DMRB VOLUME 8 SECTION 4 PART 6 - TA 92/03 - CROSSOVER AND CHANGEOVER …€¦ · ·...

November 2003

DESIGN MANUAL FOR ROADS AND BRIDGES

VOLUME 8 TRAFFIC SIGNS ANDLIGHTING

SECTION 4 TRAFFIC MANAGEMENTAT ROADWORKS

PART 6

TA 92/03

CROSSOVER AND CHANGEOVERDESIGN

SUMMARY

This document provides guidance to assist designers oftemporary traffic management systems that includechangeovers and crossovers. Geometric and non-geometric advice is given.

INSTRUCTIONS FOR USE

1. Remove existing Contents pages for Volume 8.

2. Insert new Contents pages for Volume 8 datedNovember 2003.

3. Insert TA 92/03 into Volume 8, Section 4, Part 6.

4. This Advice Note supersedes Chapter 5 ofTA 45/85 (DMRB 2.2.8).

5. Please archive this sheet as appropriate.

Note: A quarterly index with a full set of VolumeContents Pages is available separately from TheStationery Office Ltd.

TA 92/03

Crossover and ChangeoverDesign

Summary: This document provides guidance to assist designers of temporary trafficmanagement systems that include changeovers and crossovers. Geometricand non-geometric advice is given.


THE HIGHWAYS AGENCY

SCOTTISH EXECUTIVE

WELSH ASSEMBLY GOVERNMENTLLYWODRAETH CYNULLIAD CYMRU

THE DEPARTMENT FOR REGIONAL DEVELOPMENTNORTHERN IRELAND

Volume 8 Section 4Part 6 TA 92/03

November 2003

REGISTRATION OF AMENDMENTS

Amend Page No Signature & Date of Amend Page No Signature & Date ofNo incorporation of No incorporation of

amendments amendments

Registration of Amendments

VOLUME 8 TRAFFIC SIGNS ANDLIGHTING

SECTION 4 TRAFFIC MANAGEMENTAT ROADWORKS

PART 6

TA 92/03

CROSSOVER AND CHANGEOVERDESIGN

Contents

Chapter

1. Introduction

2. Traffic Management

3. Planning

4. Geometric Design

5. Non-Geometric Design

6 Sizing the Crossover

7. References

8. Enquiries

Annex A Geometric Design Tables


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Chapter 1Introduction

1. INTRODUCTION

General

1.1 This Advice Note should be used to determinethe location and design of crossovers and changeoversin temporary traffic management systems.

1.2 This Advice Note supersedes Chapter 5 ofTA 45 [DMRB 2.2.8].

1.3 Experience has highlighted the need for advice toprovide consistency in the design and layout ofcrossovers on dual carriageway roads and motorways.Use of this document should ensure appropriatelocations, and satisfactory horizontal and verticaldesign. Crossover location, and more particularlycrossover length, must not be dictated by using anexisting central reserve gap that is too short for themandatory temporary speed limit. A well-designedcrossover will minimise vehicle braking and queuing,therefore reducing the risk of nose-to-tail accidentsupstream.

1.4 The design of a crossover is a balance betweenavailable locations, economical construction cost andconvenience to road users. As crossovers form part of atemporary and often changing situation, attentionshould be given not only to the known situation andprogramme for the Works, but also to the possibility ofunplanned changes or events.

1.5 The basis of the design recommendations in thisAdvice Note allows for a reduction in normal drivercomfort within the design while maintaining acceptablelimits for sideways acceleration. Temporary trafficmanagement arrangements are prominently signed andconsequently driver concentration is heightened. In thissituation drivers are able to tolerate a tighter geometricdesign than can be accepted in a non-roadworkssituation on the assumption that the chosen designspeed is consistent with the adopted temporarymandatory speed limit.

1.6 New construction within the central reserveshould normally be retained for future use, unless thereis justification for removal at the completion of theWorks e.g. where retention creates a safety hazard oraffects the environment.

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.7 This Advice Note sets out guidance for theocation and geometric layout of crossovers andhangeovers on dual carriageway roads and motorways.t supplements the advice given in Chapter 8 (1991)aragraph 4.4.2.1 of the Traffic Signs Manual [TSM]. Aoftware macro is provided (Chapter 6) to assistesigners in sizing a crossover.

.8 This Advice Note also highlights features, suchs lighting, drainage, pavement and non-motorised userssues that require consideration in the design andperation of any crossover.

e-Use of Existing Crossover Sites

.9 Prior to re-use of an existing crossover site aesign assessment is required, even where it hasperated without problems in the past. There is noequirement to assess any existing crossover site untillans arise to re-use it. In the event that a crossover siteails this assessment, guidance is given in paragraph.4.

ew Roads and Network Improvement Schemes

.10 When designing new roads or networkmprovement schemes it is not usually desirable torovide crossover sites for the purposes of futureaintenance works. However, part of the design

rocess for a new road or improvement scheme is tonclude consideration of future maintenance needs. Thisdvice Note should be taken into account when

dentifying lengths of central reserve which could beesigned to contain the minimum of equipment to allowor the construction of crossovers, at a later date. In allases the maintaining organisation should be consultedhrough the Overseeing Organisation.

.11 BD 78 [DMRB 2.2.9] gives mandatoryequirements for crossovers at tunnels.

onstruction Records

.12 As-built drawings for the Works should indicatethe assumed design speed used in the design of acrossover. This will assist at a later date when crossoversites are considered for re-use.

