Design Speed and Target Speed

Norman W. GarrickLecture 3.1

Street and Highway Design

AASHTO DefinitionDesign Speed

A selected speed used to determine the various geometric design features of the roadway

To understand what design speed is, we need to look into i) how it is selectedii) what it is used foriii) how it is used

It is a deceptively complex concept that is essential to parse in order to understand how AASHTO design works

Selecting the Design Speed

According to AASHTO the guideline for selecting design speed is as follows:

The assumed design speed should be a logical one with respect to the topography, anticipated operating speed, the adjacent land use, and the functional classification of the highway

Every effort should be made to use as high a design speed as is practical to attain a desired degree of safety, mobility, and efficiency within the constraints of environment quality, economics, aesthetics, and social or political impacts, except for local streets where speed controls are frequently included intentionally

The selected speed should fit the travel desires and attitude of nearly all drivers that are expected to use a particular facility

What is Design Speed used For?

The guideline from AASHTO for using design speed is as follows:

Once the design speed is selected, all of the pertinent highway features should be related to it to obtain a balanced design. Above-minimum design values should be used, where practical.

Features Directly Affected by Design SpeedCurvatureSuperelevationSight distance

Features Indirectly Affected by Design SpeedLane and Shoulder WidthsLateral Clearances

1/R

Station12+00 22+00

0.0005

Curve 1

Curve 2

-0.00075

27+00 35+00

Alignment as 1/R Plot

How is Design Speed used in Practice?An Example

What is the design speed of the two roads shown?We know that both of these alignments are freeways

Therefore they most likely have DS of 60 or 70 mph (lets assume 60 mph for this exercise)

Highway A

Highway B

How is Design Speed used in Practice?An Example

Under the AASHTO procedure, the design speed is used to determine the minimum radius of curvature for the roadway section.

For a design speed of 60 mph, the minimum radius of curvature is 1300 feet

The designer can then choose to use any radius larger than this value.

We can assume that this was the procedure that was applied to these two sections of highway

Highway A

Highway B

R = 1300

How is Design Speed used in Practice?An Example

In the case of highway A, all the radii used are significantly larger than the minimum. In fact, the smallest radius used is 5,500 ft, using the AASHTO formula; this radius would be equivalent to a design of speed of about 120 mph.

We perhaps might not expect an operating speed of 120 mph, but it is clear that this entire section of road could be comfortably traversed by most drivers at speeds well in excess of the design speed.

Highway A10-mile section of alignment

11 curvesMaximum Radius = 20,000 ft

Minimum Radius = 5,500 ftAverage Radius = 10,200 ft

How is Design Speed used in Practice?An Example

The alignment for Highway B is quite different: the smallest radius here is 1,432 ft and the average is 4200 ft - less than that the smallest radius for Highway A. But again the result is the same, the operating speed would be higher that would be expected, given the design speed.

Highway B10-mile section of alignment

12 curvesMaximum Radius = 11,500 ft

Minimum Radius = 1,400 ftAverage Radius = 4,200 ft

What does this example tell us about Design Speed?

The first question it raises is whether or not the DS speed process results in a maximum or minimum limit on actually operating speed

The second point to note is that DS approach can produce very different types of facility for the same design speed

Highway AHighway B

Is design speed a maximum or a minimum limit?

This example illustrates a very important feature of the design speed approach that is not always appreciated by all designers.

The design speed sets a minimum level for the potential operating speed on a roadway.

This is not a major problem on the two roads that are used as examples here.

In both cases we have high-speed freeways where there is no risk of conflict between human activities along the road and the speed of the vehicles on the road.

This becomes, however, a big issue when designing roads in a context where high speeds affect livability and safety of other road users - including pedestrians.

Is design speed a maximum or a minimum limit?

The problem is that the design speed approach gives no guidance to the designer on how to design for an upper limit on speed for a given project.

The result is that many newer roads and streets have the look and feel of roads that are designed for 50 or 60 mph, but are sign-posted for 25 or 35 mph.

Looks and feel like a 40 mph road

Variation in design for the same design speed

In some ways, the design speed approach can be considered to be too flexible – this is illustrated by the two very different design solutions that are represented by Highway A and Highway B. Both highways are designed using more or less the same criteria, but the choices made about the alignments are very different.

Of the two roads, Highway A is more continuous, since the discontinuities between curves and tangent sections are not as sharp and the alignment is more curvilinear. Highway A is also more consistent, since all the curves are about the same radii.

However, Highway A also has the potential for much higher operating speeds because the curve radii are so large. (The actual operating speeds will depend to some extent on other design factors, such as the vertical alignment and the width.)

Highway AHighway B

The Problem of Using Design Speed in Urban Areas

Under the AASHTO approach to design, the design speed influences the choice of a host of design parameters, and not just alignment design. These include features such as lane-width, shoulder width, median width and the clear zone. Design speed is also used to help decide on whether a specific element should be part of the design for a given roadway.

As the design speed increases the scale of these features also tend to increase

The problem is that these are the very features that we found in our speed survey that help to control speed. Therefore, there is a conflict between the DS process and the need to control speeds in most urban and some rural environments

The Problem of Using Design Speed in Urban Areas

Based on a number of studies, some have pointed out that design speed is only useful for the design of freeways and other high speed highways

The concept of design speed is misapplied when used for urban streets or other streets that should operate in context time

Recently the concept of TARGET SPEED has taken hold for design and has been used in such documents as the ITE/CNU manual and the NACTO manual

The idea of target speed is to select an appropriate speed for the context and then to design to ensure that most drivers will chose to go no faster than the target speed

For more information on speed and design please click here

Target SpeedContext Time versus System Time

Design Speed Approach

Target Speed Approach

Where should target speed be used?

In most urban situations

For more information on speed and design please click here

Design Speed Approach

Target Speed Approach

Target SpeedContext Time versus System Time

Design Speed versus Target Speed for the design features

Design features affecting or affected by speedCurvatureSuperelevationSight distanceLane and Shoulder WidthsLateral Clearances

Design Speed determines the design featuresDesign features are used to indicate the Target Speed

Two very different approaches to design