

FIGURE 70 - PARALLEL LINES OF PLANTINGS SUPPORT TRACKING(Source: City Plan, J. Landa)

Carriageway width reductions

FIGURE 71 - REDUCTION OF LANES


2012R36EN

FIGURE 72 - TRAFFIC SIGNS

The carriageway may narrow because of a reduction of the number of lanes or a reduction of the width of the lanes or of an auxiliary lane. A reduction of the number of lanes, and the end of an overtaking lane, can be introduced by a traffic sign (see the Dutch example to the right), and by suitable markings such as arrows and stripes on the road (see examples in figures 71 and 72). The ending or narrowing of an auxiliary lane should be made clear with markers and reflectors. This is exemplified in figure 83. Note that non-parallel safety barriers may make the tracking task harder. Additional measures are needed if the width of the auxiliary lane is lowered to less than 2m. Oblique stripes as in figure 71 have to be added if the auxiliary lane ends.Reduction of lanes

A reduction of the width of the carriageway, for instance at an obstruction such as a narrow bridge or town entrance, can be introduced by:

• traffic signs (see for instance the Dutch, Portuguese and French traffic signs at the right, figure 68);

• An overtaking prohibition shown by traffic signs and by markings on the road as in figure 86;

• Position markers and reflector posts, as in figures 78 and 79;• Street lighting to improve the visibility of the bottleneck in darkness;• Shrubs.

Note that shrubs with thick trunks may cause serious injury in case of a crash. This can be prevented by choosing shrubs with a trunk diameter less than 7cm.

The above-mentioned lane width reductions result from insufficient space or a narrow bridge that can only be widened at very high cost. Sometimes lanes are narrowed deliberately because of a change of road category or speed limit. In this case, more suitable measures than the ones mentioned above are available, for instance a middle island such as in figure 69 below or a chicane (see chapter 3.1. about town entrances).


2012R36EN

FIGURE 73 - MIDDLE ISLAND (Source: SETRA-CETE)

Roadside objects appear to be verticalGuidelines in several countries, for example in France, Portugal and Canada, state that road signs should be vertical. However, the surroundings of the road may contain many other vertical elements that can be skewed. The fact that abutments, buildings, trees, delineators, traffic control devices or similar objects should be parallel to the road’s axis to stabilise lane- keeping is not adressed in the guidelines.

Optical framing of curvesThe importance of a frame along the outside of a curve parallel to the road’s axis is partly covered in some guidelines. There are several solutions to achieve an optical frame. A frame along the outer curve can be constructed by the use of plantings or an embankment. The Portuguese guideline (JAE P6/90 Norma de Nós de Ligação) recommends planting shrubs in the outer curve. In case of a collision, shrubs with thin trunks cause less harm than trees, which often have to be protected by safety barriers (see figure 74 on the right). However, shrubs have to be at least several metres high to offer enough guidance.


2012R36EN

FIGURE 74 - OPTICAL FRAMING OF CURVES(Source left picture JAE, 1990a, right picture: Birth, Sieber, & Staadt, 2004)

The French guidelines recommend plantings (trees and vegetation) to achieve an optical frame along the outer curve, as seen in the figure 74.

FIGURE 75 - WITHOUT OPTICAL FRAME ALONG THE OUTER CURVE

FIGURE 76 - OPTICAL FRAME ALONG THE OTHER CURVE

Before improvement After improvement introducing inclined wall

(Source: SETRA, 2006)

Figures 77 from the HF guideline show examples where the lack of an optical frame along the outer curve (left) is corrected by an embankment (right).

wrong corrected

FIGURE 77 - LACK OF AN OPTICAL FRAME CORRECTED BY AN EMBANKMENT


2012R36EN

Plantings are helpful in achieving optical guidance along the outer curve but they may not be sufficient for certain tight curves because the level of contrast of the plantings is low. Red chevron signs often have a low contrast with planting as a background. To overcome these problems at a place in Germany where injuries had occurred, a shield was constructed above the safety barriers, as shown in figure 78. The frame resulted in a decrease of the average speed.

Speed before and after the construction of an optical guiding frame in an accident curve

(b158/130 to Leuenberg)

FIGURE 78 - COURSE OF THE CURVE IS FRAMED; THE COLOURED FRAME GUIDES THE DRIVER AND REDUCES SPEED AS IS SHOWN BY THE SPEED FIGURES ON THE RIGHT SIDE

(Source: Birth, Demgensky & Wähner, 2011).

The example below of a tunnel might also be considered an optical frame along the outer curve. The brightly coloured curbs and barrier in combination with reflector studs or ‘cat eyes’ offer strong visual guidance.

FIGURE 79 - OPTICAL FRAME ALONG THE OUTSIDE OF A CURVE IN A TUNNEL (Source: CRYZAL SAS, France, E. Locquet)

Most countries have standards that recommend chevron signs for tight curves. Often the precise advice depends on the class, which is defined by the ratio of the speed in the curve and the speed preceding the curve, or a more complicated formula. In


2012R36EN

Portugal, for example, the tightest curve falls in class D, where large chevron signs are recommended in addition to other measures such as profiled marking (see figure 80). An important feature that is not addressed in all guidelines is the contrast against the background. For instance red often has only a low level of contrast against green plantings. That is why Dutch guidelines recommend fluorescent background signs for chevrons in dangerous curves (Source: CROW, 2002c). Furthermore, placement of chevrons is never a substitute for an optical frame.

FIGURE 80 - CURVE SIGNING AS A FUNCTION OF CONSISTENCY CLASS (Source: Almeida Roque, 2009d)

Unobstructed view over the inside of the curveThe inside of the curve has to be unobstructed so as to offer drivers a complete overview of the curve. In figure 81, even the chevron signs along the outside of the curve are partly covered by the anti-glare protection on top of the barrier along the inside of the curve. The problem is well addressed in most guidelines.


2012R36EN

FIGURE 81 - BAD EXAMPLE – CHEVRON SIGNS PARTLY COVERED BY ANTI-GLARE PROTECTION, AN OUTER CURVE WITH OPTICAL GUIDING LINE CUT SHORT (PLANTINGS END AT THE MOST CRITICAL APEX OF THE OUTER CURVE) AND GAPS IN THE FRAME OF CHEVRONS WHICH SHOULD BE CLOSED

(Source: EGIS, E. Locquet)

FIGURE 82 - MARKING OF CARRIAGEWAY WIDTH REDUCTIONS FOR OVERTAKING LANES(Source: SETRA, 1994)


2012R36EN

FIGURE 83 - DEFLECTING ARROWS TO INDICATE A REDUCTION OF THE NUMBER OF LANES (Source: Almeida Roque, 2009a)

FIGURE 84 - OBLIQUE STRIPING TO INDICATE A REDUCTION OF THE NUMBER OF LANES (Source: CROW, 2005b )

FIGURE 85 - BLACK AND YELLOW BOLLARD TO NOTIFY DRIVERS OF ROAD NARROWING; INCLINATION OF THE STRIPES DEPENDS ON THE SIDE OF THE ROAD WHERE THEY ARE POSITIONED

(Source: CROW, 2005b., Almeida Roque, 2009d)


2012R36EN

FIGURE 86 - BLACK-WHITE MARKER TO NOTIFY DRIVERS OF AN OBSTACLE OR NARROWING (Source: CROW, 2005b)

FIGURE 87 - POSITION MARKER POST IN A CENTRAL ISLAND (Source: Almeida Roque, 2009d)

FIGURE 88 - MARKINGS AND OVERTAKING PROHIBITION FOR A ROAD NARROWING (Source: Almeida Roque, 2009 e)


2012R36EN

FIGURE 89 - MARKERS AND REFLECTORS TO NOTIFY DRIVERS OF THE ENDING OR NARROWING OF AN AUXILIARY LANE(Source: CROW, 2005b.)


The most important recommendation would be to completely address, in guidelines, the issues regarding lane-tracking that are mentioned in the introduction. Guidelines for safe road sides often advise safety barriers that are not parallel to the road’s axis, as in figure 87. There is a good technical reason for this, but it may deceive drivers who subconsciously use the safety barriers for guidance. This problem is not addressed in any guidelines.

3.2.3. Depth of Field of View

3.2.3.1 Problem

The depth of the Field of View is crucial for drivers to be able to detect and prepare for changes and potential hazards such as intersections and (tight) curves. On curved roads, the fixation pattern follows the road geometry, whereas on straight roads, search behaviour is less active, and most of the fixations are close to the focus of expansion (Shinar, 1977). In a condition of less active visual behaviour, problems may arise if drivers hold their eyes around a bright spot that for too long supports a perception that the road continues straight, for instance because of the lack of a frame along the outside of a curve or a minor road straight ahead while the main road


2012R36EN

bends to the left. In these cases, the view axis does not correspond to the road’s axis. Eye-catching objects may also support the driving task if they attract the eye to critical points such as changes in the course of the road. Other problems may arise from the way people estimate distances.

Means other than stereo vision (available because people have two eyes) are used beyond 8 m, e.g. knowledge about the normal sizes of objects, perspective relationships and characteristics of textures. As a result, drivers are susceptible to optical illusions.

Dangers like tight curves or intersections may go unnoticed for too long because of misperceptions that result from deceptive or distracting cues in the surroundings of the road. Important are:

• an unobstructed view of the course of the road to enable drivers to notice and judge changes and hazards in time. Of particular importance is the visibility of the inside of horizontal curves to enable optimal curve scanning and negotiation. Designers have to bear in mind that perceptually, the curve negotiation process precedes the curve by several seconds.

• Dominant objects, e.g. buildings, special facilities and billboards, or bright (uncovered) spots that attract the attention and provoke fixations too far and too long away from the course of the road hinder visual search processes. On the other hand, dominant task relevant information (e.g. a conspicuous roundabout) may support visual search processes.

• Several optical illusions may hamper tracking and speed behaviour, e.g. the impression of a bottle neck caused by converging line structures along the road (i.e. a ‘distance illusion’), wrong estimation of the radii of horizontal curves in a sag or on a crest, etc.

One aspect of the depth of the field of view is the visibility of oncoming vehicles for overtaking purposes. This can be related to the 6-seconds rule, but will be treated in this paragraph.

3.2.3.2 Concept

A high-quality field of view offers cues that prevent belated or wrong perceptions of the road’s course or the presence of dangers for all drivers under all lighting conditions.


The guidelines in most countries offer few best practices to optimize the depth of the field of view. Only some of the items mentioned in the introduction are addressed in


2012R36EN

current guidelines. However, an unobstructed view along the course of the road is clearly addressed by several nationally used guidelines. The guidelines do not mention the problem of:

• eye-catching objects that disturb lane-keeping and the detection of critical points; • optical illusions e.g. those caused by non-parallel plantings, buildings, traffic

facilities or other optical guiding lines.

Course of the road is clearly visibleThe visibility of the road’s course is mostly covered by tables with stopping, anticipation or decision sight distances. See for instance Table 3 of the Canadian Guideline for Collector Roads. It defines Decision Sight Distance as the distance the driver needs wherever information may be perceived incorrectly, decisions are required, or control actions are required. This is the case with:

• complex interchanges or intersections;• locations with unexpected manoeuvres;• locations with significant changes in the cross-section;• areas where there are multiple demands on drivers’ decision-making capabilities;• construction zones.

It is particularly important to avoid obstacles that obstruct the inside of a horizontal curve. Besides that, most guidelines recommend edge-of-track markings and reflector posts.

Drivers need both long-range and short-range guidance. Present pavement markings offer only short-range guidance. Painted white lines have been found to be good for short-range guidance but not for long-range (especially in wet road conditions), while post-mounted delineators have been found to be good for long-range guidance but not for short-range (Rumar & Marsh II, 1998). The Dutch Guideline for distributor roads recommends putting reflector posts along distributor roads every 50 metres, 1 metre away from the edge line. Reflector posts can alternate with hectometre posts. These posts can be put on safety barriers if the clearance between the safety barriers and the edge line is less than 2 m. Additional measures are recommended for (tight) curves depending on the ratio of the speed in the curve and the speed preceding the curve (CROW, 2005b), for instance a smaller distance between reflector posts and chevron signs.

Dominant eye-catching objects support lane tracking and detection of critical points The Dutch guideline for distributor roads states that elements in the field of view differ in terms of conspicuousness and it advises that a study of field of view could look at whether task-relevant information is sufficiently dominant compared to other elements in the road’s surroundings. There are a number of characteristics that make an element dominant within its context, e.g. size, colour, contrast, movement and


2012R36EN

(flashing) light. It depends on the driving task whether objects like billboards become a problem. Extra caution is needed near decision points like interchanges (CROW, 2002). The guideline does not provide any further bout methodology for a study of field-of-view quality.

FIGURE 90 - LANE SEPARATOR IN FRANCE(Source: left: SETRA-CETE, right: SETRA, 1998)

A roundabout is a powerful tool to reduce speed in transition zones such as town entrances. Figures 99 and 100 show the basic roundabout design in the Netherlands and Portugal. Several guidelines, such as those of the Netherlands and Portugal, describe ways of giving drivers timely notice and catching their attention:

• Cut the driver’s line of vision through the roundabout to ensure that the roundabout will be noticed. To this end, the inner circle of the roundabout can be raised and shrubs can be planted on it. See for example figures 101 and 102.

• Provide sight distances so that drivers approaching and entering the roundabout are able to see traffic on it, e.g. see figures 93 and 94.

• Add fences and direction signs as in the images below and in figure 103.• Have middle islands ahead of the roundabout as in figures 89 and 90.• Have brightly coloured road surface materials and/or curbing. Bright curbs are also

suitable for lane separators.• Have street lighting at and shortly before the roundabout to attract drivers’ attention

at night.• Post signs before and at the roundabout.


2012R36EN

FIGURE 91 - SIGN POSTING BEFORE AND AT THE ROUNDABOUT(Source: Paul Schepers)

Below are two more examples from the Dutch guidelines.

FIGURE 92 - SIGN POSTING BEFORE AND AT THE ROUNDABOUT IN THE NETHERLANDS(Source: Paul Schepers)

Many guidelines advise advance sign posting, but there are differences with respect to sign posting at roundabouts. In the Netherlands, a post in the middle of the centre island is mostly used for distributor roads when the centre island has a radius of less than 14 m, as for instance in the image on the right above. Due to the high contrast between the sign post and the sky, it attracts more attention than the inner circle of the roundabout with shrubs, signs and red-and-white warning singns. It is an eye-catching object that attracts the eye to a critical point.

Drivers will detect this roundabout further ahead of the crossing than they would without such sign posting, which decreases the crash risk (both multiple and single-vehicle). On the other hand, if a sign post is not built to break away, it may cause injury in the event of a crash. For larger roundabouts, the Dutch guideline advises signs on the lane separators ahead of the roundabout (CROW, 1998). These are easier to use for drivers leaving the roundabout. There are different opinions about the forgivingness of these two solutions. The latter option could increase the risk because there are four obstacles instead of one and speed has not been reduced. Additionally, the likelihood of hitting the pole in the middle of the inner circle is greater.


2012R36EN

Plantings along the road can also serve as eye-catching objects. Figure 93 below, from the French guideline, shows an image of a monotonous road where drivers might stared at the focus of expansion. This problem is reduced by adding trees to catch the eye near the change in the road’s gradient (Source: SETRA, 2006b).