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Explanatory Note

1.13 This Advice Note applies to all crossovers, butthe terminology used is applicable to the first crossoverin a temporary traffic management system.

Implementation

1.14 This Advice Note should be used forthwith on allschemes for the construction, improvement andmaintenance of trunk roads, including motorways,currently being prepared provided that, in the opinionof the Overseeing Organisation, this would not result insignificant additional expense or delay progress. DesignOrganisations should confirm its application toparticular schemes with the Overseeing Organisation.Where this is confirmed, the contract documents for theWorks should be written to reference this Advice Note.

Definitions

1.15 A changeover is a change of lanes introduced todivert traffic within an existing carriageway, includingthe hard shoulder. A changeover is normally associatedwith temporary works where one or more lanes areclosed. It may occur after the traffic has been mergedinto fewer lanes than the permanent situation. Nofurther reference is made to changeovers in this AdviceNote as the criteria for crossovers apply equally tochangeovers.

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1.16 A crossover is where one or more lanes on a dualcarriageway or motorway are diverted onto theopposing carriageway. This is normally where acontraflow situation is required to carry out Works onthe primary carriageway. Crossover length is shown inFigure 1. Note that a second crossover is needed toreturn to the primary carriageway. Crossover site is thepaved area within the central reserve used as part of thecrossover.

1.17 To differentiate between existing horizontalalignment and the new horizontal curves introduced toform the crossover:

• bend is used to describe a horizontal radius in theexisting road alignment;

• curve is used to describe the crossover “S”shape;

• entry curve is used to describe the first curve ofthe “S” shape;

• exit curve is used to describe the second curve ofthe “S” shape;

• non-assisting refers to superelevation that assiststraffic in the non-works situation but adverselyaffects traffic through a crossover curve.

1.18 Longitudinal alignment is measured on thecentreline of the temporary alignment.

Figure 1: Typical layout of a crossover

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1.19 Lateral shift is used to describe the sidewayschange in position of a vehicle from one carriageway tothe opposite carriageway.

1.20 The term Large Goods Vehicles (LGV) is usedin this document to identify those vehicles (defined asover 3.5 tones gross weight) classified as LGVs forlicensing purposes in accordance with Europeanharmonisation of terminology.

1.21 Rollover factor is used to describe thepossibility of overturning, particularly in relation tolarge vehicles. Overturning depends on the height of thecentre of gravity relative to the width of the vehicle’strack. The motions which might cause a large vehicle toroll over depend very greatly on the characteristics(dimensions, loading and suspension) of the individualvehicle and the manoeuvres being carried out, butallowances have been made within the guidance givenin this Advice Note.

1.22 Temporary crossover site: paved area within thecentral reserve that is to be built, or already exists, butwill not be used again after the completion of theWorks.

1.23 Permanent crossover site: paved area within thecentral reserve that is to be built, or already exists, andwill be closed but not removed at the completion of theWorks, allowing future use.

1.24 Two-way crossover: A crossover that carriestraffic in only one direction at a time, but which may bereversed during the Works to carry traffic in theopposite direction.

1.25 One-way crossover: A crossover that carriestraffic in only one direction at a time, and which mustnot be reversed during the Works.

1.26 Primary carriageway: The carriageway onwhich the Works are being carried out, and thereforethat from which the crossover commences.

1.27 Secondary carriageway: The carriagewayopposite to the primary carriageway.

1.28 Works: The maintenance or improvementproject that requires the use of a crossover/changeover.

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Chapter 2Traffic Management

2. TRAFFIC MANAGEMENT

2.1 The requirements for traffic managementcontractors working on high speed roads are given inthe Specification for Highway Works (SHW).

2.2 Traffic management system designs shall be inaccordance with the TSM Chapter 8 and the DesignManual for Roads and Bridges (DMRB). Particularreference to TA 64 [DMRB 8.4.3] is required.

2.3 Speed cameras are an effective way of reducingspeeds within temporary traffic management systems.Where the police are willing to operate the cameras,then their provision should be considered for inclusionin the contract requirements. The OverseeingOrganisation should be consulted to determine currentpolicy before a decision to use cameras is taken.


Chapter 3Planning

3. PLANNING

3.1 The responsibility for the design of the trafficmanagement system shall be as stated in the contractdocuments for the Works.

3.2 Regardless of the status of the designer of thetraffic management system, the OverseeingOrganisation and their Agents should ensure that dueconsideration is given in the preparation of the Works tothe practicality of achieving crossover designs thatconform to this Advice Note, prior to entering into acontract for the Works. The overriding principle is thatcrossovers should be constructed to comply with theproposed temporary mandatory speed limit for theWorks.

3.3 The choice of location for a crossover is the mostcritical factor affecting both the safety in use andconstruction costs for the crossover site. It will alsohave a significant bearing on the safety, capacity anddelays for the whole traffic management system.

3.4 Ideally a crossover should be located away fromjunctions, on a straight and relatively flat section of thecarriageway, with street lighting and minimal centralreserve equipment.

3.5 The aim is to build a safe crossover while stillallowing the Works to be constructed in a cost-effectivemanner. Every effort should be made to find the bestpossible practical location that is appropriate to aparticular crossover.

3.6 Many factors have to be considered whenchoosing the location of any crossover. In some casesfinding a fully suitable location will not be possible andit will be necessary to use a location which does nothave an ideal combination of all relevant factors. Aswith the design of a road, a crossover may have to be acompromise, taking into account the limitations of theavailable locations.

