Traffic speed is a subject of considerable interest and importance to transportation and traffic enforcement agencies, safety advocates, motorists, non-motorist street and highway users (e.g., pedestrians and bicyclists), residential and commercial property occupants and the public at-large. Speed is a controversial and complex subject. Even after significant research, many questions remain unanswered about speed relationships. The labels "good" and "bad" are difficult to apply to a particular speed condition since these terms are subjective and based on preferences, which often vary amongst different individuals and stakeholder groups.
Speed is often regarded as an indicator or measure of two different transportation performance characteristics, mobility and safety. Higher speeds generally translate to lower travel times, an indication of good mobility. The relationship between speed and safety is complicated and unclear. There are very few points of consensus on the effects of speed on crash probability. One of the many complicating factors is that high-speed highways (e.g., Interstates) have low crash rates. However, since Interstate highways also have distinguishing design features (e.g., limited access control and wide clear zones), it is difficult to separate the effects of speed from other characteristics. There is general agreement that the risk of injuries and fatalities increases with speed. So, even without a complete understanding, we know that conflicts can develop between the mobility and safety objectives.
Individual vehicle speeds are selected by drivers, a very large (over 200 million in the U.S.) and diverse population. They interpret and respond to signals, both explicit and implicit, in the driving environment. The roadway alignment, cross section, roadside, advisory speeds and speed limit are all elements of the driving environment that are thought to influence speed selection. Our knowledge is limited on how speed is affected by specific features, individually and in combination. However, it is known that different information sources (e.g., speed limit and roadway geometry) often send drivers different signals on appropriate speed. Speed management principles and techniques can be applied to clarify and unify the information being provided to drivers and to balance safety and mobility objectives.
The purpose of this publication is to help engineers, planners, and elected officials to better understand design speed and its implications in achieving desired operating speeds and setting rational speed limits. It answers the following questions:
- Is the design speed the maximum safe speed?
- Can you expect to achieve target operating speeds if you select a design speed above the target speed?
- Is it safe to set speed limits above the designated design speed?
- How can the design speed concept be applied to better achieve desired speeds?
- What are some engineering measures that could be used to mitigate potential problems associated with inconsistencies in design, operating and posted speeds?
This document is written such that it can be understood by a broad audience of interested readers. Extensive background knowledge is not needed. As a result, some subjects are not covered with the depth or precision customary in specialized research publications on speed-related subjects.