FIGURE 94 - TREE ADDED AS EYE-CATCHING OBJECTS

FIGURE 93 - MONOTONOUS ROAD

The trees along the outside of the curve in the image below attract the attention to the horizontal curve. To serve as an optical frame, shrubs without gaps would have to be added at a lower level.

FIGURE 95 - TREES ALONG THE OUTSIDE OF THE CURVE ATTRACT ATTENTION TO THE HORIZONTAL CURVE

(Source: EGIS, E. Locquet)

In the images below, plantings are used, in addition to signage and markings, to attract drivers’ attention to the intersection (Source: SETRA, 1998).


2012R36EN

FIGURE 96 - USE OF PLANTINGS TO ATTRACT DRIVER’S ATTENTION TO THE INTERSECTION(Source: SETRA, 1998)

Optical illusions avoidedSome potential illusions, like the distance illusion, resulting from converging line structures along the road, are not covered in any of the guidelines that are nationally available. However, visual illusions that may result from the coordination of the horizontal and vertical alignment of the road are covered in many guidelines. The Australian guideline emphasises that alignment coordination is to be addressed in the earliest stages of any new road development proposal, and maintained throughout the planning and design process.

The main principle for obtaining good coordination between horizontal and vertical alignments is this: “Ideally vertical elements should be superimposed on horizontal ones in such a way that the intersection points practically coincide with the horizontal slightly in advance of the vertical, and the horizontal and vertical curves are of similar lengths.” The combination of superelevation and vertical profile may cause distortion in the outer pavement edges, which could confuse drivers at night, i.e. result in poor perception. Computer packages are available which enable a designer to identify such distortions in the outer pavement edges.

Figure 105 exemplifies good coordination of the horizontal and vertical alignment, while figures 106 to 109 exemplify poor coordination. Table 33 describes situations in which poor coordination of the alignments occurs, along with possible corrective action (Source: Australian guidelineQueensland Department of Main Roads, 2001). Although these principles are more related to the logic rule, they are referred to in this context in order to emphasise how important the coordination of horizontal and vertical alignment is for the field of view.

Overtaking opportunities and sight distancesOvertaking may be prohibited if Passing Sight Distances are not met. Since driving in columns for a long time may tire drivers, it is good to create overtaking opportunities. A safe design seen in Scandinavian countries and elsewhere is the 2+1 design. It has a physical lane separator (such as a guard rail) and each direction has an overtaking lane.


2012R36EN

According to guidelines in Portugal, Passing Sight Distance is used for rural two-lane roads to allow drivers to pass slower traffic by using the opposing lane. The driver of the passing vehicle must be able to see far enough ahead to complete the manoeuvre without interfering with traffic in the opposing lane. Passing Sight Distance should be based on the length needed to complete a passing manoeuvre as shown in figure 95 below, which divides the required sight distance into four parts:

d1 Initial manoeuvre distance. The initial manoeuvre period consists of a perception and reaction time and the time it takes for the passing driver to move the vehicle from a trailing position to a position of encroachment into the opposing lane of traffic.

d2 Distance travelled while the passing vehicle occupies the opposing lane. d3 Clearing length. The distance between the opposing vehicle and the passing

vehicle at the end of the passing vehicle’s manoeuvre. Observed distances vary from 30 m to 90 m.

d4 Distance travelled by the opposing vehicle after being seen by the passing vehicle. Since the opposing vehicle is assumed to be travelling at the same speed as the passing vehicle, this distance is equal to two-thirds of d2.

Minimum Passing Sight Distance equals the sum of d1 to d4. Table 34 shows the minimum Passing Sight Distances for various design speeds. Figure 112 shows Portuguese no-passing markings, details of which are derived from the passing sight distances.

FIGURE 97 - PASSING SIGHT DISTANCE IS USED (Source: AASHTO, 2004)

The Decision Sight Distances from the Geometric Design Guide for Canadian Roads are described on table 34. Additionally to this information see here the clearance in inside curves:


2012R36EN

FIGURE 98 - CLEARANCE IN THE INSIDE OF CURVES IN RELATION TO STOPPING SIGHT DISTANCE (top: curve length l greater than stopping sight distance S; bottom: stopping sight distance s greater than curve length L)

(Source: Transportation Association of Canada (TAC) 1999, updated 2007)

FIGURE 99 - APPLICATION OF REFLECTOR POSTS ALONG DISTRIBUTOR ROADS. THE REFLECTORS SHOULD BE HALF A METRE ABOVE GROUND LEVEL AND ONE METRE AWAY FROM THE EDGE LINE.

(Source: CROW, 2005b)


2012R36EN

FIGURE 100 - PRIORITY INTERSECTION WITH A TRAFFIC ISLAND IN THE MINOR ROAD AND NARROW TRAFFIC ISLAND (LANE SEPARATOR) IN THE MAIN ROAD

(Source: CROW, 2002a)

FIGURE 101 - BASIC DESIGN OF A ONE-LANE ROUNDABOUT IN THE NETHERLANDS (WITH A LANE SEPARATOR IN THE MAIN ROAD)



2012R36EN

FIGURE 102 - BASIC DESIGN OF A ONE-LANE ROUNDABOUT IN PORTUGAL, WITH ENTRANCE RADIUS TANGENT TO THE EXIT RADIUS

(Source: Bastos Silva, 2009)

FIGURE 103 - DESIGN OF A ONE-LANE ROUNDABOUT WITH A RAISED MIDDLE ISLAND IN THE NETHERLANDS


FIGURE 104 - DESIGN OF A ONE-LANE ROUNDABOUT WITH A RAISED MIDDLE ISLAND IN PORTUGAL (Source: Bastos Silva, 2009)


2012R36EN

FIGURE 105 - DIRECTION SIGNS (Source: Crow, 2005a)

FIGURE 106 - ADVANCE SIGN POSTING BEFORE A ROUNDABOUT (Source: CROW, 2005a)


2012R36EN

FIGURE 107 - IDEAL AND ACCEPTABLE CO-ORDINATION OF VERTICAL AND HORIZONTAL ALIGNMENTS (Source: Queensland Department of Main Road’s, 2001)


2012R36EN

FIGURE 108 - POOR CO-ORDINATION OF VERTICAL AND HORIZONTAL ALIGNMENTS (Source: Queensland Department of Main Road’s. 2001)


2012R36EN

FIGURE 109 - ROLLER COASTER GRADING RESULTING IN HIDDEN DIPS (Source: Queensland Department of Main Roads, 2001)


2012R36EN

FIGURE 110 - THE “OPTICAL SUMMIT” EFFECT (Source: Queensland Department of Main Roads, 2001)


2012R36EN

FIGURE 111 - POOR CO-ORDINATION OF VERTICAL AND HORIZONTAL ALIGNMENTS (Source: Queensland Department of Main Road’s, 2001)


2012R36EN

TABLE 33 - SITUATIONS IN WHICH POOR COORDINATION OF VERTICAL AND HORIZONTAL ALIGNMENT OCCURS

Problem Description Corrective actions

The vertical curve overlaps one end of the horizontal curve

If a vertical crest curve overlaps either the beginning or the end of a horizontal curve, drivers have little time to react to the horizontal curve once it comes into view. This is a particularly unsafe practice if there is a decrease in 85th percentile speed at the start of the horizontal curve. An example of this is shown in the upper diagram of figure 108.

If completely separating the curves is uneconomic, the curves must be adjusted so that they are coincident at both ends, if the horizontal curve is of short radius. If the horizontal curve is of longer radius, they need be coincident at only one end.

The vertical curve overlaps both ends of the horizontal curve

If a vertical crest curve overlaps both ends of a sharp horizontal curve, a hazard may be created because a vehicle has to undergo a sudden change of direction during passage of the vertical curve while sight distance is reduced. This creates the same problems as discussed above.

The corrective action is to make the curves coincident at one end so as to bring the crest on to the horizontal curve.

The vertical curve overlaps both ends of the horizontal curve

If there is insufficient separation between the ends of the horizontal and vertical curves, a false reverse curve may appear on the outside edge-line at the beginning of the horizontal curve, or on the inside edge-line at the end of the horizontal curve. This is a visual defect and is illustrated in the middle diagram of figure 108.

Corrective action consists of increasing the separation between the curves.

Dissimilar length horizontal and vertical geometric elements

A short movement in one plane should not be placed within a large movement in the other. A particular instance where this can lead to safety problems is when a small depression in the vertical alignment results in a ‘hidden dip’. An example of a hidden dip is shown in of figure 111. Large depressions such as those shown in the lower diagram of figure 105 are not deemed to be hidden dips.

Corrective action consists of making both ends of horizontal and vertical curves coincident, thus producing similar length curves. An alternative treatment is to completely separate the curves.

Long Flat Grades

Long straights with flat grades make it difficult for drivers to judge the distance and speed of approaching vehicles leading to overtaking accidents. An approaching vehicle more than 2,500m away on a straight seems to be standing still but the same situation on a large curve provides the driver with a changing perspective allowing some judgement of speed and distance. This situation is exacerbated at night.


2012R36EN

TABLE 33 - SITUATIONS IN WHICH POOR COORDINATION OF VERTICAL AND HORIZONTAL ALIGNMENT OCCURS

Problem Description Corrective actions

Roller Coaster Grading

Long straight sections are prone to “roller coaster” grading (see figure 109) with the added potential for hidden dips. Designers should take care that these features are not incorporated into the design by using appropriate curvature in both planes and checking lines of sight for hidden dips.

Optical Summit

Figure 110 illustrates a phenomenon known as “Optical Summit” brought about by the combination of vertical and horizontal alignments. The horizontal curve does not overlap the two vertical curves and the vertical curves are too small for the large movements in the other plane. The appearance has been improved considerably by the adjustments to both alignments as shown in the figure 110. The horizontal curve has been lengthened to encompass both of the sag vertical curves and the sag curves have been lengthened to be more in keeping with the scale of the other movements.

This example also illustrates the benefit of using perspective views during the design process to highlight potential problems while they can still be corrected at minimal cost.

Source: Queensland Department of Main Roads, 2001

TABLE 34 - MINIMUM PASSING SIGHT DISTANCE (PORTUGAL)

Operating Speed (km/h)Minimum Passing Sight Distance 0.7 PSD (m)

(rounded)Marking distance 0.4 PSD (m)

(rounded)

40 195 11050 245 14060 295 17070 340 19580 390 22590 440 250100 490 280110 540 310120 590 335


2012R36EN

FIGURE 112 - PASSING SIGHT AND MARKING DISTANCES IN PORTUGAL(Source: Carlos de Almeida Roque)


The most important recommendation would be that guidelines should fully address the issues mentioned in the introduction regarding depth of the field of view. None of the guidelines address the problem of a bright spot suggesting that the road continues differently from the way it actually does for some considerable distance, for example due to an opening in the plantings along the outside of a curve (i.e. where the view axis diverges from the road’s axis).

3.3. LOGIC RULE

3.3.1. Change of road function without corresponding change in design and optical characteristics (e.g. Town Entrance)

3.3.1.1 Problem

Drivers need to adapt their driving behaviour when entering a built-up urban area. They need to decrease speed and be more attentive because more decisions and reactions are required within built-up areas. Generally, there should be unambiguous visual cues for change of function, for instance a horizontal swing in the road’s course, optical sight barriers, planted central islands, special speed-reducing markings or a combination of these.

3.3.1.2 Concept

Drivers need visual cues to recognize the change of function.


2012R36EN


The Dutch guideline ‘Handboek Wegontwerp Gebiedsontsluitingswegen buiten de bebouwde kom’ describes a best practice to induce the required driving behaviour (CROW, 2002a).

Location of the town entranceA town entrance should be located near the boundary between a rural and a built-up area where the built up area is recognizably larger compared to a rural area. The distance between buildings and the road axis should be three times the height of the adjacent buildings at most. If these requirements are met, than the exact location can be chosen taking the following characteristics of the road or its surroundings into account:

• A location where the landscape changes considerably is preferred.• Road or surrounding characteristics can support the image of the town entrance. • The town entrance needs to be visible at a sufficient distance so that the driver can

anticipate and slow down.• Short-term developments such as new buildings have to be taken into account.

Source: Handboek Wegontwerp Gebiedsontsluitingswegen, p. 202 (CROW, 2002a)

DesignThe design of the town entrance depends on the road categories (speed limits, number of lanes) preceding and following the town entrance. Combining the town entrance with a junction, preferably a roundabout or crossing with traffic lights, supports the transition. Drivers should drive through the town entrance before entering the intersection or roundabout. The sign indicating an entrance to a built-up area (or the entrance to a speed zone) is placed at both sides of the road if the width of the road is over 5 m (see figure 111).

FIGURE 113 - SIGNS FOR TOWN ENTRANCE AND TOWN EXIT IN THE NETHERLANDS (Source: Crow 2005a)


2012R36EN

Normally a distributor road outside a built-up area with a speed limit of 80 km/h is connected with a distributor road within a built-up area with a speed limit of 50 km/h. A transition zone of about 200 m with a speed limit of 50 km/h is needed if a distributor with a speed limit of 80 km/h is to be connected with an access road with a speed limit of 30 km/h. Such a zone is not needed if a roundabout is chosen to support the transition (CROW, 2002a p. 202 and 203).

Another good practice for town entrances is the chicane. Chicanes are primarily designed to reduce the speed of drivers to a maximum of 50km/h in urban contexts and they can also introduce a change in carriageway width at the entrance of a town or in an urban context. A chicane can include a central island. A typical layout in French guidelines is sketched in figure 113.

The Dutch Design guideline provides three examples for the design of a town entrance. These are presented in figure 112. Figure 113 exemplifies a chicane that is used to introduce a town entrance in France. Figure 114 shows the marking that can be used as a speed-reducing measure. Figure 115 exemplifies a town entrance in the Netherlands (CROW, 2002a).

FIGURE 114 - EXAMPLES OF TOWN ENTRANCE WITH SPEED-REDUCING MEASURES (Source, CROW, 2002a)


2012R36EN

FIGURE 115 - THE DESIGN OF A CHICANE (Source: CERTU, 2006)

FIGURE 116 - MARKING TO REDUCE THE SPEED OF VEHICLES APPROACHING A TOWN ENTRANCE (Source: CROW, 2005b)


2012R36EN

FIGURE 117 - A TOWN ENTRANCE COMBINED WITH A MIDDLE ISLAND AND MARKING TO WARN THAT SPEED SHOULD BE DECREASED FROM 80 KM/H TO 50 KM/H

(Source: Paul Schepers)


A change of road function for town entrances is well addressed in some guidelines, like the Dutch guideline for rural distributor roads. Further improvement for changes in function may be possible through coordination between alignments and the holistic overall impression of spatial perception (CROW, 2002a).

3.3.2. Effect of pre-programmed habits and routines

3.3.2.1 Problem

Visual perception is not a linear, mechanistic process. The ability of drivers to react to a hazardous object or event depends on their experience and on previous conditions along the road. For example, drivers travelling on a roadway with few access points and wide curves may be unprepared to stop for a slow-moving vehicle ahead. The time they need to react safely will substantially exceed the time they would have needed in a busy urban traffic environment (the amount of needed time will be even higher if there is a stopping prohibition on that road, as on a motorway). The ability to judge and predict traffic depends on long-term a priori expectations and short-term ad hoc expectations. A priori expectations are built up during months and years of experience (what in general to expect on a motorway for instance), while ad hoc expectations depend on the logic that drivers derive from previous impressions (for instance, expectations about the presence of curves based on the average bendiness on previous stretches of road, Rainer, et. al., 1999). Eye-catching objects far ahead on the road support the adoption of ad hoc expectations.