3.7 The main considerations and limits on the designof the crossover in terms of the effect on the vehicle anddriver are:

• The maximum acceptable sideways acceleration.

• The maximum overturning force (affectingrollover of large vehicles).

• Stopping Sight Distances.

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Layout Design Influences

3.8 The layout of any crossover, in terms of entry andexit lane position, will often be determined exclusivelyby the requirements of the Works being carried out.However, when there is a degree of flexibility in thelayout this may be used to the advantage of the designand cost of the crossover.

3.9 Crossover sites at narrow central reservesgenerally result in minimal areas of pavementconstruction.

3.10 Layouts with large lateral shifts (e.g. where thecrossover runs from the normal nearside lane orhardshoulder of the primary carriageway to the normalnearside lane on the secondary carriageway) minimisethe cost of new pavement and other works on thecentral reserve.

3.11 The longest length of construction on the centralreserve occurs when the crossover runs from the laneadjacent to the central reserve on one carriageway tothe equivalent lane on the opposing carriageway e.g.Lane 3 to Lane 3 on a D3 road. Lane 3 may be an offsettemporary Lane 3 for certain standard trafficmanagement layouts.

3.12 Most layouts will fall between the extremes inparagraphs 3.10 and 3.11. Fully symmetrical crossovercurves provide the opportunity for designing curvescompletely within the existing carriageway by allowinga short straight alignment across the central reserve.This obviates the problems of changes of camber,crossfall and gradient at a point of curvature, which formost arrangements will need to be catered for asdescribed in paragraph 4.12.

3.13 Any existing Departures From Standard maycompound problems with the crossover design, andtherefore locations that contain such Departures shouldbe avoided, unless it can be demonstrated that they donot adversely affect the crossover design for theproposed temporary mandatory speed limit.

3.14 The detailed layout constraint effects areconsidered in Chapters 4 and 5 of this document.Further guidance on pre-contract planning is given inTA 64 [DMRB 8.4.3]. However, there are some generalconstraints as follows:

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Chapter 3Planning

a) Where a two-way crossover is to be designed thesuitability of the location and potential problemsmust be looked at for traffic in both directions.

b) Any structure at or near a proposed crossovermay cause difficulties or preclude the use of thatlocation completely. Even where the structuredoes not interfere directly with the layout orvisibility, all safety aspects should be consideredand appropriate measures employed, e.g.protection against impact, siting of signs etc.

c) A sharp horizontal bend immediately before thecrossover is undesirable and could rule out theuse of the location by reducing visibility throughthe traffic management system. It will also affectthe layout of the crossover.

d) High gradients, steep crossfalls and large leveldifferences between carriageways should beavoided.

e) Crossovers with their associated trafficmanagement signs, plant and operatives arepoints where drivers have a heavy informationload to assimilate. Care should be exercised ifexisting roadside features cause driver distractionand close attention should be paid to any likelyeffect on safety.

f) Noise: Crossing a central reserve may createadditional noise, particularly if designed for useby LGVs. If practicable, crossover sites shouldbe located away from residential property.

g) Statutory Undertakers' plant, lighting andcommunication cables, drains: where the centralreserve is to be excavated, records of buried plantand equipment should be checked and suchlocations should be avoided where possible toreduce costs and minimise risks.

h) Overhead power lines can present a hazard,particularly if temporary lighting is to beprovided at the crossover.

i) Soft central reserves may be valuable habitats forflora and fauna and specialist advice should besought before removing areas of the soft estate.There may also be reasons why the soft estateneeds to be re-established at the conclusion of theWorks.

j) An assessment of the strength and suitability ofthe adjacent carriageway and central reservepavement (if paved) will be required.

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) Where a Private Means of Access (PMA) isaffected by the crossover location or length ofcontraflow, the landowner should be approachedin advance to determine if alternativearrangements for the PMA can be made.Similarly, laybys may need to be closed off forthe duration of the works and such a factor mayinfluence the position of the crossover.

) Wide Loads: Crossovers may present difficultieswith the movement of wide loads, particularly ondual two-lane roads. Guidance is available in“The Movement of Abnormal Indivisible Loadsthrough Planned Roadworks” available from theOverseeing Organisation.

) The movements of non-motorised users shouldbe considered at the outset of the planning andany necessary diversion routes arranged withother highway authorities and privatelandowners. Ideally adverse effects, includingdiversions, should be avoided when planning thelocation of crossovers.

) Traffic volumes and composition: the successfuloperation of the crossover will depend onmaintaining a sufficient number of lanes to caterfor expected traffic demand. An assessment ofLGV flow will also be required to ensure thatsufficient lanes are available.

) Crossovers should be located so as to avoid orminimise traffic diversions and be as far awayfrom junctions as possible. This is particularlyimportant where restrictions for LGVs are inplace for one or more lanes, or where certainlanes are dedicated to a particular destination atthe next exit downstream.

) The location and number of crossovers has adirect effect on the physical length of thetemporary traffic management system. The lengthwill have a bearing on driver stress and will alsoaffect the arrangements for recovery ofbroken-down vehicles. The OverseeingOrganisation should be consulted to determinecurrent policy on the length and spacing ofroadworks.

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Chapter 4Geometric Design

4. GEOMETRIC DESIGN

Design Speed and Speed Limits

4.1 The temporary mandatory speed limit to be usedfor a particular project is a matter to be determined byreference to TSM Chapter 8. The design speed to beadopted for a crossover is directly related to thetemporary mandatory speed limit imposed by theTraffic Regulation Order for the overall Works, inaccordance with Table 1 (see Annex A for tables).

