The Role of Speed for Street and Highway Design Norman W. Garrick Lecture 2.1 Street and Highway Design Norman W. Garrick Lecture 2.1 Street and Highway Design 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 Context Time Social Behavior Governs Multi-functional Culturally defined Personal Diverse Unpredictable The design should be such that the speed of vehicles is controlled In many European 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 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 Speed and Traffic Safety Speed affects the severity of crashes Not necessarily the number of crashes Chance of Pedestrian Fatality vs. Impact Speed Annual US Traffic Fatalities 20304050607080900008 50,000 40,000 30,000 20,000 10,000 12,155 52,627 50,000 40,000 30,000 20,000 10,000 Source: NHTSA, FHWA 33,808 09 Courtesy of Jim Charlier Changes in Traffic Fatality Rate in USA Changes in Traffic Fatality Rate in 17 OECD Countries Changes in Traffic Fatality Rate in 4 OECD Countries Sweden Netherland Germany USA Population 1970 USA 205 million All 16 Others Countries 417 million Population 1970 USA 309 million All 16 Others Countries 523 million Changes in Traffic Fatality: 1970 to 2010 Population 1970 USA 205 million All 16 Others Countries 417 million Population 1970 USA 309 million All 16 Others Countries 523 million Changes in Traffic Fatality: 1970 to 2010 USA versus 16 Other OECD Countries Speed and Flow How Do We Talk about Speed in Street and Highway Design? What are some terms that are used for characterizing speed? How do we conceptualize speed for design? Common Description of Speed Speed Limit Operating Speed Running Speed Design Speed Target Speed Is design speed equal to running speed? or operating speed? or target speed? What should be the relationship between these various types of speed? AASHTO Definition of Speed AASHTO defines three different types of speed Operating Speed Running Speed Design Speed AASHTO Definition Operating Speed The speed at which drivers are observed operating their vehicles during free-flow conditions In other words, this is the chosen speed of drivers at that point in the road way (not encumbered by congestion or weather or other adverse conditions) This is roughly what I am trying to get at when I ask What speed would the average driver choose? Typically AASHTO suggests using the 85th percentile observed speed to represent the operating speed AASHTO Definition Running 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? AASHTO Definition Design 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 conventional (AASHTO) design works We will discuss this in more details next class