2012R36EN

3.3.2.2 Concept

Drivers need a road environment that is in accordance with their expectations derived from previous impressions. Conditions that represent a fundamental change from the logic of the preceding stretches of road need to be introduced in time and with enough cues, e.g. eye-catching objects.


Guidelines in several countries provide best practices regarding pre-programmed habits and routines.

Best practice at the strategic level: functional classification of roadsDrivers are supported by providing them with road categories having a consistent design and composition of traffic so that they know what to expect. According to the Sustainable Safety Vision, the Netherlands has a hierarchical road categorization system consisting of three functional categories: main roads or ‘through roads’ to help traffic flow as smoothly as possible, ‘access roads’ to provide access to destinations, and ‘distributor roads’ connecting the first two categories to create a proper transition between ‘providing access’ and ‘ facilitating traffic flow’. One of the most important goals is to have priority marked by signs or traffic lights at intersections of distributor roads. Drivers on distributor roads always have priority over traffic on minor roads (CROW, 2002). This supports a priori expectations.

• Types of intersections

This document is focused on distributor roads outside built-up areas. There are two types of distributor road outside built-up areas in the Netherlands, depending on the number of lanes and carriageways: 2 x 2 lanes (type I) and 2 x 1 lanes (type II). Type I connects to access roads with roundabouts or intersections with traffic lights (possibly with traffic calming measures). Type II crosses access roads with roundabouts or priority intersections. Because of potential conflicts at crossings, large distances between intersections are advised along distributor roads. The minimum distance is 300 m if one sign post is placed (200 m ahead of an intersection) and 500 m if two sign posts are placed ahead of the intersection. Distributor roads are connected to ‘through roads’ (motorways) by interchanges.

Access to private properties should be avoided along distributor roads. It is recommended that access roads if needed run parallel to a distributor road (CROW, 2002a).


2012R36EN

• Transition zones

Normally a distributor road outside a built-up area with a speed limit of 80 km/h is connected to a distributor road within a built-up area with a speed limit of 50 km/h. A transition zone of about 200m with a speed limit of 50 km/h is needed if a distributor with a speed limit of 80 km/h is to be connected to an access road with a speed limit of 30 km/h. Such a zone is not needed if a roundabout is used to support the transition (CROW, 2002a).

• Markings

Standard markings for road categories support a recognizable design and a predictable alignment so that road users know which behaviour is expected from them and what they can expect from other users (CROW, 2004). Road authorities in the Netherlands have agreed upon standard markings as shown in figure 119.

Road alignment conforms to drivers’ expectationsMany national guidelines demand continuity for the layout of roads. For example, the German guideline for line management contains design principles for continuous spatial line management; see figure 121 (Forschungsgesellschaft für das Straßen- und Verkehrswesen e. V.). It explains that two curves may have the same small radius in two different alignments. The curve may be expected on a winding road and unexpected on a road with long straight sections. It is important to be able to see over longer distances preceding curves and other critical points.

Most national guidelines include requirements for the maximum length of straight sections (see for instance table 35) and the radii of subsequent curves (see for instance figure 122). Two curves in the same direction need particular attention because the second curve can be difficult to detect in time. According to the Dutch guideline for distributor roads, the horizontal alignment preferably consists of horizontal curves (with large radii) about 500 m long. Lengthy straight sections should be avoided. A curvilinear alignment is preferable because it reduces monotony, the view ahead is constantly changing, and steering to the focus of expansion is avoided. It supports judgments about the distance to a vehicle that is approaching or slowing down because of a queue. For overtaking manoeuvres, this applies to left-turning curves. An approaching vehicle more than 2500 m away on a straight section seems to be standing still. Also, night driving is more comfortable in that the approaching headlight glare is significantly reduced (Queensland Department of Main Road’s, 2001).

Drivers have to negotiate horizontal curves. To be sure that they can detect and estimate horizontal curves in timely fashion, the horizontal and vertical alignments should be coordinated. Unfavourable combinations of horizontal and vertical curves can result in apparent discontinuities in the alignment, even though the horizontal and


2012R36EN

vertical designs each comply separately with the provisions for horizontal and vertical curves. Vertical elements should be superimposed on horizontal ones in such a way that the intersection points practically coincide with the horizontal slightly in advance of the vertical, and the horizontal and vertical curves are of similar length. Poor driver perception may result from the combination of superelevation and vertical profile. Computer packages are available which enable a designer to view the roadway from the driver’s perspective. Figure 105 shows examples of ideal and acceptable coordination of horizontal and vertical curves. Figures 106 and 107 show poor examples. The Australian Road Planning and Design Manual describes several specific problems resulting from poor coordination between the horizontal and vertical alignment (see table 33) (Queensland Department of Main Roads).

Drivers will adopt a speed related to the horizontal alignment and the terrain. If the horizontal alignment can be designed to match the terrain, the road will be consistent with driver expectations, aesthetically pleasing and economical. Figure 123 shows two alignments through the same hill, one generally following the contours and producing a more pleasing result than the other. Figures 105 to 107 exemplify coordination between the horizontal and vertical alignments as described in an Australian guideline.

Changes are introduced in timely fashion The need for a different behaviour arising from an intersection or tight curve should be introduced in timely fashion. Advance warnings and eye-catching objects are useful in supporting the transition.

• Approach to intersections

Advance signposting that is provided ahead of an intersection allows motorists to anticipate a change in traffic conditions. The Dutch guideline recommends advance sign posting be placed 200 m before, and if necessary also 400 m and 600 m before an intersection (if the number of directions to be shown exceeds six) (CROW, 2002); see figure 125. Painting a warning triangle on a minor road helps warn drivers who approach an intersection with a priority road (see figure 126), in addition to warning signs (see figure 124). Street lighting can be used to enhance discontinuities such as intersections, roundabouts, level crossings, etc.

• Approach to speed zones

Speed-reducing measures such as middle islands where lanes bend outwards are useful in supporting a transition, for instance at the entrance to a speed zone (i.e. a zone where a lowered speed is desirable). Two examples of speed-reducing measures are shown below. Bright curbs or fences can be added to increase visibility.


2012R36EN

FIGURE 118 - SPEED-REDUCING MEASURES(Source: SETRA-CETE)

Vertical elements such as plantings are suitable to improve the visibility of middle islands. This is exemplified by the simulation image below from France. Note that the road on the right is also less monotonous than the road on the left. Other similar examples with middle islands are included in section 3.1. about town entrances.

Most important is to design a continuous curvilinear alignment. Standard sight distances may be insufficient to alert drivers on time if this requirement is not met for a specific curve. Guidelines in most countries recommend additional warnings. Research shows that curve warnings which contain perceptual components or emphasise the physical features of the curve work best, such as chevrons with good contrast (Charlton, 2004). Through consistent application of such measures, explicit information in warning areas is provided. Drivers can consider their meaning if they have enough experience and explicitly attend to them, for instance a Chevron sign with two arrows means that the curve will be tight but not extremely dangerous.

The Dutch guideline for markings advises that dangerous horizontal curves should be treated with advance warning signs and beaconing. The measures to be taken depend on the K-ratio: (design) speed in the curve / speed along the road section preceding the curve. For curves with the highest K-ratio, it is recommended to add yellow, fluorescent background shields to the chevron warning signs and to use these signs for the whole curve instead of only at the start. More detailed information derived from the Dutch Guideline is shown in figures 127 - 129. Figure 130 is a good example that is derived from a Portuguese guideline.


2012R36EN

A warning sign and recommended speed are needed for sharp curves, according to the Dutch Traffic Code. CROW (2010) describes the recommended advisory speed depending on the design speed in the curve and the speed limit (see table 37). It also depends on the superelevation and radius. A warning sign should only be used if the speed limit differs by more than 20 km/h from the recommended speed. According to the Dutch Traffic Code, advance warning signs should be placed where they serve traffic safety best. CROW (2010) advises 100 to 200 m for roads with a speed limit of 80 km/h.

Figure 119 presents the standard for marking of road categories in the Netherlands (for single carriageway roads)2, while figure 120 shows a road with an 80 km/h speed limit. Figure 121 contains examples of consistent and inconsistent horizontal curves derived from the German standard RAS-L (FGSV, 1995). Figure 122 includes requirements for radii of subsequent curves, derived from a Dutch Guideline. Figures 127 to 130 show examples of advance sign posting and advance warning signs. Figure 131 describes the relationship between bendiness and speed.

FIGURE 119 - STANDARD MARKINGS FOR ROAD CATEGORIES AND SPEED LIMITS FOR SINGLE CARRIAGEWAY ROADS


2 On 80 km/h and 100 km/h roads, a dual carriageway road is preferred from a safety point of view, i.e. to avoid head-on collisions.


2012R36EN

FIGURE 120 - A DISTRIBUTOR ROAD WITH AN 80 KM/H SPEED LIMIT IN THE NETHERLANDS(Source: Paul Schepers)

FIGURE 121 - CONSISTENT AND INCONSISTENT HORIZONTAL CURVES IN A ROAD (Source: Forschungsgesellschaft für das Straßen- und Verkehrswesen e. V. ,1995)


2012R36EN

FIGURE 122 - ALLOWABLE VALUES FOR SUBSEQUENT CURVE RADII (Source: CROW, 2002a)


2012R36EN

FIGURE 123 - EXAMPLES OF ROAD-LANDSCAPE COMPROMISE (Source: Queensland Department of Main Road’s, 2001)

FIGURE 124 - ADVANCE WARNING SIGNS (Source: CROW, 2005b)


2012R36EN

FIGURE 125 - EXAMPLES OF ADVANCE SIGN POSTS IN THE NETHERLANDS(Source: CROW 2005a)

FIGURE 126 - ADVANCE WARNING TRIANGLE FOR DISTRIBUTOR ROADS (LEFT), ACCESS ROADS (MIDDLE) AND BICYCLE PATHS (RIGHT)



2012R36EN

FIGURE 127 - K-VALUES FOR CATEGORIZING HORIZONTAL CURVES IN ORDER TO DECIDE ON ADDITIONAL SIGNING; TREATMENTS FOR EACH CATEGORY


FIGURE 128 - ADDITIONAL SIGNING AND BEACONING FOR EACH CURVE CATEGORY (Source: CROW, 2005b)


2012R36EN

FIGURE 129 - PLACEMENT OF CURVE WARNING SIGNS LIKE CHEVRON SIGNS ON THE ROAD SHOULDER (Source: CROW, 2005b)

FIGURE 130 - LOCATION AND SPACING OF DELINEATORS AND CHEVRONS IN A PORTUGUESE GUIDELINE (Source: Almeida Roque, 2009d)


2012R36EN

FIGURE 131 - RELATIONSHIP BETWEEN BENDINESS (BH) OF THE ROAD AND SPEED (V85) FOR A ROAD WIDTH BETWEEN 6.5 AND 7.5 M

(Source: CROW (2002a))

TABLE 35 - ADVISORY SPEEDS1

SuperelevationRadius -2,5% -2% -1% 0% 1% 2% 2,5% 3% 4% 5%

10 16 16 16 17 17 17 18 18 18 1850 33 34 35 35 36 37 37 38 38 38100 45 46 47 48 49 50 51 52 53 54150 54 54 56 57 59 60 61 61 63 64200 60 61 63 65 66 68 68 69 71 72250 66 67 69 71 72 74 75 76 77 79300 71 72 74 76 78 80 81 82 83 85350 75 76 78 80 83 85 86 87 89 90400 79 80 82 85 87 89 90 91 93 95450 82 84 86 88 91 93 94 95 97 100500 85 87 89 92 94 97 98 99550 88 90 92 95 97 100600 91 92 95 98650 93 95 98700 96 97750 98 99800 100(Source: CROW, 2010) 1 The speeds should be rounded up to the nearest 10 km/h


2012R36EN


Most guidelines advise against the use of long straight sections and recommend a curvilinear alignment consisting of large radius horizontal curves instead; an example would be the Australian guideline (Queensland Department of Main Roads). Although this is a considerable improvement over a design with long straight sections, the design speed may still considerably exceed the speed limit. Increasing the radius of horizontal curves, and giving consideration to the consistency of the geometric alignment, can foster a credible speed limit. Figure 131 describes the relationship between the average bendiness of a road, where Bh is the sum of the absolute changes in angle per unit length in gon/km and L is the length of the road. Designers could strive for a certain level of bendiness to achieve a credible speed limit.

Several guidelines for curve warning signs, such as the Dutch guideline (CROW, 2005), differentiate between a number of curve categories. These categories, with varying geometric consistency, are often based on the unimpeded speed distribution, and on their variation from one road section to another. There are more advanced methods that use the length of the straight section, the average bendiness and the increase in the expected number of accidents to calculate the inconsistency class of horizontal curves (see Cardoso, 2001).

3.3.3. Multiple critical points occur concurrently

3.3.3.1 Problem

A driver’s attention and ability to process information is limited. Driving requires the performance of multiple tasks, such as control, guidance and navigation. Drivers can focus on one piece of information at a time and multiple distractions or critical points which can include signage, intersections, sharp curves, etc. Increase the driver’s workload and distraction and have a potential to decrease the driver’s performance. Too many decision points in a short time frame can overload information processing capacity and result in safety risk.

3.3.3.2 Concept

Ensuring that the road is designed in a consistent manner, so that it meets the driver’s expectations, has adequate Decision Sight Distances and avoids or minimizes multiple critical points, will provide increased driver comfort and facilitate the safe vehicle operation.


2012R36EN


In order to avoid multiple, inconsistent and surprising critical points that in turn can greatly increase driver workload, adequate road design and appropriate signage are needed. The safety and efficiency of a road network depends largely on the quality of operation and design of the intersections. The following human factors must be considered: usual behaviour and decision capabilities of drivers, surprise effect or driver’s expectations, perception and reaction times, natural travel paths and visual appearance of intersections from the driver’s and pedestrian’s perspective. Characteristics of existing intersections should be taken into consideration so that consistency may be ensured along a road (see figure 3 and the principles of the 6-Seconds Rule – Perception and Visibility).

Multiple critical points avoidedAvoidance of multiple critical points will prevent driver overload and increase the safety and efficiency of the road network. In order to prevent multiple at-grade intersections, it is recommended to design large residential areas with access roads that run parallel to a main road. The minimum distance between at-grade intersections for 90 km/h rural roads is 3 km. For 80-90 km/h suburban roads, the minimum distance is 1 km. For interchanges, the minimum distance is 4 km for rural dual carriageway roads and 1 km for suburban dual carriageway roads. Private access to main roads should not be allowed. Service roads, built parallel to the main roads, can collect all private accesses and provide a single connection to the main road at a selected intersection.

A well designed horizontal and vertical alignment will also reduce the frequency of unexpected sharp horizontal curves. However, right-of-way or economic restrictions sometimes make this difficult. A number of rules need to be respected for proper coordination of horizontal and vertical alignment:

• Alignment should be integrated into the environment; • The overlap of curves in horizontal and vertical alignments normally improves the

visual aspect of the road. However, the consequences of this overlap should address the safety and operation of traffic;

• Short vertical curves should not coincide with horizontal curves. Whenever possible, vertical curves should be long and the bisectors of the horizontal and vertical curves should match each other;

• Since short curves seen from a distance seem to have a very small length, the radius should be as large as possible in order to avoid the appearance of a break;

• Give preference to curves of great length over than a long straight section with curves of small length;

• Opportunities should be provided for safe overtaking on two-lane roads. This can often limit the selection of the best horizontal and vertical alignment combinations.