4.2 The selection of design speed is not affected bythe status of the crossover site as permanent ortemporary and hence geometric design features areunaffected by the designation.

4.3 It is not acceptable to attempt to reduce costs byselecting a crossover design speed and correspondingtemporary speed limit lower than TSM Chapter 8 wouldnormally recommend.

4.4 Where, exceptionally, a crossover cannot bedesigned for the prevailing “blanket” temporarymandatory speed limit in place for the whole length ofthe Works, it should be designed for an appropriatelower design speed and the temporary mandatory speedlimit may be further reduced locally at the crossover. Inthe case of existing crossover sites, shown byassessment to be sub-standard, the designer will need toconsider the cost implications of enhancement. Wheresuch costs are likely to be disproportionate,consideration should be given to providing a lowertemporary mandatory speed limit and adopting acorresponding design speed. Reducing the design speedby one step (Table 1) is preferable to two steps, and atwo-step reduction should only be considered inextreme cases where the costs of adopting a one-stepreduction remain disproportionately high. In nocircumstances should reductions of more than two stepsbe allowed as it is unlikely that traffic will obey suchextreme speed limits.

4.5 The advice of TSM Chapter 8 (1991 para 2.3.4.2)normally recommends that a mandatory speed limit isundesirable where the length of restriction is less than800m. However, in the case of a crossover, the length ofany locally lower mandatory speed limit (see paragraph4.4) will normally be less than 800m. This is acceptableand enforceable because the crossover is evident todrivers as being a good reason to slow down further andbecause the overall length of speed controlled zone (i.e.the “blanket” temporary mandatory speed limit) is

November 2003greater than 800m.

Design Philosophy

4.6 The design philosophy adopted for crossovers isbased on providing acceptable horizontal radii for an“ideal” crossover and where various adverse factorsexist, reducing the risk by increasing the radii in stepsto continue to provide an acceptable layout. Annex Agives design tables to determine the curve radii to beused for a particular situation.

Horizontal Alignment

Existing Bends

4.7 Horizontal curvature on the existing carriageway,i.e. a bend at the proposed crossover location, maycause difficulties in the design. It is preferable that theradius of curvature of the entry curve into the crossoveris the same as that of the exit curve. The effect of anyexisting bends on the ability to provide this has to betaken into account.

4.8 The provision of equal entry and exit curve radiiposes no difficulty if the carriageway is straight.

4.9 Any existing horizontal bend has an adverseeffect on the curves forming the crossover. An existingright hand bend results in a tighter entry radius thanwould be the case with a straight road. Similarly anexisting left hand bend causes tightening on the exitcurve.

4.10 An existing bend may also effectively form athird curve either before entry to the crossover or on theexit. Although this occurs entirely on the existingcarriageway, it will cause a ‘triple’ bend to be created.Introducing a third change of direction for drivers tonegotiate is undesirable.

4.11 Where a bend on the approach to a crossover hassuperelevation in any direction, the radius of thecrossover entry curve should be increased inaccordance with Table 3 to allow a greater margin ofsafety when drivers are already experiencing differentsideways forces before entering the crossover.

4.12 Where the entry and/or exit curves are locatedpartly on the existing carriageway and partly on thecentral reserve there may be a change of vehiclesideways acceleration. Crossover radii should be

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increased in accordance with Tables 3 and 4 whenacceleration changes occur on any curve. The extremecase is a change from assisting superelevation toadverse camber.

4.13 An extreme case is where the existing bend is ofthe same hand and its radius is approaching the designlimit for the radius of the entry curve. In this case theentry curve approximates so closely to the radius of theexisting bend that, effectively, no ‘lateral shift’ can takeplace. In these circumstances the crossover should besited elsewhere.

4.14 The use of unequal curve radii through thecrossover is undesirable and it should be avoided, butwith one exception: where a right-hand bend exists, theradius of the exit curve can be greater than that of theentry curve, provided that the crossover will only beused in this direction. If the crossover is to be used laterin the other direction then undesirable “tightening”through the crossover would occur. In this situation thecrossover must be designed and constructed with equalradii curves. Alternatively the crossover must bereconstructed for the reverse direction.

4.15 The use of crossovers with equal entry and exitcurve radii will result in long crossover lengths if theyare situated on a significant bend in the road and suchcrossovers will be asymmetric. However, this ispreferred as this will not be particularly noticeable to adriver whereas any change of radii would.

Transition Curves

4.16 Generally transition curves are not required in thedesign of a crossover and the ‘tightening radius’ effectcreated is undesirable.

4.17 The use of a transition curve will cause theoverall length of the crossover to increase. This willnormally increase the length of the central reserve gapin most designs with a resulting increase in constructioncosts.

4.18 Where the crossover cannot be better located ordesigned in accordance with the recommendations ofthis document, the use of a transition curve may berequired to reduce sideways acceleration or overturningproblems.

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Vertical Alignment

General

4.19 The vertical alignment to be considered in thedesign of the crossover is that lying on the vehicle pathfor the temporary layout, rather than that of thecentreline of the normal carriageway.

4.20 The longitudinal vertical curve to be consideredis the approximate curve that can be fitted between thetwo existing vertical curves, one for each carriageway,allowing for any temporary construction. Figure 2 givesan example of Fitted Curve diagrams for a particularcase where the secondary carriageway is higher than theprimary carriageway.

