Driver Risk and Perception
Drivers’ speed choices impose risks that affect both the probability and severity of crashes. Speed is directly related to injury severity in a crash. The probability of severe injury increases sharply with the impact speed of a vehicle in a collision, reflecting the laws of physics. The risk is even greater when a vehicle strikes a pedestrian, the most vulnerable of road users. Although injury to vehicle occupants in a crash can be mitigated by safety belt use and air bags, the strength of the relationship between speed and crash severity alone is sufficient reason for managing speed.
Speed is also linked to the probability of being in a crash, although the evidence is not as compelling because crashes are complex events that seldom can be attributed to a single factor. Crash involvement on interstate highways and non-limited-access rural roads has been associated with the deviation of the speed of crash-involved vehicles from the average speed of traffic. Crash involvement has also been associated with the speed of travel, at least on certain road types. For example, single-vehicle crash involvement rates on non-limited-access rural roads have been shown to rise with travel speed.
Speeding is a pervasive behavior, with about three-quarters of drivers reporting in a recent national survey they drove over the speed limit on all types of roads within the past month, and one-quarter reporting speeding over the limit on the day of interview.4 Speed data collected by the Federal Highway Administration (FHWA) indicates that, on average, 70 percent of motorists are exceeding the posted speed limits.5
According to the National Survey of Distracted and Drowsy Driving Attitudes and Behaviors: 2002,6 drivers believe they can travel between 7-8 mph over the posted speed limit, on average, before police would normally give them a ticket. However, they believe the tolerances should be 2-3 mph higher, suggesting that speed limits are about 10 mph below what motorists believe to be appropriate. However, when questioned directly, 83 percent of the drivers say speeds limits are about right on city and neighborhood streets and 61 percent say the same for interstate freeways.
Many of the drivers surveyed believe that the speed limits on interstates should generally be higher, that they would drive faster than the speed limit even if the limits were raised, and that they themselves speed at least sometimes. However, 68 percent of drivers feel that other drivers’ speeding is a major threat to their own personal safety. More than three-quarters of drivers feel that it is at least somewhat important that something be done to reduce speeding on all road types. This suggests a strong “it’s not me, it’s the other guy who is a problem” mentality among many drivers.Speeding is not only a problem in the United States, but has been identified by many countries as a key risk factor in road traffic injuries. In other industrialized countries, speed is a factor in around 30 percent of highway deaths, a rate which is similar to the United States.
Internationally, the World Health Organization has identified a number of interventions synthesized from international practice that are effective in the management and control of vehicle speed7:
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Setting and enforcing speed limits are two of the most effective measures in reducing road traffic injuries.
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Posted speed limits will only have a minimal effect on reducing travel speeds unless accompanied by sustained, visible enforcement of these limits.
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Speed cameras are a highly cost-effective means of reducing road crashes.
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Variable speed limits are responsive to local conditions and traffic circumstances, and are therefore more likely to be obeyed.
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Speed levels can also be affected by developing a safer infrastructure.
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Traffic-calming measures can be particularly useful where enforcement of speed control laws may be ineffective.
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Design features used to mark transition zones on busy roads approaching towns and villages can influence drivers’ speed. Slower-speed zones and modern roundabouts are examples of features that are useful in reducing the speed of vehicles.
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Appropriate speed can be achieved through design features that limit the speed of the vehicle itself. This is already being done in many countries with heavy load vehicles and coaches.
The issue of what constitutes appropriate driving speeds will persist as long as there are individual drivers making choices about risk and time efficiency. Ultimately, decisions about appropriate speed limits depend on judgments about society’s tolerance for risk, valuation of time, and willingness to police itself.
Technological advances may offer additional techniques for controlling driving speeds on all types of roads. For example, technology could help establish limits that are more sensitive to actual changes in road conditions and thus provide drivers with better information. With modern vehicles becoming quieter and more comfortable at higher speeds, technology installed on the roadside or onboard vehicles could alert drivers and control vehicle speeds that are approaching the design limits of the road. Finally, technology could help improve the efficiency, effectiveness and safety of enforcement efforts. Further development, demonstration, and evaluation are needed for many technologies to realize their potential.4. Dawn Royal, National Survey of Speeding and Unsafe Driving Attitudes and Behavior: 2002, Volume II, (DOT HS 809 688), October 2003, NHTSA, Washington, DC.
5. D. L. Harkey, H.D. Robertson, and S.E. Davis, Assessment of Current Speed Zoning Criteria, Research Record 1281, Transportation Research Board, Washington, DC, 1990.