2012R36EN

All critical points visibleSee the description “Critical point visible and clearly identifiable” under the 6 Seconds Rule - Perception and Visibility.

Driver progressively informed of multiple critical pointsWhen providing information to the driver regarding upcoming critical points, it is vital to place the most important information first, to help drivers reduce their workload by reducing unnecessary cues or unrelated tasks.

Drivers can only process a limited amount of information and they often rely on past experience to manage the new information they must process while driving. Drivers can process information best when it is presented in accordance with expectations, sequentially, to maintain a consistent level of demand and in a way that helps drivers prioritise the most essential information. When drivers are not provided with or do not accept information in a timely fashion, when they are overloaded with information, or when their expectations are not met, slowed responses and mistakes may occur.

Providing a driving environment that is consistent with drivers’ expectations is critical and can be achieved by drawing on the concept of a “self-explaining road”. See the section on the 6 Seconds Rule - Perception and Visibility.

To be effective, all traffic control devices must meet five basic requirements. These are to:

a. fulfil a need; b. command attention; c. convey a clear and simple meaning; d. provide adequate time for a proper response; e. command the respect of road users.

A traffic sign should be placed where it supports traffic safety best. Stopping Sight Distance (SSD) should be used when there is no need to provide a longer sight distance (DSD or PSD). However, while signage is critical to communicating important information to the driver, it can also be distracting if it is not well thought out with respect to spacing, legibility and visibility. Too much signage can distract the driver. See the description in “Critical point visible and clearly identifiable” under the 6 Seconds Rule - Perception and Visibility for additional considerations regarding signage.

CROW 2010 explains the optimal location of traffic signs in the following way, see figure 132. The optimum place S1 (distance in advance of the action point) is calculated as S3-S2 (see figure 132). S3 is the distance travelled during the process of recognizing the sign, deciding and adjusting (braking). There is a table for S3


2012R36EN

(rows: speed limits; columns: needed speed reduction); for instance, S3 is 330 m for a driver who has to stop at a road with a speed limit of 120 km/h (240 m for 100 km/h; 160 m for 80 km/h). S2 is the distance at which the traffic signs can be recognized. For instance, without sight restrictions, it is about 100 m for an advance warning sign for a driver approaching a priority intersection. Along distributor roads, advance sign posting is placed 200 m before and if necessary also 400 m and 600 m before the junction. In some situations, a warning triangle is painted on the road to warn drivers who are approaching a priority road. Advance warning signs for level crossings and movable bridges are combined with flashing lights depending on traffic intensity and the length of traffic jams.

Traffic signs should be placed where they are most beneficial to road safety. The road authority has to determine the location in relation to the action point. A distance of 100 to 200 m is given as an indication for 80 km/h roads. In general, a sign has to be placed ahead of the action point at a distance S1, so that drivers are able to comply with the information in time or rule on the sign. This distance (S1) is the difference between the distance needed to recognize the sign (S2) and the distance that is travelled during the adaptation and decision-making process (S3). The distance needed for recognition should be determined assuming poor visibility conditions. The adaptation is strongly dependent on the required speed reduction.

recognition

adaptation

S2

S3

S1

decision making

FIGURE 132 - DISTANCE NEEDED FOR RECOGNITION(Source: CROW 2010)

In the past, experiments have been conducted to determine the recognition distance for traffic signs. A retroreflective right-of-way sign (a sign that meets the standards) as in the image above will be recognized at a distance of about 100 m.

Information must be presented to the driver in a consistent manner in order to maintain an appropriate workload. The information needs to be presented in a proper sequence, ensuring a proper order of priority where safety-related information comes first. Clear sight lines and adequate sight distances must be provided to allow time for decision-making. Because Decision Sight Distance allows drivers to manoeuvre


2012R36EN

their vehicles or vary their operating speed rather than stop, Decision Sight Distance is greater than Stopping Sight Distance and is dependent on design speed. Designers should use Decision Sight Distance to address issues of critical points whenever information may be perceived incorrectly and decisions regarding alternate driving manoeuvres need to be made.

TABLE 36 - DSD FOR SEVERAL DRIVING MANOEUVRES Design Speed

(km/h)Decision Sight Distance for Avoidance Manoeuvre (m)

A B C D E

50 75 160 145 160 20060 95 205 175 205 23570 125 250 200 240 27580 155 300 230 275 31590 185 360 275 320 360100 225 415 315 365 405110 265 455 335 390 435

120+ 305 505 375 415 470Avoidance Manoeuvre A: stop on rural roadwayAvoidance Manoeuvre B: stop on urban roadwayAvoidance Manoeuvre C: speed/path/direction change on rural roadwayAvoidance Manoeuvre D: speed/path/direction change on suburban roadwayAvoidance Manoeuvre E: speed/path/direction change on urban roadway(Source: Transportation Association of Canada (TAC), 1999, Table 1.2.5.6)

When using these sight distances, the designer should consider eye and object height appropriate for specific applications.


Most of the design standards address the issue of intersection spacing and adequate placement of signage in order to avoid or compensate for multiple critical points.

3.3.4. Deficiencies in traffic control devices

3.3.4.1 Problem

Because of higher traffic volumes, roads have to be equipped with traffic control devices for safety. Along with traffic rules, traffic control devices organise the behaviour of road users. Traffic signs should never be covered by plantings or other structures and they should be visible, legible and detectable under all lighting conditions and against all optical backgrounds.


2012R36EN

That is because the effect of mimesis may sometimes make even bright, oversized signs invisible to the driver. Signs should be in accordance with drivers’ expectations, and the road alignment should be consistent and in accordance with the traffic control devices. Otherwise drivers may be misled or confused, and accidents may result.

All these requirements seem to be obvious, but many discrepancies are seen during safety inspections. Discrepancies can lead to surprised drivers, incorrect information, late information, or no information, regulation or warning at all.

3.3.4.2 Concept

Providing adequate demand, regulation, warning and information at all points on the road in time to react without critical change of driving routine is the basic condition for safe traffic flow with higher density and speed. In absence of traffic control devices in the correct location and with correct, advance information, the capacity of the road system will collapse. Drivers should slow down to a speed where they are able to react to sudden changes in all possible road parameters.

By the Vienna Convention, traffic signs are uniform as much as possible (“Convention on Road Signs and Signals of 1968, European Agreement Supplementing the Convention and Protocol on Road Markings, Additional to the European Agreement -2006 consolidated versions”, also known as the “Vienna Convention”) with a preference for pictograms and a uniform shape of shield, but there are still differences between European, Irish, American and Japanese styles. Other countries have more or less adopted some of these dominant styles. The differences appear in the shape of “Danger” warning signs. These are considered in the Vienna Convention in the following terms:

In the figure below there is an example from the 1995 IRF survey on vertical signs that was conducted in 12 European countries.

FIGURE 133 - VERTICAL SIGN SURVEY (Source: Version 1.00, 1995/07/20, IRF)


2012R36EN

Traffic control devices have to be visible against backgroundIn most countries there is a manual stating where and at what distance (depending on speed and type of control device) the devices have to be posted. The position is fixed in plan and the devices are erected accordingly. But there is potential for serious mistakes:

• Device is hidden behind other poles or trees;• Device is hidden behind a bridge etc.;• Device is covered by leaves and branches of trees and bushes, with poor maintenance;• Device is not clearly visible among a group of other signs or advertisements;• Device is not oriented to the user’s direction;• Device does not contrast with the background – buildings, trees, sky, sun.

The problem of the visibility of the sign against the background, in the context of the mimesis effect, is not directly addressed. The basic advice is to use retro-reflective material and yellow frames to create visibility against the background.

FIGURE 134 - INVISIBLE RED-WHITE CHEVRONS (LEFT) AND IMPROVED SITUATION WITH YELLOW FRAMES (RIGHT)

(Source: Birth, 2004b)

If there are signs, the following should be checked:

• location (carefully checked before installation);• visibility from the direction of the user, especially when the sun is low;• use of retro-reflective material and yellow frames to create visibility against the

background;• use of yellow/orange framing and of more visible signs through reflective sheeting

and full cube technology; • use of anti-glare shield;• use of anti-phantom grid;• whether there is regular control, maintenance and clearing, and an exchange of the

sign shield (retro-reflective).


2012R36EN

Traffic control device has to be appropriate for the road characteristicsDrivers need correct and proper information, restrictions, warnings etc. The most important items for safety are priority signs and red lights. These should be very carefully and systematically installed, controlled and maintained, because otherwise there could be an increase in accidents.

Second in importance are prohibitory or restrictive signs (regulatory). A lack of such signs could lead to head-on or side collisions. Third in importance are danger warning signs. A lack of such signs will lead to speeding, loss of control and crashes. Other categories of signs are not closely connected to an imminent danger of accident.

The lack of or wrong location of advance direction signs and directional signs (and of lane selection signs - lateral or overhead signs, not covered by the Vienna Convention but thoroughly used in most countries in two carriageway roads) might give rise to unnecessary lane-changing manoeuvres in conflict with traffic in other lanes, due to the indecision or even perplexity of the driver as a result of a lack of information, continuity in the information or non-correspondence between sign and infrastructure.

Examples of lane selection signs are given in figures 135 and 136.

United Kingdom: Portugal:

FIGURE 135 - LANE SELECTION SIGNS IN UNITED KINGDOM

FIGURE 136 - LANE SELECTION SIGNS IN PORTUGAL

All sudden and unexpected changes in road characteristics need to be signed in advance, in particular:• the first horizontal or vertical curve of small radius after a long straight alignment;


2012R36EN

• any narrowing of the road;• low underpasses;• pedestrian crossings;• the entrance to a village.

Traffic control devices have to be in accordance with driver’s expectation.

All traffic control devices have to be located at the standard position on the right-hand side (the left-hand side in countries where driving is on the left), at the correct height and distance from the pavement, and in specific cases they must be repeated on the gantry or on the left- hand side. All traffic control devices should be in accordance with international convention, with the same rules and colours and uniform within the country’s borders. It is best to use pictograms instead of text so that foreigners will understand. Advance directional signs enable drivers to take the necessary precautions for exiting, including switching lanes, double-checking whether they have the right exit, and slowing down. Signs are needed on expressways and motorways, where the first approach sign for a motorway exit should be at least 1 000 m from the actual interchange, followed by one or two additional advance directional signs before the actual exit point.

The commonest mistakes are:

• An important sign is missing (not installed, damaged or stolen);• There are no danger warning signs on the road;• Informative signs are not systematic, give no information about the continuation of

the road, typical for offset intersection.• Low reliability of information/warnings on variable message signs due to lengthy

process of information input – processing – transfer – display.

Signs and letters have to be unambiguous and readableAll traffic control devices should be of the standardized dimensions approved for three different purposes and speeds. For urban roads with a speed of 100 km/h, the “normal” size is used. For expressways/motorways, there are larger signs, especially for priority, regulation and direction. They are installed at exit/entry ramps and often in an overhead position.

Smaller signs are used for low speeds in urban areas, for cyclists, to indicate no-stopping, no-parking and tow-away areas.

Traffic lights also usually have three standard sizes:

• Normal – for urban and interurban use;• Large – very often red light on the normal size signal head, used in different

countries by some producers – red light is more visible;


2012R36EN

• Small – repeating signal in front of first driver when signals are elevated on gantries or cantilevers, or for cyclists.

Letters are also standardized in height, width, shape, density and different dimensions depending on the size of the sign. Letters should be placed on the sign in such a way as to make it clear which information pertains to straight ahead and which to turns. There are two problematic options:

• Standardized letters on a standardized panel with short text, where letters should be larger to use the space available (compare “BRNO” with “MORAVSKE BUDEJOVICE”)

• Because there is so much text, signs cannot be read.

The specific area where there are variable message signs, where letters and pictograms are limited by technology, often means that images are simplified or in negative colours, and can sometimes be too bright. The technology is developing very quickly though there are some mechanical problems with prismatic VMS technology, in that some prisms are not in the correct position, making the signs difficult to read.

This technology is hardly used any longer (see Easyway’s vms-2 esg4_vms ver 2010) but it is still used on directional signs where parts of the message and arrows are variable.

FIGURE 137 - MESSAGE SIGNS LIMITED BY TECHNOLOGY


2012R36EN

FIGURE 138 - PICTOGRAMS ON MESSAGE SIGNS

Road alignment has to be consistent with traffic control devicesThis is the opposite requirement to the requirement that traffic control devices be consistent with road characteristics. Each road class should fulfil some expectation, connected to the importance and road category. It mostly works on newly built lengthy sections of motorways and bypasses, but even when the original sections are twenty years old, their equipment and level of safety are different. Because of the important rule “Never surprise the driver”, roads of the same class and importance should have the same level of safety and equipment by homogenized standard.

A regular road safety inspection should point out the deviations from uniformity and qulaity and quality of traffic control equipment.

All newly installed traffic control devices should be installed with respect to safety zone, with deformable/frangible support or using safety barriers to protect road users from them, the first solution being preferable since a safety barrier is always an obstacle in the safety zone.


FIGURE 139 - RAILWAY CROSSING IN AN URBAN AREA IS NEARLY INVISIBLE AND THERE IS NO RED LIGHT. PEDESTRIANS ON THE SIDEWALK ARE UNPROTECTED. .



2012R36EN

FIGURE 140 - ADVANCE WARNING SIGNS AND MARKING ON 2 + 1 ROAD IN SWEDEN. . (Source: Matt Petersson)

FIGURE 141 - EXAMPLE: NEW JERSEY BARRIER (Source: Tomás Jilek)

Figure 141: Advance warning, gradual traffic flow changes and congestion control are critical for work zones on motorways. The terminals of the New Jersey barrier do not seem to be adequate for the situation, as they are vertical and the traffic is not protected from them.

There is a similar situation described (guidance for two way opposing traffic for temporary work zones) in the Portuguese Manual on Temporary Signing ( figure 142).

FIGURE 142 - EXAMPLE: PORTUGUESE MANUAL ON TEMPORARY SIGNING (Source: JAE 1997)


2012R36EN

FIGURE 143 - POORLY VISIBLE RAILWAY CROSSING; WARNING SIGN BELONGS TO TRAFFIC LIGHT AFTER THE CROSSING .(Source: Jindrich Fric)

FIGURE 144 - RIGHT-HAND SIDE TRAFFIC SIGNAL HIDDEN BEHIND POLE(Source: Jindrich Fric)


2012R36EN

FIGURE 145 - RAILWAY CROSSING SIGNALS INVISIBLE FROM DIRECTION OF ACCESS (Source: Jindrich Fric)

FIGURE 146 - WRONG USE OF SIGN, WRONG COMBINATION OF SIGNS, INVISIBLE RAILWAY CROSSING WITHIN A CITY .

(Source: Jindrich Fric)


Traffic control devices are well addressed in guidelines and traffic decrees.

One thing that needs improvement is the contrast between signs and devices and their background. Specific attention is needed for colour-blind people who cannot distinguish red from green (e.g. by white edges around the red part of the sign), and a small portion of colour-blind people who are not able to see red items on variable


2012R36EN

message signs unless red is mixed with orange (in which case they are able to see differences in brightness).