4.21 Consideration must be given, where there areadverse factors through the crossover, to the effects ofthe crest curve on the limits of adhesion or theoverturning of large vehicles. See Tables 3 and 5.

4.22 Consideration must be given to the effects of asag curve causing an unacceptable reduction in comfortlevel or distances illuminated by headlights. See Tables3 and 5.

4.23 Taking the fitted curve as shown in Figure 2, thevertical alignment through the crossover should bedesigned to comply with Table 5.

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Figure 2: Fitted Vertical Curve Diagrams (Example)

a. Crest curve

b. Sag curve

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Gradient

4.24 Designers are reminded to check effects for two-way crossovers when assessing gradient.

4.25 Crossovers should not be located where thedownhill gradient exceeds the desirable maximum asgiven in TD 9 [DMRB 6.1.1]. The maxima arereproduced below:

3% Motorways

4% AP Dual Carriageways

4.26 Where downhill gradients steeper than those inTD 9 [DMRB 6.1.1] are unavoidable, additionalconsideration must be given to the measures to beemployed for reducing speed e.g. speed enforcementcameras. Downhill gradients may cause undesirableadverse camber effects (Table 2) and horizontal radiimay need to be increased (Table 3).

4.27 The effect of an uphill gradient on approach tothe crossover is generally beneficial when the gradientcomplies with paragraph 4.25. However, if the uphillgradient or height risen is near to the advice limits forthe start of provision of climbing lanes in TD 9[DMRB 6.1.1] then traffic throughput may be reducedto unacceptable levels.

Crossfall And Height Differences BetweenCarriageways

4.28 The longitudinal alignment should be checkedalong the centreline of the temporary lanes through thecrossover, including entry and exit curves. The effectivealignment along this path will involve both the existingfitted vertical curve (Figure 2) and the effects ofcarriageway crossfall and level differences between thecarriageways. Situations may arise where the gradientof the fitted curve and the crossfall combine to give anunacceptable resultant value of adverse camber. Theresultant value of adverse camber and downhill fittedgradient is calculated using Table 2. Along the full S-curve there may be various different values of resultantadverse camber. The worst value should be determinedand inserted into Table 3. As gradient will usually varyalong the fitted curve, the worst case instantaneousdownhill gradient that occurs along the length of routeat which adverse camber is being considered should beused. Relieving effects of uphill gradients are not takeninto account.

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4.29 The section of road being considered should haveminimum crossfalls to both carriageways whereverpossible. In circumstances where the road falls to thenearside verge of each carriageway, the crossfalls willform an adverse camber on the curves through thecrossover. It will only be in the circumstances where theroad falls towards the central reserve that it will assistwith one or both curves through the crossover.

4.30 Whenever possible, the crossover location shouldhave no significant difference in the levels between theedges of each carriageway across the central reserve.

4.31 Where the level of the secondary carriageway islower than that of the primary carriageway, assuming asuperelevation not greater than 7% is formed, this willnormally assist with the entry into the contraflowsection, but will normally create an adverse camber atthe exit curve, and the latter case should be used todetermine the horizontal design.

4.32 Where the level of the secondary carriageway isgreater than that of the primary carriageway, an adversecamber is formed which must not exceed the resultantvalues given in Table 2. This will generally be thelimiting factor when deciding what level differencebetween the two lanes can be accepted.

4.33 Where a significant difference of level betweenthe two carriageways exists and there is no alternativelocation for the crossover, additional design andconstruction may be necessary, possibly in the form ofan overlay to raise the level of one of the carriageways.The fitted curve can then be adjusted.

4.34 The side profiles of drainage channels can causeadverse camber to increase while travelling through thecrossover entry or exit curve. Where such drainageitems are present then the crossover should be locatedand designed so that any profile does not adverselyaffect the design and performance of the crossover.Where this is not possible, the termination of channelsand use of alternative drainage methods is advised. Theelimination of adverse conditions by filling withwearing course material is not recommended, unless asuitable bond can be guaranteed, because a loss ofwearing course filler may create a hazard.

4.35 Linear drainage channels systems affect both thelateral and longitudinal alignment levels and need to beconsidered under both categories when using Table 2.

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Rollover

4.36 The worst case for rollover is when changingfrom a 7% adverse camber in one direction to a 7%adverse camber in the other. This does not supplyenough energy for a large goods vehicle to roll-overalthough it is close to the limit. Camber changes ofgreater than 10% (e.g. –5 to +5%) are notrecommended so as to allow a margin of safety.

4.37 The main factors to be taken into considerationwhen checking the design are:

a) Crossovers from one 7% adverse camber toanother must be avoided.

b) Crossovers should cross the central reserve at ashallow angle of no more than 20 degrees, wherepossible, to the existing road. This factor isimportant whatever the width of the centralreserve.

c) Changes in level or camber should be gradualand preferably on a straight section.

Stopping Sight Distance

4.38 Stopping Sight Distances (SSD) should bemeasured to the low object in accordance with Fig 3 ofTD 9 [DMRB 6.1.1], but for each temporary lane.Sighting outside the line of cones (i.e. across the WorksArea) is not permitted when measuring SSD.

4.39 Table 1 gives the absolute minimumrequirements for SSD through crossovers for thetemporary mandatory speed limit adopted. Cones andbarriers should be placed so that Table 1 values areachieved.

