1. Design Speed and Target Speed Norman W. Garrick Lecture 3.1 Street and Highway Design Norman W. Garrick Lecture 3.1 Street and Highway Design

2. 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

3. 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

4. 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

5. 1/R Station 12+00 22+00 0.0005 Curve 1 Curve 2 -0.00075 27+0035+00 Alignment as 1/R Plot

6. 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

7. 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

8. Highway A Highway B R = 1300

9. 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

10. 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

11. 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

12. 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.

13. 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.

15. Looks and feel like a 40 mph road

16. 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

17. 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

18. 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

19. 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 herehere Target Speed Context Time versus System Time Design Speed Approach Target Speed Approach

20. Where should target speed be used? In most urban situations For more information on speed and design please click herehere Design Speed Approach Target Speed Approach Target Speed Context Time versus System Time

21. 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