4. PROPOSALS FOR MISSING LINKS

4.1. OPTICAL DENSITY OF THE FIELD OF VIEW: MONOTONOUS SURROUNDINGS OF ROADS AVOIDED

Optically diversified surroundings in the visual periphery, in contrast to monotonous surroundings, improve driving behaviour. One of the cues for speed perception is optical flow: the visual motion of contrasts that is perceived as the observer moves relative to them. Contrast reduction decreases the rate of optical flow, and thereby leads to a lower perceived speed and activation of driver physiology, which in turn causes fatigue and a lack in concentration and alertness. The consequence of such contrast reduction is a subconsciously produced higher speed (Snowden, Stimson, and Ruddle, 1998)3. The rate of optical flow increases by surroundings that are rich of bright and colour contrast. Moreover, a diversified environment is stimulating for drivers. Together with a curved alignment, it keeps drivers alert.

The following measures in the road’s surroundings are suitable to avoid monotony:

• Increase contrast on structures along the road such as noise barriers and tunnel walls:

– Vary the height of the top of a noise barrier, the plantings on or behind the barrier rising above it, or the paintings on tunnel walls.

– Change the brightness, colours and texture of the surface of the structure. Paintings and shapes can be used for this purpose.

– The same also applies to buildings. Buildings with different heights and appearances (i.e. different levels of brightness or surface structure) result in a pleasant optic flow.

– Diversify plantings by varying types, intervals and heights of the plantings but always parallel to the road’s edge. This also results in pleasant optical flow.

The optical flow is especially high when a darker line of buildings, plantings etc. contrasts well with the bright sky and varies in height frequently (see the principle in figure 152).

3 Note that real fog leads to lower speeds as fog masks distal portions of a scene, leaving only proximal parts with higher angular velocities visible. As a result, the global rate of optical flow will indicate a higher speed (Pretto, Chatziastros, 2006). Also, fog will be consciously perceived by the driver, possibly leading to risk compensation for reduced vision by decreasing speed.


2012R36EN

4.1.1. Best practices for noise barriers

Figure 147 shows low contrast noise walls with a monotonous height, resulting in a monotonous driving environment. Several solutions are available, e.g. putting texture on the surface of the structure (see figure 148), alternating the height of the top of the noise barrier (see figure 149), using different types of planting (see figure 149), trees which rise above the structure and have the effect of opening and closing the view of the sky, and enrichment of the surface using paintings (see figure 150). The latter solution is also possible for tunnel walls, as exemplified in figure 152. Paintings should not be too surprising for drivers and should not be used excessively near decision points like entrances where the driving task is more demanding. Windows can be used to offer a view of the surroundings of the road, but they do not always increase the level of contrast, as is shown in the image on the right in figure 147. This can be solved by using differently coloured windows, e.g. figure 151. The risk of glare through reflected sunlight should be checked.

FIGURE 147 - LOW CONTRAST NOISE WALLS RESULTING IN A MONOTONOUS DRIVING ENVIRONMENT(Source: Paul Schepers)

FIGURE 148 - NOISE WALLS WITH IMPROVED CONTRAST BY ADDING TEXTURE ON THE SURFACE AND BY ALTERNATING THE HEIGHT OF THE STRUCTURE (IMAGE ON LEFT)



2012R36EN

FIGURE 149 - NOISE WALLS WITH IMPROVED CONTRAST BY ADDING PLANTINGS ON OR BEHIND THE STRUCTURE WITH VARYING HEIGHTS


FIGURE 150 - NOISE WALLS WITH IMPROVED CONTRAST THROUGH PAINTINGS ON THE SURFACE(Source: left picture: Tchona Idossou, right pictures: EGIS, Eric Loquet)

FIGURE 151 - NOISE WALLS WITH DIFFERENTLY COLOURED AND SIZED WINDOWS(Source: Paul Schepers)


2012R36EN

FIGURE 152 - TUNNEL WALLS WITH LITTLE CONTRAST (LEFT) AND INCREASED CONTRAST (RIGHT)(Source: Paul Schepers)

4.1.2. Best practices for planting

A lack of plantings in a rural area, as in the left-hand image on figure 155, results in a low optical density of the field of view and higher speeds. There are few objects that contrast with their environment. In the right-hand image, the optical density of the road’s environment is increased by trees that vary in height and in the distance between them (but not too much in distance from the road because this could distort the tracking task). Planting lines without variation in brightness and height, such as the images at the top of figure 154, also result in a low optical density. This is improved by putting different sorts of trees and shrubs along the road and including open spaces in the planting line; see for instance the images in figures 154 and 155. This opens up the view onto the surroundings every now and then and results in a more active state of alertness.


2012R36EN

wrong eliminated

FIGURE 153 - THE IMAGE ON THE LEFT SHOWS AN ENVIRONMENT WITH A LOW LEVEL OF CONTRAST, WHICH IS IMPROVED BY PLANTINGS IN THE IMAGE ON THE RIGHT.


wrong

reduced

FIGURE 154 - THE IMAGES ON TOP SHOW A MONOTONOUS ENVIRONMENT (LOW LEVEL OF CONTRAST, LONG STRAIGHT SECTION), WHICH IS IMPROVED IN THE IMAGES BELOW


low bushes (up to about 1.50 m)

medium-height bushes (up to about 3-4 m)

tall trees (4 m and over)

FIGURE 155 - PLANTINGS THAT VARY IN INTERVALS AND IN HEIGHT(Source: Bielenberg, et al, 2002).


2012R36EN

4.1.3. Works of art along the road

To conclude, works of art can be used along lengthy stretches of road to avoid monotony. These should not be too surprising for drivers and should not be used excessively along stretches of road such as entrances where the driving task is more demanding, because that could result in overly long glances off the road at the wrong moment.

FIGURE 156 - WORKS OF ART THAT HELP TO PREVENT MONOTONY(Source: Daniel Aubin, Google Earth)

4.2. TRANSITION ZONES ATTRACT THE ATTENTION OF THE DRIVER

The driver has to be prepared in time for a change in road category or an intersection that calls for a different type of driving. A transition zone needs to be conspicuous in order to alert the driver and focus attention on information that may be critical. Several measures are available to achieve this:

• Pavement markings and vertical elements that attract the attention and notify the driver.

• Many of these measures, such as raised middle islands, have already been mentioned in chapter 3. Coloured pavement in combination with vertical elements may be useful to draw even more attention.

• Structures like arches or a rope with decorations stretched over the road.


2012R36EN

Examples of paint on the road surface in Great Britain (Birmingham) to notify drivers of the presence of a roundabout or an intersection are shown in figure 157. Paint is applied to the area preceding the roundabout or intersection, and it becomes visible to drivers several seconds before the road feature is reached (as compared to paint that is applied only within the feature).

FIGURE 157 - PAINT ON THE ROAD ATTRACTS THE ATTENTION TO AN INTERSECTION OR ROUNDABOUT(Source: Daniel Aubin, Birmingham)

FIGURE 158 - A RAISED INTERSECTION AREA PAINTED IN BRIGHT YELLOW (GRONINGEN)(Source: Paul Schepers)

Figure 158 shows a raised intersection area that is painted in bright yellow to make it more conspicuous. The design speed should be taken into account when designing raised intersections. Paint on the road can also be used to draw more attention to a curve; see for instance figure 159. Note that the paint is bright (like the white colour used for edge lines) to guarantee visibility at night. The human eye is unable to detect colour differences at night but it is able to perceive differences in brightness.


2012R36EN

FIGURE 159 - A CHANGE IN SURFACING TO STRENGTHEN THE PERCEPTION OF THE CURVE(Source: AXIMUM, P: ANELLI)

Countries with high levels of cycling as in Northern Europe could benefit from markings and colours on the road to notify drivers of cycling paths. Several studies show that speed-reducing measures such as elevated cycling paths, possibly combined with markings, are very effective in reducing speed, improving subconscious visual scanning activity, and reducing the risk of collision (Summala, et al, 1996; Gårder, et al., 1998; Schepers, et al, 2011).

If there are no speed-reducing measures, but only painted white rectangles or a painted area at the cycle crossing (e.g. figure 160), there is no clear decrease in collision risk (Schepers, 2011). In a field test on the visibility of cycle crossings marked with white rectangles at night, Nygårdhs et al. (2010) found that cyclists were always seen from longer distances than the cycle crossing itself, which only became visible at a very late stage. To conclude, figure 161 exemplifies a cycle crossing that improves the visibility of both the crossing and the cycling path.


2012R36EN

FIGURE 160 - THIS BICYCLE CROSSING IN FRANCE IS PAINTED GREEN AND MARKED WITH WHITE RECTANGLES TO ATTRACT DRIVERS’ ATTENTION

(Source: SETRA – CETE)

FIGURE 161 - THIS RAISED BICYCLE CROSSING IN THE NETHERLANDS ATTRACTS ATTENTION AND REDUCES SPEED(Source: Paul Schepers)

Vertical elements can be used to support a transition zone, for instance a town entrance, as was previously described in chapter 2. Structures like arches are suitable for dramatising the transition zone, as seen in figure 162. Figures 162 and 163 exemplify a town entrance that is dramatised by two symmetrical artworks next to the road. Gallenne et al. (2010) refer to Lamberti et al. (2009), who applied a gateway


2012R36EN

at a town entrance in a simulator study (see figure 164). Gateway treatments appear to be highly effective in reducing speed because they catch driver’s gaze to the right critical point and interrupt the unbroken view to the horizon which subconsciously increases speed.

FIGURE 162 - AN ARCH DRAMATISES A TRANSITION ZONE SEVERAL HUNDRED METRES AHEAD OF A ROUNDABOUT(Source: Paul Schepers)

FIGURE 163 - TWO SYMMETRICAL ARTWORKS DRAMATISE A TOWN ENTRANCE(Source: Paul Schepers)


2012R36EN

FIGURE 164 - A GATEWAY TO DRAMATISE A TOWN ENTRANCE IN A SIMULATOR STUDY BY LAMBERTI ET AL. (2009)

Preconditions for managing field of view When improving the quality of the field of view, it is very important that other Human Factors demands be addressed as well as basic provisions for safe road sides:

• Planting lines, building lines, tunnel walls and noise barriers should run parallel to the road’s axis, and lines at both sides of the road should have about an equal distance to the road surface to ensure a symmetrical appearance.

• Planting lines should be placed outside the clear zone that is needed given the design speed of the road. Low plantings, e.g. shrubs, can be added to the edge of the clear zone provided that no trunks grow outward to become obstacles over the course of time (7cm at most). Plantings adjacent to the clear zone may be used to cover ditches that create a risk of drowning.

4.3. FIXATION OBJECTS IN THE LATERAL ROAD SIDE ENVIRONMENT SUPPORT OPTIMAL LANE TRACKING

Field dependent people in particular rely on information at the side of and above the road for spatial orientation and guidance that plays a major role in the tracking task. Tasks such as walking on a slope or riding on a road with a high superelevation are well supported on condition that vertical structures are perpendicular and overhead structures are level. Vertical elements such as pillars, posts and trees that are tilted or superstructures that are slanted tend to hamper lane-keeping in the field-dependent drivers; see for instance the crash location in figure 165. Figure 166 shows a good example where both arches of the bridge are perpendicular and symmetrical. An eye-catching object like this bridge offers a landmark for driver’s estimation of road’s course and general orientation.


2012R36EN

FIGURE 165 - CRASH LOCATION WITH TILTED ROWS OF TREES ALONG THE ROAD(Source: Paul Schepers)

FIGURE 166 - BOTH OF THE BRIDGE’S ARCHES ARE PERPENDICULAR AND SYMMETRICAL(Source: City Plan, J. Landa)

Leaning vertical structures are unproblematic as long as structures at both sides of the road are symmetrical as in figure 167. Non-symmetrical structures may hinder visual guidance, for instance due to clearances of varying width between the road surface and planting lines. The example in figure 168 would badly hamper lane-keeping.

FIGURE 167 - IMAGINARY EXAMPLE OF AN ARTWORK WITH ELEMENTS THAT ARE OUT OF TRUE BUT SYMMETRICAL (Source: Paul Schepers)

(Note that this type of artwork could support a transition zone where drivers have to slow down, but it would be a dangerous obstacle along a straight section with higher speeds.)


2012R36EN

FIGURE 168 - IMAGINARY EXAMPLE OF AN ARTWORK WITH ELEMENTS THAT ARE OUT OF TRUE, NON-SYMMETRICAL AND CONFUSING FOR DRIVERS


4.3.1. Structures over the road

A crossover, constructed to allow a downward road to cross another road at a different level, as in figure 169, results in a slanted structure. This will tend to hamper lane-keeping with some of the driver approaching the crossover. Field-dependent drivers try to avoid the optical bottleneck at the right side with the lower height - resulting in slight steering manoeuvres to the left and movements to correct their course. It is recommended to conceal the structure’s inclination with a horizontal shield if the problem cannot be prevented through design (or only at high cost).

This solution is exemplified in the right-hand image of figure 169. The shield’s most important purpose is to support good lane-keeping. This will be achieved with a shield that is perceived as a clear, coherent visual hold and gives a symmetrical impression of the crossover. The shield can be made of material that allows wind and light to pass through, provided that the material draws more attention than the structure of the bridge which has to be concealed. Thus it has to be optically dominant compared to the crossover.

FIGURE 169 - THE LEFT IMAGE SHOWS THE PLAN OF A VIADUCT OVER A DISTRIBUTOR ROAD; THE FRONTAL VIEW IN THE MIDDLE IMAGE SHOWS THE PROBLEM OF UNEQUAL HEIGHTS OF THE

ABUTMENTS THAT WILL CAUSE FIELD-DEPENDENT DRIVERS TO DRIFT TO THE LEFT; THE RIGHT IMAGE SHOWS THE SAME VIADUCT COVERED BY A SHIELD

(Source: Birth, Sieber & Staadt, 2004).


2012R36EN

Another potential problem with structures over the road has already been described in chapter 4.2: abutments of a viaduct that do not run parallel to the road axis. The most favorable solution for drivers is to achieve abutments that run parallel to the road axis as in figure 170, possibly by using artificial abutments. Sometimes this is difficult because the angle under which two roads cross is too small. Figure 171 is a best practice example where the problem was solved by placing a railway track diagonally on a crossover. The abutments of the structure run parallel to the edge of the road. In figure 172 the problem of non-parallel abutments is avoided with pillars parallel to road’s edge. Driver orientation cannot be disturbed with these pillars because they are round.

FIGURE 170 - ABUTMENTS THAT RUN PARALLEL TO THE ROAD AXIS(Source: Paul Schepers)

FIGURE 171 - A RAILWAY RUNS OVER A BRIDGE DIAGONALLY TO ACHIEVE ABUTMENTS PARALLEL TO THE ROAD

(Source: Daniel Aubin, Google Earth)


2012R36EN

FIGURE 172 - PILLARS INSTEAD OF ABUTMENTS UNDER A VIADUCT(Source: Paul Schepers)

4.3.2. Vertical elements along the road

Planting lines, building lines and rows of lighting posts provide peripheral information which drivers subconciously use in addition to information provided by the road and markings in the central part of the field of view. To support the tracking task, vertical elements and lines along the road should be:

• Perpendicular → rows of vertical elements along the road should appear perpendicular

• Symmetrical → vertical elements on both sides of the road should be symmetrical with respect to the distance from the road and its slope.