4.40 The specification for the traffic managementsystem should be written to include details of therequired SSD. Sometimes visibility greater than theabsolute minimum SSD requirement can be partiallyachieved if plant is carefully located to allow somelimited enhancement of sightlines across the WorksArea.

4.41 Where width is limited and narrow lanes and lowsetbacks to cones or barriers are to be adopted, it maybe necessary to increase horizontal radii purely for thereason of achieving the required SSD. Where adversefactors exist (Table 3), the step increases in radii (Table4) will also automatically help to improve SSD. Usingsmall radii may compromise SSD in certain

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circumstances. In all cases SSD should be checkedlocally, including vertical effects, to ensure that Table 1values are achieved. Due to the large number ofvariables at a crossover, using radii from Table 4 doesnot guarantee SSD compliance in the horizontal plane.

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Chapter 5Non-Geometric Design

GN
5. NON-GEOMETRIC DESI
Lighting

5.1 It is preferable to locate a crossover within a litsection of road.

5.2 The presence of existing lighting columns on thecentral reserve may reduce the desirability of proposinga crossover at such a location. However, it may befeasible to temporarily remove the lighting columns andsupplement the remaining street lighting withtemporary replacement lighting, usually in the nearsideverges.

5.3 Where existing lighting is to be disrupted,temporary lighting should be designed to provide, so faras is practical, an equal intensity and uniformity of lightwhen compared to the existing permanent arrangement.Temporary lights should normally consist ofconventional street lighting columns and lanterns.Floodlighting should only be considered whereconventional street lighting is impractical. Floodlightsmay dazzle drivers, create isolated pools of light andshould normally not be considered. Retention of vergelighting for future use should be considered in the caseof permanent crossovers.

5.4 Where a crossover is to be located on an unlitsection of road, the publication “Guidance for SaferTemporary Traffic Management” gives advice. If adecision to light the crossover is made then the designconsiderations of paragraph 5.3 should be taken intoaccount.

Vehicle Restraint Systems

5.5 Layouts for vehicle restraint systems (VRSs) atcrossovers are given in the Overseeing Organisation’scurrent standard. Where a new crossover site isconstructed, or an existing crossover site is proposed tobe re-used, the VRS arrangements shown in the currentstandard should be installed before the crossover site isbrought into operation. This may require amendment tothe existing layouts.

5.6 On completion of the works, and if the crossoversite is to be retained for future use, the VRS in the“closed” position should be constructed to the detailsgiven in the current standard, regardless of the currentprovision, where sufficient space exists.

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Non-Motorised Users On All Purpose Roads

5.7 Crossovers should not be located where there is alikelihood that non-motorised users (NMUs) will becrossing the road within the crossover length. Thepresence of bus-stops should be taken into account inthis assessment. Temporary closure of bus-stops shouldbe avoided where possible when selecting the crossoverlocation.

5.8 Following a risk assessment, considerationshould be given to providing a dedicated NMU bypassroute, either outside or within the highway boundary, sothat NMUs can avoid the crossover altogether.Dedicated NMU bypasses which involve a substantialadditional travelling distance will be unattractive tousers. If a bypass is not practicable, NMUs must beadequately provided for on the carriageway. The TrafficRegulation Order will need to be drafted accordingly.Signing will need to be planned carefully to informNMUs of the most appropriate route.

5.9 Traffic Advisory Leaflet 15/99 gives guidance onsafety for cyclists at roadworks, including selection ofappropriate lane widths. In carrying out the riskassessment (see above), designers will need to take intoaccount the available lane widths for cyclists.

Surface Water Drainage

5.10 Additional or temporary drainage may berequired as part of the Works. Steps should be taken toensure that there are no surface water drainageproblems at the proposed location. The construction ofa new pavement may restrict or even effectively removethe existing drainage arrangements. Problems mayinclude:

• Standing water.

• Water running across the carriageway.

• Blocked or inadequate drainage.

• Open linear drainage systems.

5.11 The existing drainage and any new drainagerequired for the Works should be checked to ensure thatit conforms with good surface water drainage practice[DMRB 4.2] appropriate for the class of road. In

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Chapter 5Non-Geometric Design

particular, areas of minimal crossfall that createstanding water or combinations of crossfall andgradient that cause ‘streams’ of water to flow across thecarriageway or have the potential for the formation ofice should be avoided.

5.12 If slot drains or linear drainage systems withcontinuous gratings are used, the effect on rollovershould be checked.

5.13 Linear drainage systems that could “trap” thewheel of a vehicle should not be trafficked.

5.14 All drainage elements that are to be traffickedmust be designed to meet the minimum requirement ofClass D400 of BS EN 124.

Road Pavement

5.15 The design of the pavement for crossover sitesshould be in accordance with HD 26 [DMRB 7.2.3].Construction should be in accordance with theSpecification for Highway Works [SHW]. Any existingfoundations or pavements should be assessed for theircontribution to requirements and be incorporated, ifpossible, into the new construction. It may be necessaryto match the pavements of existing carriageways that,because of previous overlays or other treatments, maybe thicker than that required by HD 26 [DMRB 7.2.3].

5.16 Consideration needs to be given to the designlength of service for the crossover site. For temporarycrossover sites, design traffic loading may be calculatedusing the advice contained in HD 24 [DMRB 7.2.1].Traffic flow data obtained for the main carriagewaypavement design should be used and an estimate of thedesign traffic calculated using the programmed lengthof service for the crossover site in question.

