Traffic speeds are relevant and of interest to nearly everyone. Our preferences and judgments of appropriate speed are strongly influenced by setting and perspective. The speed we prefer as a driver or passenger traveling long-distance along a rural Interstate highway is much different than what we prefer as a pedestrian crossing an urban street. Context is everything.

Speeds have fuel consumption, emissions and traffic noise consequences. Although these are important matters, most of the public interest and agency actions related to traffic speeds involve consideration of mobility and safety.

SPEED MOBILITY RELATIONSHIP

Speed is used directly and indirectly as a measure of mobility. For motorists, shippers and the commercial transportation industry, high operating speeds are considered desirable. Several other mobility measures (e.g., delay and travel time) are also linked to speed. For example, travel time along a specified route can be determined on the basis of distance and average speed. For this reason, facilities that emphasize mobility (e.g., Interstate highways and other freeways) accommodate high speed.

SPEED SAFETY RELATIONSHIP

The effects of speed on safety are complex and only partially known. Despite a long and sustained research effort, only a limited number of consistent and reliable speed-safety relationships have been established, including:

• Many factors affect highway safety.
• Safety is fundamentally dependent on successful human performance.
• Relationships of safety performance to specific driver-related factors (e.g. education, impairment, perception-reaction time, visual acuity) have been investigated.
• Studies have also investigated the impacts of vehicle characteristics (e.g., size, weight, braking), roadway type and configuration, traffic conditions (e.g. vehicle, pedestrian and bicycle volumes) on crash probability and severity.
• Complicated interactions between human and non-human subsystems have made it difficult for researchers to isolate the effect of speed or other selected factor(s) on safety.

A clear understanding of how speed affects safety has also been obscured by the use of different speed-related indicators and a wide range of study methods. Different studies have evaluated the safety effects of speed limits, operating speeds and design speed, with some studies making the unsubstantiated assumption that these characteristics are related to each other. Many of the speed related studies are based on data from high-speed highways. Too often, the results and conclusions of these studies are inappropriately applied to all roads and streets. Despite the complexity and challenge of understanding speed-safety relationships, useful research conclusions have been reached. Key research studies and conclusions are summarized below.

Effect of Speed on Crash Severity

There is clear and convincing evidence that crash severity increases with individual vehicle speed. This finding is supported by theory and statistical analysis.

A vehicle’s kinetic energy is proportional to its velocity squared. When a crash occurs, all or part of the kinetic energy is dissipated, primarily through friction and mass deformation. As kinetic energy increases exponentially with speed, so does the potential for mass deformation, including humans that are inside and outside of the vehicle. Analysis of crash statistics (2) have shown that the probability of being injured in a crash increases as the change in speed at impact increases, particularly when this change in speed occurs over a short time duration.

A direct and positive relationship has been found between speed and the severity of pedestrian injury in vehicle-pedestrian crashes. The results of one United Kingdom study (3) are shown in the second column of table 1. A different study (4), conducted in the United States, also found a positive correlation between speed and severity of pedestrian injuries. The results, stratified by pedestrian age group are presented in columns three through five of table 1.

Table 1. Probability of pedestrian death resulting from various vehicle impact speeds.

*Source: Ref (3); ** Source: Ref (4)