Perpendicular vertical elementsRoad signs and other road furniture are often well maintained, resulting in perpendicular vertical elements. However, trees are sometimes out of true due to years of wind blowing from a certain direction. A row of vertical elements tilting to the left, as seen in figure 173, disturbs the organs of balance in field-dependent drivers. This can result in a complete destabilisation of visual perception. A field-dependent driver will try to compensate for the perception that the road inclines to the left, by steering to far to the right. This will add to the risk caused by the lack of a safe clear zone. Installing safety barriers will reduce the severity of crashes but will not really reduce the risk of a crash occurring. Correct perception of the road is supported through the addition of perpendicular elements along the road which are both sufficiently conspicuous and symmetrical, as seen in the image on the right in figure 173.


2012R36EN

FIGURE 173 - ACCIDENT POINT IN GERMANY ON A ROAD WITH ROWS OF TREES THAT TILT TO THE LEFT (LEFT) AND ITS SIMULATED CORRECTION (RIGHT)

(Source: Birth, 2003 a)

Symmetry of objects / orientation lines on the lateral roadsideThe following recommendations support a symmetrical appearance of vertical elements along the road:

• Planting lines, building lines, safety barriers, and lines of lighting posts along both sides of the road should be at about the same distance from the road edge.

• Vertical elements that stick out of a planting line should be removed, or should be complemented by a similar element on the other side of the road, if the clearance between the obstacle and the road is sufficiently wide.

• Vertical elements out of true should be removed or complemented with similar elements on the other side of the road.

FIGURE 174 - ACCIDENT SPOT IN GERMANY WHERE LETHAL ACCIDENTS OCCUR DUE TO A DIFFERENCE BETWEEN THE POSITION OF THE CENTRE LINE AND THE OPTICAL MIDDLE OF THE ROAD

(Source: Birth, Demgensky, & Wähner, 2011)

In figure 174, the coulisse of trees on the two sides of the road is not symmetrical (about 2.50 m on the left, but about 1.80 m on the right). The optical middle of the


2012R36EN

road deviates from the centre line and field-dependent drivers will tend to drift into oncoming traffic. In figure 175, a simulated correction is shown using vertical elements in different colours. Such different colours should be used in limited amounts when the monotony of the road is high and drivers speed up because of the small optical flow.

FIGURE 175 - COUNTERMEASURE FOR FIGURE 177: CORRECTION OF THE OPTICAL DEVIATION FROM THE CENTRE LINE USING COLOURED GLARE PROTECTION 1.50M ABOVE THE CRASH BARRIER.

(Source: Birth, Demgensky, & Wähner, 2011)

Figure 176 shows a road with clearances of unequal width between the road surface and the abutments of the crossover. Underneath, planting is shown as a corrective measure. The open left side under the crossover is covered to achieve a more symmetrical appearance.

wrong

reduced

FIGURE 176 - ASYMMETRICAL CROSSOVER AND SIMULATED CORRECTION USING PLANTINGS(Source: Birth, Sieber & Staadt, 2004)


2012R36EN

4.3.3. Risk of optical bottlenecks

Vertical elements that stick out of a planting line will hamper the tracking task and, depending on the distance between the obstacle and the edge of the road, cause considerable harm to crash victims. We therefore strongly recommend removing these obstacles. If this is impossible or undesirable, they should be complemented by similar elements on the other side of the road, taking into account the requirements for safe road sides.

Two examples are shown in figure 177. The trees seen here will make drivers slow down or drive too close to the middle of the road. A vertical element could be put next to the road on the other side, but this would still cause a bottleneck where drivers will slow down. This negative effect can be reduced by adding elements such as clearly visible safety barriers, possibly increased in size with shields, on both sides of the road starting some tens of metres ahead of the tree, similar to what was seen in figure 175. Drivers will slow down earlier and will have a visual hold while passing the tree. Some of the solutions for carriageway reductions as described in chapter 4.2 are also useful.

FIGURE 177 - EXAMPLES OF TREES THAT STICK OUT OF A PLANTING LINE AT TWO ACCIDENT SPOTS(Source: Birth, 2003b)

4.3.4. Distance illusion

The distance illusion results from the way humans perceive perspective over longer distances. Stereoscopic vision can only be used over shorter distances of up to about 8 m. Over longer distances, one of the cues is the orientation of lines. If two lines converge slightly, the end seems to be further away because items of constant size are perceived to be smaller (i.e. cover a smaller area on our retina) if they are further away. As the distance between the two lines is even smaller at the end (because they converge), it seems that the end is further away. This perceptual trick has been used for centuries by architects to achieve an impression of splendour and make properties look larger, as seen on figure 178, following page.


2012R36EN

FIGURE 178 - DISTANCE ILLUSION (1784): THE CONVERGING SIDES OF THE APPROACH TO A MONASTERY MAKE IT APPEAR THAT THE APPROACH IS LONGER AND THE PROPERTY LARGER THAN THEY REALLY

ARE, THUS IMPRESSING VISITORS (Source: Notas Historicas Sobre La Integratcion De La Carretera En el Medio Ambiente (1750-1925)

For good technical reasons, safety barriers sometimes bend towards the road edge. This change will be especially visible to drivers in the left lane of a dual carriageway road, where the clearance from the safety barriers is small. This is shown in figure 179, following page, where the safety barriers converge toward the road’s edge, resulting in a distance illusion. The distance to the oncoming curve is overestimated because our information processing system interprets converging lines as a proof that objects are further away than they really are.

Besides a distorted perception of distance, there may also be problems with lane-tracking when drivers pass the viaduct. Figure 180, following page, shows the problem situation and figure 181, following page, shows a correction made by concealing the non-parallel guiding line.


2012R36EN

FIGURE 179 - DISTANCE ILLUSION AT AN ACCIDENT POINT IN THE NETHERLANDS: THE NON-PARALLEL CRASH BARRIERS DISTURB THE ORIENTATION TO THE APPROACHING SECTION OF A BRIDGE IN A LONG

RIGHT-HAND CURVE (Source: Birth, 2008)

FIGURE 180 - DISTANCE ILLUSION: THE NON-PARALLEL ORIENTATION LINE (SUCH AS A FENCE, PLANTING LINE OR CRASH BARRIER) LEADS TO AN OVERESTIMATION OF DISTANCES


FIGURE 181 - THE DISTANCE ILLUSION (CAUSED BY NON-PARALLEL SAFETY BARRIERS) IS REMOVED THROUGH USE OF A PARALLEL ORIENTATION LINE


wrong

reduced


2012R36EN

4.3.5. Eye-catching objects

Lane-tracking is supported if the view axis coincides with the road’s course, apart from short glances at the surroundings that are part of an alert driver’s normal spatial perception. A bright digital billboard with moving images is likely to catch the attention of drivers more frequently and longer (SWOV, 2009), and relying on peripheral vision for too long could cause problems.

Another problem is extremely bright spots in the field of view; these may catch the driver’s subconscious fixation and disturb correct perception. At one accident point in the Netherlands, a bright spot supported a wrong perception of the course of the road, as seen in figure 182. There are four openings under the viaduct and the bright spots under the two left openings suggest that the road continues straight on whereas in fact it bends to the right. To correct that impression, attention should be drawn to the right opening, which could be done by placing bright paint on the crossover. Plantings could be added in the surroundings to cover the horizon behind the two left openings and conceal the bright eye-catching spots.

FIGURE 182 - ACCIDENT SPOT IN THE NETHERLANDS: THE BRIGHT SPOT UNDER THE CROSSOVER CAUSES A WRONG PERCEPTION OF THE COURSE OF THE ROAD

(Source: Birth, 2008)

4.4. DEPTH OF THE FIELD OF VIEW - DOMINANT EYE-CATCHING OBJECTS SUPPORT DETECTION OF CRITICAL POINTS

Drivers scan the road far ahead to prepare for possible alterations in the course of the road or other events that require a change in driving behaviour. In order for critical information, e.g. a horizontal curve or intersection, to be detected in time, it is important that it be at least as dominant as other nearby information, like attractive buildings, bright spots on the horizon, or commercial billboards. If critical information is inconspicuous and other nearby information is highly conspicuous, that will decrease the quality of the field of view.


2012R36EN

There are several ways to improve the quality of the field of view:

• cover dominant eye-catching objects which distract the eye away from the road axis or from critical points;

• increase the conspicuousness of critical information to guide the eye to the right spot.

4.4.1. Dominant eye-catching objects support detection of critical points

Critical points are decision points such as roundabouts, intersections, private accesses, bus stops or curves. In figure 183, the intersection further down the road may go unnoticed because of eye-catching objects in the surroundings in combination with sight restrictions. This can be solved by using other eye-catching objects that draw the attention to the intersection. Several ways to achieve this are already covered in guidelines; two of these are the gateway solution (see figure 164) and raised middle islands.

FIGURE 183 - THE INTERSECTION MAY GO UNNOTICED BECAUSE OF EYE-CATCHING OBJECTS IN THE SURROUNDINGS AND SIGHT RESTRICTIONS (LEFT). A SIMULATION WITH A POSSIBLE

CORRECTION (RIGHT).(Source: City Plan, J. Landa)

Vertical elements other than the ones mentioned in this chapter (section III) are possible as well. Figure 184, following page, shows a railway crossing that attracts less attention than the church on the left, especially in this relatively open landscape. The railway crossing should be equipped with level-crossing barriers as in figure 185, following page, including lights that start flashing when a train passes (in the case of a distributor road, a grade separation should be considered).

The flashing lights are recommended in most guidelines, but some of these crossings still have accidents. In the case of the crossing in figure 184, an arch would also draw the driver’s attention to the railway crossing, as seen in figure 186.


2012R36EN

FIGURE 184 - DOMINANT EYE-CATCHING OBJECT – THE CHURCH ON THE LEFT – IN COMPETITION WITH A SUBDOMINANT RAILWAY CROSSING

(Source: Birth, Sieber& Staadt, 2004)

FIGURE 185 - RAILWAY CROSSING EQUIPPED WITH LEVEL-CROSSING BARRIERS AND FLASHING LIGHTS

FIGURE 186 - USE OF AN ARCH TO DRAW ATTENTION TO A RAILWAY CROSSING(Source: Birth, Sieber & Staadt, 2004)

wrong

reduced


2012R36EN

A best practice for use of eye-catching objects is shown in figure 187. It is not specifically designed to draw attention to a critical point. Rather it serves road safety by breaking the long-distance perspective and giving the driver a nearer fixation point, resulting in lowered speed.

FIGURE 187 - THE LONG DISTANCE PERSPECTIVE IS BROKEN BY A BUILDING WITH AN ARCHWAY THAT SERVES AS AN EYE-CATCHING OBJECT

(Source: City Plan, J. Landa)

4.4.2. Change of road’s direction despite a dominant eye-catching orientation line (e.g. city by-pass dilemma)

Drivers need optical guidance to realize that the road’s direction is changing despite dominant eye-catching orientation lines. For instance a change of direction has to be supported by concealing a misleading view / orientation axis.

It has been found that at black spots, dominant eye-catching objects like a line of trees, buildings or straight road sections impede the correct anticipation of road’s course even though signing is present. Road features that mislead spatial perception cause technically “unexplainable” accidents.

This subject is not sufficiently described in most of the national standards. Only a special guide from one of the German states (HVO, Brandenburg) gives good advice on this problem.

A change in the road’s direction should be clearly and unambiguously visible and not disturbed by eye-catching objects or guiding lines. The change of direction has to be supported by concealing a misleading view/orientation axis and through clear enhancement of a changing course using embankments, planting lines or other optical guiding measures.


2012R36EN

FIGURE 188 - PROBLEM SITUATION (LEFT) AND PARTLY CORRECTED SITUATION (RIGHT) FOR MISLEADING VISUAL ORIENTATION(Source: Birth, Pflaumbaum & Sieber, 2006).

The goal is to guide driver’s fixation unmistakably to critical points. Misleading features can easily be concealed or corrected with planted embankments or – if that is not possible - with a safety barrier that provides additional optical guidance in the form of glare protection (min. 120 cm) on top of the rail.

In the German “HVO” [Advice for Guiding Traffic and Giving Optical Orientation by Planting], the problem of the driver being misled by optical guiding lines in the wrong direction is set out. The HVO emphasises the need to “break” the wrong orientation by a shift in horizontal alignment combined with optical barriers in the form of planted embankments and groups of shrubs/trees.

FIGURE 189 - EYE-CATCHING OBJECTS ARE USED TO FOCUS ATTENTION ON THE CHANGED ALIGNMENT OF A ROAD

(Source: Bielenberg, et al.,2002)

old road new road


2012R36EN




planted embankment



FIGURE 190 - NEW MINOR ROAD HAS A BROKEN VIEW TO THE OLD ALIGNMENT AS A RESULT OF A PLANTED EMBANKMENT

(Source: Bielenberg, et al.,2002)

The Dutch guideline for markings states that plantings in the outside of a curve can be used in combination with chevron signs to prevent a deceptive impression. The use of chevron signs should depend on the ratio of the speed in the curve and the speed preceding the curve (CROW, 2005). The Dutch guideline for distributor roads outside urban areas describes the use of guiding elements such as line structures. It says that rows of lighting posts offer guidance at night. From the Human Factors point of view, this only works when it is dark. In daylight, the contrast against the background is low and the distance between the light posts is too large for the poles to create the impression of a holistic frame.

One German solution for improving black spots should be mentioned. If there is not enough space to use plantings or embankments to correct driver spatial perception, glare protection (green, yellow or orange) can be installed on a safety barrier. This simple and effective countermeasure is a proven optical guiding element for the correction of misleading eye-catching orientation lines.


2012R36EN

FIGURE 191 - USE OF GLARE PROTECTION TO ENSURE PROPER SPATIAL PERCEPTION(Source: Birth, Demgensky, & Wähner, 2011)

FIGURE 192 - USE OF NOISE BARRIERS AND EMBANKMENTS TO CONCEAL A MISLEADING OLD PLANTED COURSE (BLUE) AS A RESULT OF A HUMAN FACTORS AUDIT OF A NEW OVERFLYER IN A NATIONAL

ROAD IN THE NETHERLANDS(Source: Birth, 2009)


2012R36EN

5. CONCLUSIONS

5.1. INTRODUCTION

The interaction between drivers and vehicles is well described in psychological and ergonomic standards used by the automobile industry. Literature on the interaction between road users and road features is often limited to specific design elements and is less systematic.

In the previous PIARC cycle from 2008 to 2011, the “IST Checklist 2008” was used to identify the degree to which the Human Factors items on the Checklist are explicitly or implicitly addressed in the 9 currently used national design standards and guidelines for rural distributor roads. The checklist contains about 100 validatet Human Factors criteria for spatial perception that were derived from an extensive literature search focused on reliable experimental studies and about 1,500 road safety inspections at accidents spots in Brandenburg, Germany that where technically unexplainable. During these road safety inspections, it was found that often the laws of spatial perception were violated through specific mistakes in optical guidance and mismanagement of the field of view as well as serious mistakes in preprogramming driver’s habits, routines and expectations (Birth, Sieber & Staadt, 2004).