5.17 For permanent crossover sites it is likely that thepavement design will be related to the maincarriageway design. However, when designing to aminimum construction requirement, it will be necessaryto consider the design life of the main carriageway andthe likely usage of the crossover site during this period.The design traffic for a permanent crossover site shouldbe assumed to be no less than 1.0 million standard axlesor 5% of the design traffic for the main carriageway,whichever is the greater.

5.18 There is no restriction on the type of material orpavement design to be laid at a crossover site, providingthe material conforms with the requirements of SHWand the design is in accordance with HD 26 [DMRB7.2.3]. Further advice on pavement options is containedin HD 27 Chapter 2 [DMRB 7.2.4.2].

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5.19 Foundations should be designed in accordancewith HD 25 [DMRB 7.2.2].

5.20 Advice on the compatibility of adjacent pavementtypes is available in HD 27 Chapter 2 [DMRB 7.2.4.2].

5.21 Particular attention should be paid to theimplications of new pavement construction on drainagepaths. Pavement designs should, where possible, matchthe total thickness of the main carriageway pavement sothat formation levels are in the same plane and drainagepaths are not interrupted. Where drainage features passthrough or lie within the area of a proposed crossoversite, the design should incorporate provision for thesefeatures within the construction. Surface water drainageis discussed at paragraphs 5.10 to 5.14.

5.22 Attention should be given to the interruption ofdrainage flows when overlaying hardened centralreserves containing linear drainage systems. In suchcases, to accommodate water that previously flowedthrough the crossover site, it will be necessary toprovide alternative outfalls.

5.23 Surfacing materials should be in accordance withHD 36 [DMRB 7.5.1], with the exception that abrushed, burlap dragged or tined concrete surface ispermitted for a temporary crossover site.

5.24 Skidding Resistance: The psv of the aggregateused in the surfacing on a new crossover site shall be60. For existing sites an assessment should be carriedout to ensure that the skid resistance is equivalent to aSCRIM value (MSSC) of not less than 0.45 (unless SiteCategory G2 applies, when value will be 0.50). Theremay be exceptional circumstances where new orexisting crossover sites cannot comply with the aboverequirements without significant disruption ordisproportionate cost. In such cases, and with prioragreement of the Overseeing Organisation to aDeparture from Standard, lower values may beconsidered. For new sites, an aggregate of psv valuehigher than 60 may be used in exceptionalcircumstances at the discretion of the designer where therisks are considered to be high, or where economies resultthrough bulk ordering and laying of similar material.

5.25 The skidding resistance of an existing crossoversite should be investigated using the Portable SkidResistance Tester (Pendulum Test) which provides ameasure of skidding resistance that may be converted inaccordance with HD 28 [DMRB 7.3.1 Para 3.79] tocomparable SCRIM values.

5.26 Prior to opening any crossover site to traffic, itssurface should be clean and free of any debris, detritusor fuel spillage.

November 2003


November 2003

Chapter 6Sizing the Crossover

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6. SIZING THE CROSSOVER

6.1 Sizing the gap in the VRS, the length of thepavement in the central reserve and the overall length ofcrossover involves complex geometric calculations anda software macro has been developed specially by TRLfor the Overseeing Organisation. The software givesMicrosoft Excel output, including graphics. Theattached excel application allows designers to size acrossover layout (as described in Chapter 6 of TA92).Before entering any parameters, users must first enteran excel filename to allow the program to depositoutput data. The program has a Help function included.Excel software and licences must be held to use thesoftware. If reading this document on CDROM thenclick here to access software. Otherwise please [email protected] and a copy will besent by return.

6.2 Tables 3 and 4 are used to select the desiredradius, with due recognition of SSD considerations.Designers wishing to use the software will need to inputradius, lane widths and offsets of lanes and VRS. Themacro assumes a straight road.

6.3 The software is primarily aimed at early designstages to aid early understanding of the size of thecrossover required. Designers should ensure thatdetailed checks are carried out to satisfy themselvesabout the adequacy of the design e.g. the type of VRSand the necessary setbacks and working widths willrequire detailed checks.

6.4 Within the software, the user must enter an Exceloutput file name at the opening screen before thecalculations can commence.


Chapter 7References

7. REFERENCES

1. References to Design Manual For Roads andBridges

Volume 2

BD 78 Design of Road Tunnels

Volume 4

Section 2 Drainage

Volume 6

TD 9 Highway Link Design

Volume 7

HD 24 Traffic Assessment

HD 26 Pavement design

HD 27 Pavement Construction Methods

HD 28 Skidding Resistance

HD 36 Surfacing Material for New andMaintenance Construction

Volume 8

TA 64 Narrow Lane and Tidal FlowOperations at Road Works onMotorways and Dual CarriagewayTrunk Roads with Full WidthHardshoulders.

2. Other References

DETR Traffic Advisory Leaflet 15/99: Cyclists atRoadworks.

Department of Transport (1991) The TrafficSigns Manual – Chapter 8 ‘Traffic SafetyMeasures and Signs for Road Works andTemporary Situations’.

Specification for Highway Works (MCHW).

“The Movement of Abnormal Indivisible Loadsthrough Planned Roadworks”: joint HighwaysAgency/ACPO policy statement 2001.

November 2003

“Guidance for Safer Temporary TrafficManagement”: joint Highways Agency/Healthand Safety Executive/County Surveyors Societypublication 2002.

BS EN 124: Gully tops and manhole tops forvehicular and pedestrian areas. Designrequirements, type testing, marking, qualitycontrol.

