Design Speed and Target Speed
Norman W. Garrick
Lecture 3.1
Street and Highway Design
AASHTO Definition
Design 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 selected
ii) what it is used for
iii) 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 Speed
Curvature
Superelevation
Sight distance
Features Indirectly Affected by Design Speed
Lane and Shoulder Widths
Lateral Clearances
Alignment as 1/R Plot
1/R
Curve 1
0.0005
27+00
12+00
35+00
22+00
Station
-0.00075
Curve 2
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
R = 1300
Highway A
Highway B
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 A
10-mile section of alignment
11 curves
Maximum Radius = 20,000 ft
Minimum Radius = 5,500 ft
Average 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 B
10-mile section of alignment
12 curves
Maximum Radius = 11,500 ft
Minimum Radius = 1,400 ft
Average 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 A
Highway 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 A
Highway 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
Target Speed
Context Time versus System Time
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
Target Speed Approach
Design Speed Approach
For more information on speed and design please click here
Target Speed
Context Time versus System Time
Where should target speed be used?
In most urban situations
Target Speed Approach
Design Speed Approach
For more information on speed and design please click here
Design Speed versus Target Speed
for the design features
Design features affecting or affected by speed
Curvature
Superelevation
Sight distance
Lane and Shoulder Widths
Lateral Clearances
Design Speed determines the design features
Design features are used to indicate the Target Speed
Two very different approaches to design