It was shown in a blind test that the Human Factors judgement correlated with road crashes, with a validity of about 0.65. A reliable prediction of accident spots is thus possible through road safety inspections and road safety audits (Birth & Pflaumbaum, 2005). The checklist has been successfully used for the investigation of black spots (i.e. high risk locations) and during road safety audits in recent years. The focus of the current PIARC cycle was to support the application of knowledge about the interaction between drivers and road features at an earlier stage, i.e. in road design. Many design elements like curve radii are difficult to adjust once the road has been built and accident concentrations become manifest.

The goal of this project was to determine to what degree the Human Factor items on the “IST-Checklist 2008” are explicitly or implicitly addressed in current nationally used design standards and guidelines. The project was focused on guidelines for rural distributor roads because of their high average crash risk. The Human Factors and the examples are of course generally valid for other road categories such as access roads, motorways and through roads. Guidelines from the following countries were audited for the spatial perception criteria on the “IST-Checklist 2008”: Portugal, Canada, Australia, Japan, China, Hungary, Czech Republic, France and the Netherlands. As a result of long-standing knowledge exchange within organizations like PIARC, the guidelines in developed countries show a high degree of convergence, allowing us to draw general conclusions. Design guidelines in developing countries are mostly less advanced, indicating opportunities to benefit from best practices in


2012R36EN

the guidelines of developed countries, of course taking their starting point and differences in traffic systems into account.

5.2. CONCLUSIONS

Based on the audits of national design guidelines, the following conclusions were drawn:

1. The need for the driver to anticipate any critical point in the driving environment and adequately respond to that is well described in the standards of developed countries. Of the HF demands, 49% were met, 28% were partly met and 24% were not met.

For each road category and design speed, clear standards are formulated for sight distances, the visibility of markings, reflector posts, traffic lights, the need for intersection design with design features such as a raised middle island, and so on.

The need for an anticipation sight distance (of about 6 seconds or more to prepare for a different driving program) is explicitly described in only a few cases. The practice to give the driver an advance warning section in case of more complex changes should be addressed more directly in the guidelines.

The need to provide a clear continuous optical frame along the outside of curves, for reliable guidance and anticipation, is not described clearly in the standards. It was judged that the management of horizontal and vertical alignment is not efficient enough to provide optical guidance without any other optical guiding elements.

The perception and visibility of critical points with regard to human limitations is poorly described in most standards and guidelines.

2. Management of the field of view to ensure appropriate speed and lane tracking is poorly addressed in the design standards. Of the HF demands, 9% were met, 31% were partly met and 59% were not met.

The subject of spatial perception underlying the HF demands seems to be a blind spot in the field of road design. The value of parallel plantings and guiding structures for the lane-tracking task and to avoid optical illusions is described in only one design standard; it is partly covered in two and not covered in seven.

The need for contrasts in the lateral visual periphery to avoid monotony and manage the speed perception of drivers is not addressed in the standards or only in a rudimentary way (100%) and this matter should be incorporated in all of them.


2012R36EN

The possibility of using eye-catching objects to support lane-tracking and the detection of critical points is not addressed in the standards (69%) or only partly addressed (19%). Only 13% of the standards mention the need to avoid misleading and misperception. But there is no clear specification for designers about how to achieve this.

Management of the field of view is thus not specified well enough to be put into practice by designers.

3. The HF demands to pre-programme driver’s behavoiour correctly show a differentiated situation. Of the HF demands, 34% were met, 27% were partly met and 39% were not met.

The demands concerning traffic control devices and changes in road features (e.g. a reduction in the number of lanes) in relation to driver expectations are well described in 67% of the standards, in 22% only partly and in 11% they are not described. Requirements are formulated for subsequent curve radii to limit the chance that drivers will be surprised by a sharp curve. With respect to traffic control devices, the mimesis effect (invisibility of signs against their background) is not always well incorporated in the standards.

In most guidelines, drivers’ expectations are not well covered. Changes of function or direction despite the dominant optical characteristics of the road are not mentioned (60%) or only in rudimentary way (40%). There is broad agreement in saying “Never surprise the driver”. However, it seems to be difficult to integrate this demand with practical requirements into standards because the underlying principles of the subconscious regulation of driving actions, reasoning and learning are not well known.

Important in this respect is the design of the road network by a hierarchy of basic functional roads which helps to formulate specific criteria (for each road category) and to avoid large transitions. For instance, Dutch guidelines differentiate between access roads, distributor roads and through roads, and these correspond to special distinctive optical road features. Through roads are connected to access roads via distributor roads. Private accesses and parking places are not allowed directly along distributor roads. The chances of a driver being surprised (e.g. by a large abrupt transition or a vehicle suddenly stopping to park along the road) are greatly reduced if such a system is systematically applied.

5.3. DISCUSSION

There are several possible reasons for our findings. Although psychologists know from experimental studies that spatial perception and subconscious regulation of driving actions play a role, it was until now difficult to quantify these processes and to translate them into specific design recommendations.


2012R36EN

In the second place, most of the well addressed HF items such as sight distances and visibility of markings refer to “ focal view”, a process of which drivers are more conscious and which are clearer for non-psychologists. HF items such as the importance of parallel peripheral building/planting structures for the tracking task or the problem of eye-catching objects refer to “ambient view”, which is largely processed subconsciously.

Items related to conscious visual processes can be understood better by non-psychologists and therefore have a higher chance of being included in guidelines. But the items related to subconscious visual processes have to be understood by designers so that they can use a small number of very efficient guiding elements and eye-catching objects for reliable optical guidance and anticipation.

This implies that incorporation of HF safety rules is unlikely to be successful unless civil engineers are trained in human spatial perception during their basic education. Without a comprehensive understanding of the Human Factors principles of spatial perception, a self-explaining, safe road design with low accident risk cannot be achieved.

5.4. RECOMMENDATIONS

The result shows that management of the field of view in particular, as well as preprogramming driver’s behaviour, should be more clearly incorporated in standards. Inclusion of these Human Factors is essential if the likelihood of accidents on new roads is to be reduced. The Human Factors can also be used as a kind of checklist in “on-the-spot” investigations of accident points or during road safety inspections. Road safety auditors can make use of them to validate planning and design processes when performing road safety audits.

Based on the audit results, the following recommendations can be formulated:

• Train future road designers and auditors in the interaction between drivers and road design (including subjects like spatial perception, management of the field of view and preprogramming driver’s behaviour).

• Train future road designers and auditors in the basics of self-explaining road design, design-for-all, and the basic principles of a hierarchical system of road categories as part of their basic education.

• Use best practices from other countries’ design standards and guidelines, as described in chapter 3, to supplement one’s own guidelines with HF items that are not yet addressed.

• Develop the proposals for missing links, as described in chapter 4, in cooperation with experts on spatial perception, landscape architects and engineers (e.g. in the field of forgiving road sides). The design guidelines need systematic improvement concerning:


2012R36EN

– the principles of spatial perception – the management of the field of view to ensure appropriate speed and lane tracking – the preprogramming of driver’s behavoiour correctly – other more specific features like the visibility of critical points, use of eye-catching objects and so on, as set out in chapter 4.

The Human Factors Subgroup of Technical Committee TC1.1 “Safer Road Infrastructure” is convinced that this report will be a first step toward a better understanding of drivers’ subconscious spatial perception and toward inclusion of Human Factors in design standards.

It will also help to focus road authorities and government ministries on the need to initiate psychological training for future road designers as a basic part of their education. This would be the realization of a long ago overdue step that was gone in the design of machines and technical systems about 20-30 years ago to train students in industrial design and also in the basic psychological needs of users of their designed systems.

The authors and editors of this report hope that responsible parties with an interest in safer roads will use these initial steps to help make the world’s roads as safe as possible.


2012R36EN

6. REFERENCES

• Almeida Roque, C. (2010). “Criteria for Uniform Designing and Signing of No-Passing Zones in Portugal”. In: TRB 4th International Symposium on Highway Geometric Design. Podium presentation. June 2nd-5th 2010. Valencia. Spain.

• AASHTO (2004). “A Policy on Geometric Design of Highway and Streets Austroads Incorporated” (2008). “Guide to Traffic Management Part 8: Local Area Traffic Management”. Austroads: Sydney, Australia

• Standards Australia/Standards New Zealand. (2010) AS/NZS 1158. Lighting for roads and public spaces. Sydney: Standards Australia

• Bassan, S. (2011). “Decision Sight Distance review and evaluation”. Traffic Engineering and Control, 52(1), p. 23-26.

• Berlyne, D. E. (1971). “Aesthetics and psychobiology”. New York: Appleton-Century-Crofts.

• Bielenberg, H., Birth, S., Sporbeck, O., Staadt, H. Stauff, M.; (2002). “Hinweise zur Verkehrslenkung und optischen Orientierung durch Bepflanzung (HVO) an Bundes- und Landesstraßen (außerorts) im Land Brandenburg”. [Advice for Guiding Traffic and Giving Optical Orientation by Planting along Federal and District roads (outside municipalities) in Brandenburg]. Potsdam: Ministerium für Stadtentwicklung, Wohnen und Verkehr.

• Birth, S. (2004a). Report “Human Factors Profiling Unfallstelle B1 „Kurve Ortseingang Geltow”. Potsdam: MSWV des Landes Brandenburg.

• Birth, S. (2003a). Report “Human Factors Unfallanalyse B109/140”. Potsdam: Ministerium für Infrastruktur und Raumordnung.

• Birth, S. (2003b). Report “Human Factors Profiling Unfallstelle L291”. Potsdam: Ministerium für Infrastruktur und Raumordnung.

• Birth, S. (2004b). Report “Beurteilung der Wirksamkeit fluoreszierender und reflektierender Verkehrszeichen: Untersuchung einer unfallauffälligen Kurve der B 167”. Potsdam: Intelligenz System Transfer.

• Birth, S. (2004c). Report “Psychological preconditions for fly-over junctions”. Potsdam: Ministerium für Infrastruktur und Raumordnung.

• Intelligenz System Transfer GmbH [Birth, S.] (2008). “Human Factors Guideline for Safer Road Infrastructure”. Report 2008R18 of Technical Committee C3.1 Road Saftey, Paris: PIARC. Available at: http://www.piarc.org/en/member-edit.htm?dl=/ressources/publications/5/6165,2008R18WEB.pdf [Accessed 06.12.2010]

• Birth, S., Pflaumbaum, M. & Sieber, G. (2006). “HF-Training for Engineers”. Potsdam: Intelligenz System Transfer.

• Birth, S., Pflaumbaum, M. (2006). “Human Factors in Road Design: a way to self-explaining roads. Validation of the „IST-Checklist 2005“: Project Report of Work Package 3: Expert Assistance for Safety Review of Rural and Urban Roads (single carriageway roads).In: Ranking for European Road Safety (RANKERS) Research Project funded under the 6th Framework Program of the European Community.


2012R36EN

• Picture: Course of the road is not visible, the misguiding optics are irritating, Available at: http://norwegen.kaifler.net/bilder/kurioses.shtml [Accessed 06.12.2010]

• Birth, Dr. S., Staadt, Prof. Dr. H., Sporbeck, O. (2002). “Guideline for optical orientation by planting, (HVO)”. Brandenburg: Ministry of Infrastructure and Regional Development

• Birth, S. & Aubin, D. (2009). “Space Perception and Road Design for Vulnerable Road Users (VRU)”, presentation of working group results for PIARC, Cape Town.

• Birth, S., Sieber, G. & Staadt, H. (2004). “Straßenplanung und Straßenbau mit Human Factors”. Ein Leitfaden. Potsdam: Ministerium für Infrastruktur und Raumordnung. Abt. 5, Straßenwesen, Straßenverkehr.

• Birth, S. (2009a). “Human Factors Design Features Supporting Space Perception”, lecture for the training of accident commissions in Brandenburg, Germany, Potsdam: Intelligenz System Transfer.

• Birth, S. (2009b). “Human Factors Design Features Supporting Space Perception”, lecture in 4th International Conference on Safer Road Infrastructure, Prague.

• Birth, S. & Aubin, D. (2009). “Space Perception and Road Design for Vulnerable Road Users (VRU)”, lecture in PIARC International Seminar: Promoting Road Safety for Vulnerable Road Users, Cape Town

• Birth, S., Demgensky, B. & Wähner, U. (2011). “Bericht zur Evaluation der Maßnahme: Aufbau einer Kurvenkulisse in der Unfallkurve Leuenberger Grund der”. B 158.Report für die Unfallkommission Märkisch-Oderland. Strausberg.

• Birth, S. (2008). Report “Human Factors Accident Profiling on four accident bends in the Netherlands: A28” Nieuwleusen Staphorst (Bend 1 & 2), A6 Oosterzee Sint Nicolaasga (Bend 3), A27 Breda, Junction Annabosh (Bend 4). Rijkswaterstaat Dienst Verkeer en Scheepvaart. Potsdam: Intelligenz System Transfer

• Birth, S. (2009c). Report “Human Factors Audit for two constructional drafts in the Netherlands: A4 continuation Steenbergen, N33 doubling Veendam”. Rijkswaterstaat Noord Brabant. Potsdam: Intelligenz System Transfer

• Burkhardt, C. (1965). “Fahrbahn, Fahrzeug und Fahrerverhalten”. In: Hoyos, C. G. (1965). Psychologie des Straßenverkehrs. Bern: Hans Huber

• Campbell, J.L. & Graham, J. (2008). NCHRP Report 600A: “Human Factors Guidelines for Road Systems”, Collection A. Washington D.C.: Transportation Research Board

• TAC (Transportation Association of Canada) and the Canadian Institute of Transportation Engineers (1998). “Canadian Guide to Neighbourhood Traffic Calming”.

• Transportation Association of Canada (TAC) (2006). “Guide for the Design of Roadway Lighting”.

• Transportation Association of Canada (TAC) (1998). Manual of Uniform Traffic Control Devices for Canada (MUTCD). (Updated 2008)

• Cardoso, J.L. (2001) “Detection and low-cost engineering improvement of inconsistent horizontal curves in rural roads”. Paper presented at the 12th International Conference Road Safety on Three Continents, FERSI/VTI/TRB, Moscow.


2012R36EN

• Cardoso, J.L. (2005). “Safety assessment for design and redesign of horizontal curves”. Paper presented at the 3rd International Symposium on Highway Geometric Design, Transportation Research Board, Chicago.

• CERTU (2006). “Maîtrise des vitesses par l’aménagement”, Fiches n°03• CÉTÉ-NC. (1990). “Analyse de comportement en entrée d’agglomération : Le cas

de la Couture-Boussey”. Division Exploitation Sécurité Gestion des Infrastructures, Grand-Quevilly, Seine-Maritime, France.

• Charlton, S.G. (2004). “Perceptual and attentional effects on drivers’ speed selection at curves”. Accident Analysis and Prevention, 36, 877 884

• Charman, S., Grayson, G., Helman, S., Kennedy, J., de Smidt, O., Lawton, B., & Tu ka, P. (2010). “Self-explaining roads literature review and treatment information”. Deliverable Nr 1, June 2010, SPACE - Speed Adaption Control by Self Explaining Roads.