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November 2003 8/1

8. ENQUIRIES

All technical enquiries or comments on this Advice Note should be sent in writing as appropriate to:

Divisional DirectorThe Highways AgencyRoom 4B/01Federated HouseLondon Road A PICKETTDorking RH4 1SZ Divisional Director

Chief Road EngineerScottish ExecutiveVictoria QuayEdinburgh J HOWISONEH6 6QQ Chief Road Engineer

Chief Highway EngineerTransport DirectorateWelsh Assembly GovernmentLlywodraeth Cynulliad CymruCrown Buildings J R REESCardiff Chief Highway EngineerCF10 3NQ Transport Directorate

Assistant Director of EngineeringDepartment for Regional DevelopmentRoads ServiceClarence Court10-18 Adelaide Street D O’HAGANBelfast BT2 8GB Assistant Director of Engineering

Chapter 8Enquiries


November 2003 A/1

Annex AGeometric Design Tables

ANNEX A: GEOMETRIC DESIGN TABLES

Use of Tables

1. Use Table 1 and TSM Chapter 8 to select design speed.

2. Assess geometry through centre of proposed temporary route (including use of Tables 2 and 5) and insertfindings into Table 3.

3. Use Table 4 to select radius.

4. Check SSD meets Table 1 requirements and confirm appropriate radius.

5. If required, use software to size the crossover (see Chapter 6).

Table 1: Design Speeds And Stopping Sight Distances For Crossovers

TEMPORARY MANDATORY DESIGN SPEED FOR ABSOLUTE MINIMUMSPEED LIMIT (MPH) CROSSOVER SSD REQUIRED

(DERIVED FROM (KPH) (METRES)TSM CHAPTER 8)

30 60 50

40 70 70

50 85 90

Note: see paragraphs 4.1 to 4.5 for details of design speed & paragraphs 4.38 to 4.41 for details of SSD.


November 2003A/2


Table 2: Resultant Value Of Adverse Camber For Gradient-Crossfall CombinationsFor Downhill Fitted Curves

ADVERSE CROSSFALL [C]

DOWNHILLGRADIENT OF 2.5% 3.5% 5% 7%

FITTED VERTICALCURVE [F]

1% 2.7% 3.6% 5.1% X

2% 3.2% 4.0% 5.4% X

2.5% 3.5% 4.3% 5.6% X

3% 3.9% 4.6% 5.8% X

3.5% (*) 4.3% 4.9% 6.1% X

4% (*) 4.7% 5.3% 6.4% X

5% (**) 5.6% 6.1% X X

6% (**) 6.5% 6.9% X X

Notes

1. Formula used is Resultant = √[C2 + F2].

2. X = not recommended for design.

* = not recommended for new motorways, but may be present where crossover is envisaged.

** = not recommended for new dual carriageways or motorways, but may be present where crossover isenvisaged.

3. C is adverse when the existing crossfall falls from central reserve to nearside verge on either carriageway.Take worst case of entry and exit curve.

4. For uphill situations insert value of C directly into Table 3.

5. Table 2 also to be used to check acceptable level difference between carriageways.


November 2003

Table 3: Number Of Step Increases In Radii (See Table 4) Required ifAdverse Conditions Exist Through Crossover

Reference Adverse Condition No. StepsIncreaseRequired

No Adverse Factor 0

4.28 & Worst Case Resultant Adverse Camber ≥2.5% and ≤5% 1Table 2

Worst Case Resultant Adverse Camber >5% and ≤7% 2

Worst Case Resultant Adverse Camber >7% X

4.12 Any change of value of Superelevation/Adverse Camber Through Entryor Exit Curve 1

Change from assisting Superelevation to Adverse Camber Through Entryor Exit Curve 2

4.21 & Desirable Minimum Crest K Value 0Table 5

One Step below Minimum Crest K Value 1

4.22 & Absolute Minimum Sag K Value 1Table 5

4.11 Approach Bend with Superelevation of 2.5% 1

Approach Bend with Superelevation >2.5% and ≤7% 2

4.36 Rollover: Change in Camber ≥5% and ≤7% 1

Rollover: Change in Camber >7% and ≤10% 2

Rollover: Change in Camber > 10% X

Notes:

1. The number of step increases is cumulative for all tests and is the same for all design speeds.

2. ‘Bend’ and ‘curve’ have specific meanings: see Definitions.

3. X denotes “Not recommended”.

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November 2003A/4


Table 4: Horizontal Curve Radii For Design Speeds

STEPS FROM DESIGN SPEEDTABLE 3

85 kph 70 kph 60 kph

0 510 m 360 m * 255 m *

1 720 m 510 m 360 m *

2 1020 m 720 m 510 m

3 1440 m 1020 m 720 m

4 or more 2040m 1440m 1020m

Notes:

1. * denotes curve widening may be required in accordance with TD 9 [DMRB 6.1.1] if a lane used by LGVsis less than 3.65m wide and radius is less than 400m. If lane widening is not practical then radius may needto be increased.

2. Refer to Chapter 4 for SSD requirements which may affect use of Table 4.

Table 5: Limiting ‘K’ Values Of Vertical Curvature For Crest & Sag Curves

DESIGN SPEED 85 kph 70 kph 60 kph

Desirable minimum crest K value 55 30 17

One step below desirable minimum crest K value 30 17 10

Absolute minimum sag K value 20 20 13

Note: The K values given in Table 5 should be checked against the fitted curves shown in Figure 2.

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