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Synthesis of Safety Research Related to Speed and Speed Management - Turner-Fairbank.. Page 1 of- 25 INTRODUCTION This document provides a review of safety research related to speed and speed management. This review builds upon a similar synthesis prepared in 1982. This synthesis highlights the relationships among vehicle speed and safety; factors influencing speeds; and the effects on speed and crashes of speed limits, speed enforcement, traffic calming and other engineering measures intended to manage speed. Despite the substantial social and technological changes that have occurred since the original speed synthesis was published, vehicle speed remains an important public policy, engineering, and traffic safety issue. Speed is cited as a related factor in 30 percent of fatal crashes and 12 percent of all crashes (Bowie and Walz, 1994). Based on on-scene investigations of over 2,000 -crashes in Indiana by teams of trained technicians, excessive speed for conditions was identified as the second most frequent causal factor out of approximately 50 driver, vehicle, and environmental factors (Treat et al., 1977). Excessive vehicle speed reduces a driver's ability to negotiate curves or maneuver around obstacles in the roadway, extends the distance necessary for a vehicle to stop, and increases the distance a vehicle travels while the driver reacts to a hazard. The following pages present the results of a systematic review of the literature concerning safety research related to speed and speed managemenf. Initial listings of citations were generated using multiple keyword filters on several bibliographic databases. The most productive databases were those of the National Technical Information Service (NTIS), the Knight-Ridder Transportation Resources Index, and the Transportation Research Information Service (TRIS). The initial inventory of approximately 700 citations was supplemented by searches of the Institute of Transportation Engineers (ITE) index and more than 100 items that either predated the on-line data bases or otherwise were known to be pertinent. SPEED-SAFETY RELATIONSHIPS Speed is the quintessential traffic safety issue, probably due to the clearly perceived relationship between vehicle velocity and human capabilities and limitations. Even inexperienced drivers usually recognize the merit of reducing their speed in uncertain or hazardous conditions to provide additional time for decision-making and action; driving experience affirms this natural tendency for self-preservation. Good judgment, however, is not uniformly applied by the operators of motor vehicles, nor are skills and abilities possessed in equal measure by all drivers. For these reasons, vehicle speed could be related to traffic safety in two ways: (1) the greater a vehicle's velocity the less time available for the operator to react to a hazard or for other motorists, bicyclists, or pedestrians to react to the vehicle; and (2) the physical relationship of mass and speed to energy. If the first relationship exists, it would be expressed in the relative incidence of crashes at different speeds. If the second relationship exists, it would be expressed in the relative severity of crashes at different speeds. Research concerning these relationships is reviewed in the following paragraphs. Speed and the Incidence of Crashes In a landmark study of speed and crashes involving 10,000 drivers on 600 miles (970 kilometers) of rural highways, Solomon (1964) found a relationship between vehicle speed and crash incidence that is illustrated by a U-shaped curve. Crash rates were lowest for travel speeds near the mean speed of traffic, and increased with greater deviations above and below the mean. The estimated travel speed from the accident records were compared to the speeds measured at representative sites within each study section. The comparisons showed that crash-involved drivers were over-represented in both high- and Iow- speed categories of the speed distribution. Crash-involvement rates decreased with increasing speeds up to 65 mi/h (105 km/h), then increased at higher speeds. Further, Solomon reported that the results of his study showed that "low speed drivers are more likely to be involved in accidents than relatively high speed drivers." Cirilto (1968) in a similar analysis of 2,000 vehicles involved in daytime crashes on interstate freeways confirmed Solomon's results, extending the U-shaped curve to interstate freeways, as http://www.ntl.bts.gov/ntl/DOCS/speed/speed.htm 07/19/2000