• Cohen, A. S.; Imholz, J.; Siegrist, J. (1996). “Erforderliche Abweichung zwischen Blick- und Fahrtrichtung für die sichere Fortbewegung beim Befahren von Engpässen”. In: Schlag, B. (1997). Fortschritte der Verkehrspsychologie 1996. Deutscher Psychologen Verlag

• Cohen, A. S.; Zwahlen, H.T. (1989). “Blicktechnik in Kurven. Wissenschaftliches Gutachten”. Schweizerische Gesellschaft für Unfallverhütung, bfu-report 13

• Cohen, A.S. (1987). “Blickverhalten und Informationsaufnahme von Kraftfahrern”. In: Bericht zum Forschungsprojekt 8306/3 der BaSt, Bergisch Gladbach, Nr. 168

• CROW (1998). “Eenheid in rotondes” publicatie 126. Ede.• CROW (2002a). “Handboek Wegontwerp Gebiedsontsluitingswegen” [Handbook

for Distributor Road Design], CROW-publicatie 164c. Ede.• CROW (2002b). “Handboek Wegontwerp; delen Basiscriteria, Stroomwegen,

Gebiedsontsluitingswegen en Erftoegangswegen”. Ede• CROW (2002c). “Richtlijn Bewegwijzering”. CROW-publicatie• CROW (2004a). “Richtlijn essentiële herkenbaarheidkenmerken van

weginfrastructuur: wegwijzer voor implementatie”. Ede: CROW.• CROW (2004b). “Richtlijn Essentiële Herkenbaarheidskenmerken van

weginfrastructuur” [Guide for Essential Recognizability Characteristics of road infrastructuur], CROW-publicatie 203. Ede.

• CROW (2005a). “Richtlijn bewegwijzering” [Guide on signposting], CROW-publicatie 222. Ede.

• CROW (2005b). “Richtlijn voor de bebakening en markering van weg” [Guide for beaconing and marking of roads], CROW-publicatie 207. Ede.

• CROW. (1999). “Richtlijn Veilige Inrichting van Bermen” [Guideline on safe road sides], CROW-publicatie.

• CROW (2010). Verkeerstekens. Available at: http://kennisnet.crow.nl/verkeerstekens/_t63_p0_m7_i3359.htm#2 [Accessed December 2010]

• Decreto Regulamentar n.º 22-A/98 “Regulamento de Sinalização do Trânsito” (Highway Code Regulation Law)


2012R36EN

• Forschungsgesellschaft für das Straßen- und Verkehrswesen e. V. (FGSV), Hrsg. (1995), Köln: Richtlinien für die Anlage von Straßen (RAS-L) Teil Linienführung.

• Forschungsgesellschaft für Straßenwesen Arbeitsgruppe Strassenentwurf. (Hrsg.). (1985). Empfehlungen für die Gestaltung von Lärmschutzanlagen an Straßen, Köln: Forschungsgesellschaft für Straßen- und Verkehrswesen

• Forschungsgesellschaft für Straßenwesen Arbeitsgruppe Strassenentwurf. (Hrsg.). (2005). Empfehlungen für die Gestaltung von Lärmschutzanlagen an Straßen, Köln: Forschungsgesellschaft für Straßen- und Verkehrswesen

• Gallenne, M.L. (2010). Clôture du projet français SARI ou « comment informer plus efficacement les conducteurs et les gestionnaires routiers d’un risque élevé de perte de contrôle ? ». Transport, Environnement, Circulation, 207, 2-6.

• Ganneau,F. (2006). “Paysage et lisibilité de la route - Eléments de réflexion pour une démarche associant la sécurité routière et le paysage”, (Road Environment and Legibility), Sétra

• Gårder, P., Leden, L. & Pulkkinen, U. (1998). “Measuring the safety effect of raised bicycle crossings using a new research methodology”. In: Transportation Research Record 1636. Transportation Research Board, Washington D.C.

• Gegenfurtner, K.; Sharpe, L. T. (1999). “Color vision: from genes to perception”. New York: Cambridge University Press.

• Transportation Association of Canada (TAC) (1999). “Geometric Design Guide for Canadian Roads” (Updated December 2007)

• Forschungsgesellschaft für das Straßen- und Verkehrswesen e. V. (FGSV), Hrsg. (Draft 3/2008) “Richtlinien für die Anlage von Landstraßen”. Köln

• Forschungsgesellschaft für das Straßen- und Verkehrswesen e. V. (FGSV), Hrsg. (in Publication). “Richtlinien für die Anlage von Landstraßen RAL”. Köln

• Girard, Y. (2006). “Modération de la vitesse et ergonomie routière”. Colloque - La route autrement : concevoir des routes incitant à une conduite apaisée, Maison de la Mutualité, Paris Ve, Jeudi 9 mars 2006, Paris, France.

• Goldstein, E. B. (1997). “Wahrnehmungspsychologie: Eine Einführung”. Heidelberg, Berlin, Oxford: Spektrum, Akad. Verl.

• Herberg, K.-H. (1994): “Auswirkungen des Straßenbildes und anderer Faktoren auf die Geschwindigkeit”. In: Flade et.al. Mobilitätsverhalten. Bedingungen und Veränderungsmöglichkeiten aus umweltpsychologischer Sicht. Weinheim: Beltz, Psychologie Verlags Union.

• http://www.autobahnatlas-online.de/Bildergalerie/L511OerHerten2.h t m & u s g = _ _ 7 N K M U r M Q 5 J E M m r Q J V- e 6 p Wa - c h k = & h = 4 0 9 & w =600&sz=73&hl=de&star t=53&zoom=1&tbnid=Y7GevdoHrWSLb -M:&tbnh=147&tbnw=202&ei=28UVTsmdJ8_Msgb0sYzjDw&prev=/search%3Fq%3Dfahrbahnverengung%26um%3D1%26hl%3Dde%26sa%3D-N%26biw%3D749%26bih%3D795%26tbm%3Disch&um=1&itbs=1&i-act=hc&vpx=344&vpy=459&dur=26&hovh=185&hovw=272&tx=115&-ty=130&page=5&ndsp=12&ved=1t:429,r:10,s:53[Accessed 07.12.2010]


2012R36EN

• Almeida Roque, C. (2009a). “Disposição Técnica - Marcas Rodoviárias - Dispositivos Retrorreflectores Complementares” (Technical Disposition - Road studs and delineators). InIR.

• Almeida Roque, C. (2009b). “Disposição Técnica - Sinalização de Proibição de Ultrapassagem” (Technical Disposition -Signing of no-passing zones). InIR.

• Almeida Roque, C. (2009c). “Disposição Técnica - Sinalização Vertical, Critérios de Utilização” (Technical Disposition - Criteria for use of road signs). InIR.

• Almeida Roque, C. (2009d). “Disposição Técnica - Sinalização Vertical, Critérios de Colocação” (Technical Disposition - Criteria for location of road signs). InIR.

• Almeida Roque, C. (2009e). “Marcas Rodoviárias - Características Dimensionais, Critérios de Utilização e Colocação” (Technical Disposition - Dimensions and criteria for use and location of road markings). InIR.

• Bastos Silva, A. . (2009). “Dimensionamento de Rotundas - Documento síntese” (Roundabout Design - Guideline). InIR (Portuguese Institute for Road Infrastructures).

• JAE (1990a). “JAE P6/90 Norma de Intersecções” (Interchanges Standard). • JAE (1990b). “JAE P5/90 Norma de Intersecções” (Intersections Standard). • JAE (1999). “Norma de sinalização turística” (Tourist signage standard). • JAE (1994). “JAE P3/94 Norma de Traçado” (Design Standard). Junta Autónoma

de Estradas. • JAE (1997). “Manual de Sinalização Temporária”.• Jensen, S.U. (2007). “Safety effects of blue cycle crossings: A before-after study”.

Accident Analysis and Prevention, 40, 742-750.• Katz, B. (2007). “Peripheral transverse pavement markings for speed control”.

Ph.D. dissertation, faculty of the Virginia Polytechnic Institute and State University, Blacksburg, Virginia.

• Klebelsberg, D. (1963). “Eine Methode zur empirischen Ermittlung des psychologischen Vorrangs an Straßenkreuzungen und -einmündungen”. In: Leutzbach, W.; Papavasiliou, V (1988). Wahrnehmungsbedingungen und sicheres Verhalten im Straßenverkehr: Wahrnehmung in konkreten Verkehrssituationen. Bericht zum Forschungsprojekt 8306 der Bundesanstalt für Straßenwesen, Bereich Unfallforschung, Nr. 177.

• Lamberti, R., Abate, D., De Guglielmo, M.L., Dell’Acqua, G., Esposito, T., Galante, F Pernetti, M. (2009). “Perceptual measures and physical devices for traffic calming along a rural highway crossing a small urban community: speed behavior evaluation in a driving simulator”. In Proceedings of the 88th TRB Annual Meeting, 11-15 january 2009, Transportation Research Board of the National Academies Washington, D.C., USA.

• Lozano, E. E. (1988). “Visual needs in urban environments and physical planning”. In: J.L. Nasar (Ed.). Environmental Aesthetics. Theory, research & applications. Cambridge: Cambridge University Press.

• M.L. Gallenne, F. Rosey, L. Désiré (2011). “Bibliographical review about human factors and road design self explaining road and landscape and legibility concept”.


2012R36EN

• Ministerie van Verkeer en Waterstaat. (1994). Besluit administratieve bepalingen inzake het wegverkeer (BABW) [Administrative Provisions (Road Traffic) Decree]. Den Haag.

• Baier, R., Heinz, H., Rücker, J., (2000). OD-Leitfaden 2011. “Leitfaden für die Gestaltung von Ortsdurchfahrten in Brandenburg”. Potsdam: Ministerium für Infrastruktur und Landwirtschaft (MIL),

• National Authority for Road Safety. (2009). “Instrução Técnica sobre a utilização da Sinalização de Mensagem Variável”. (Technical Instruction on Variable Message Signs). Autoridade Nacional de Segurança Rodoviária.

• Notas Historicas Sobre Le Integatcion De La Carretera En el Medio Ambiente (1750-1925)

• NP EN (1871). “Road marking materials - Physical properties”.• CERTU (2006). “Maîtrise des vitesses par l’aménagement” Fiches n°03. • Nygårdhs, S., Fors, C., Eriksson, L., Nilsson, L., 2010. “Field test on visibility at

cycle crossings at night”. Linköping: VTI.• Ministry of Transportation (2001). “Ontario Traffic Manual - Book 7: Temporary

Conditions”. Toronto, Ontario: Ede.• Otten, N.; Schroiff, H.-W. (1988). “Untersuchungen zu Determinanten der

Geschwindigkeitswahl”. Bericht zum Forschungsprojekt 8525/2 der Bundesanstalt für Straßenwesen, Bereich Unfallforschung, Nr. 169

• Pretto, P, Chatziastros, A. (2006). “Changes in optic flow and scene contrast affect the driving speed”. DSC 2006 Europe.

• Queensland Department of Main Roads (QDMR). (2001). “Road planning and design manual: chapter 10: alignment design”. Brisbane, Australia: Queensland Department of Main Roads

• Retting, R.A. & Farmer, C.M. (1998). “Use of pavement markings to reduce excessive traffic speeds on hazardous curves” ITE Journal

• Rainer, C., Delhomme, P., Kaba, A., Mäkinen, T., Sagberg, F., Schulze, H., Siegrist, S., (1999). “GADGET Guarding Automobile Drivers through Guidance Education and Technology”. Vienna: Kuratorium für Verkehrssicherheit (KfV).

• Roth (1973). In Cohen, A.S. (1984). “Einflussgrößen auf das nutzbare Sehfeld”. Bericht zum Forschungsprojekt 8005 der BaSt, Bereich Unfallforschung Nr. 100

• Santos, J. A., Berthelon, C., & Mestre, D.(2002). “Perception of road users’ motion”. In R. Fuller & J. A. Santos (Eds). Human Factors for Highway Engineers (pp. 115-130). Oxford, UK, Elsevier.

• Schepers, J.P., Kroeze, P.A., Sweers, W., Wüst, J.C. (2011). “Road factors and bicycle motor vehicle crashes at unsignalized priority intersections”. Accident Analysis and Prevention, 43:853-61.

• SETRA (1994). “Aménagement des Routes Principales”.• SETRA (1998). “The design of interurban intersections on major roads, at-grade

intersections”.• SETRA (2001). “Signalisation Touristique”.


2012R36EN

• SETRA. (2003). « Paysage et lisibilité recueil d’expérience approches paysages et sécurité routière » Service d’études sur les Transports, les Routes et leurs Aménagements, Bagneux, France.

• SETRA. (2006a). “Comprendre les principaux paramètres de conception géométrique des routes” (Document technique n°4044). France.

• SETRA. (2006b). “Paysage et lisibilité de la route : Elément de réflexion pour une démarche associant la sécurité routière et le paysage”. France.

• SETRA. (2008). Rapport d’études - « L’application du concept de la route autrement pour une conduite apaisée » à la desserte de l’aéroport du Grand Ouest près de Nantes. France.

• Snowden, R., Stimpson, N., Ruddle, R. (1998). “Speed perception fogs up as visibility drops”. Nature, 392, 450.

• Summala, H., Pasanen, E., Räsänen, M., en Sievänen, J. (1996). “Bicycle accidents and drivers’ visual search at left and right turns”. Accident analysis and prevention, 28, 147-153.

• SWOV (2009). SWOV-factsheet “Roadside advertising and information”. Leidschendam.

• Sieber, G., (1986). “RWE Programm für die Ausbildung von Human Factors Opti-mierungskoordinatoren in Kernkraftwerken”. [unpublished programm] München: Intelligenz System Transfer

• Theeuwes, J., & Godthelp, H. (1995). “Self-explaining road”. Safety Science, 19, 217-225.

• Gouvernement du Québec (1999). “Traffic Control Devices”.• Unknown author (1750-1925) “Notas Historicas Sobre La Integratcion De La

Carretera En el Medio Ambiente”. • Van Elslande, P., Fouquet, K., Michel, J.E., & Fleury, D. (2004). “Analyse

approfondie de l’accidentologie en aménagements urbains : Erreurs, facteurs, contextes de production”. Institut National de Recherche sur les Transports et leur Sécurité,, Salon de Provence, France.

• Vollpracht, H.-J. (2006). Report “Road Safety Inspection in Romania”. National Company for Motorway and National Roads, Technical Assistance of European Union

• Zwielich, F., Reker, K.; Flach, J. (2001). “Fahrerverhaltensbeobachtungen auf Landstraßen am Beispiel von Baumalleen. Eine Untersuchung mit dem Fahrzeug zur Interaktionsforschung Straßenverkehr”. Berichte der Bundesanstalt für Straßenwesen, Reihe Mensch und Sicherheit, Heft M 124

• Yerkes R.M., Dodson J.D. (1908). “The relation of strength of stimulus to rapidity of habit formation”

• JAE “Norma de Sinalizacao Turistica” (Portugal)• SETRA “Signalisation touristique-Guide” (France)• JAE P6/90 Norma de Nós de Ligacao. Portugal • Forschungsgesellschaft für das Straßen- und Verkehrswesen e. V. (FGSV). (1995)

RAS-L. Köln


2012R36EN

• Picture: Spectral sensitivity of the eye in dark conditions (blue) and day conditions (red), Available at: http://www.osram.ch/osram_ch/DE/Tools_%26_Services/Training_%26_Wissen/lichtlexikon_popups/images/pop_35_1_de.gif [Accessed 06.12.2010]

• Rumar, K. & Marsh, D.K. (1998). “Lane Markings in Night Driving”. UMTRI-98-50. Michigan: The University of Michigan, Transport Research Institute

PIARC factores humanos en diseño de carretera

Documents

Transcript of PIARC factores humanos en diseño de carretera