The Role of Speed for Street and Highway Design

1. The Role of Speed for Street and Highway Design Norman W. Garrick Lecture 2.1 Street and Highway Design

2. Speed and The Role of the Designer Speed is perhaps the most important parameter for the design of both streets and highways As we saw in the speed survey, design has a huge impact on the speed chosen by drivers The role of the designer with respect to designing for speed varies depending on if we are designing for context time or system time

3. Context Time Social Behavior Governs Multi-functional Culturally defined Personal Diverse Unpredictable The design should be such that the speed of vehicles are under control In many Europe cities the maximum speed in built-up areas is 18 mph This is not the tradition in the USA, but we are moving in that direction In any case, the design should be consistent with the desired speed

4. System Time Traffic Behavior Governs Single purpose Regulated Impersonal Uniform Predictable The design should be such that vehicles can operate safely at high speeds In system time the operating conditions needs to be uniform and predictable for safe high speed operations

5. Speed and Traffic Safety Speed affect the severity of crashes Not necessarily the number of crashes

6. Chance of Pedestrian Fatality vs. Impact Speed http://webarchive.nationalarchives.gov.uk/+/http://www.dft.gov.uk/foi/responses/2005/nov/203040message/paperaboutthedepartments20302445

7. Annual US Traffic Fatalities Courtesy of Jim Charlier 52,627 50,000 50,000 40,000 40,000 30,000 33,808 30,000 20,000 20,000 10,000 10,000 12,155 ‘20 ‘30 ‘40 ‘50 ‘60 ‘70 ‘80 ‘90 ‘00 ‘08 ‘09 Source: NHTSA, FHWA

8. Traffic Fatalities in USA: 1970 to 2010 USA 1970 205 million All Others 1970 417 million USA 2010 309 million All Others 2010 523 million

9. Changes in Traffic Fatality Rate in 16 OECD Countries

10. Changes in Traffic Fatality Rate in 16 OECD Countries

11. Changes in Traffic Fatality Rate in 16 OECD Countries

12. Speed and Flow

13. How Do We Talk about Speed in Street and Highway Design?

14. What are some terms that are used for characterizing speed? • How do we conceptualize speed for design?

15. Common Description of Speed • Operating Speed • Running Speed • Design Speed • Target Speed

16. Is design speed equal to running speed? … or operating speed? … or target speed? What should be the relationship between these various types of speed?

17. AASHTO Definition of Speed AASHTO defines three different types of speed • Operating Speed • Running Speed • Design Speed

18. AASHTO DefinitionOperating Speed The speed at which drivers are observed operating their vehicle during free-flow conditions In other words, this is the chosen speed of the driver on that point in the road way (not encumbered by congestion or weather or other adverse conditions) This is the speed that I am trying to get at when I ask – What speed would you choose? Typically AASHTO suggest using 85 percentile observed speed to represent the operating speed

19. AASHTO DefinitionRunning Speed The speed at which an individual vehicle travels over a highway section The running speed is not necessarily in the control of the driver since it is affected by congestion and weather What is the relationship between operating speed and running speed for the same section of roadway?

20. AASHTO DefinitionDesign Speed A selected speed used to determine the various geometric design features of the road way Say what? To understand what design speed is, we need to look into i) how it is selected, iii) what it is used for, and iii) how it is used It is a deceptively complex concept that is essential to parse in order to understand how AASHTO design works