5.2. SR 07-24-2000 - Laserfiche WebLink

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Synthesis of Safety Research Related to Speed and Speed Management - Turner-Fairbank.. Page 7 of- 25 Of all drivers involved in fatal crashes, young males are the most likely to have speed as a collision factor. In 1995, nearly 40 percent of the fatal crashing involving male drivers 15 to 20 years old were speed related (NHTSA 1995). The relative proportion of speed-related crashes to all crashes decreases with increasing driver age. A recent study of the behavioral cues associated with driving while intoxicated (DWI) found that drivers who were exceeding speed limits by 10 mi/h (16 km/h) or more were DWI (BAC30.08) only in 9 percent of all nighttime enforcement stops, but those driving more than 10 mi/h (16 km/h) under the limit were found to be DWI in 48 percent of the stops (Stuster, 1997); driving under the speed limit does not include maneuvers that require slow speed. A previous study of motorcyclist DWI detection found that 10 percent of speeding motorcyclists have BACs of 0.08 or greater (Stuster, 1993). These probabilities of DWI are Iow compared to other behaviors, such as weaving, turning with a wide radius, or drifting during a curve (all with probabilities of DWI greater than 50 percent). Driving with excessive speed is a risk-taking behavior that often is found in association with other risk-taking behaviors. For example, in 1995, only 37 percent of passenger vehicle drivers under 21 years old who were involved in fatal crashes related to speed were wearing, safety belts at the time of the crash. In contrast, 56 percent of drivers in the same age group were properly restrained when speed was not a factor. For drivers 21 years and older, the percentage of drivers involved in speed-related fatal crashes who were using restraints at the time of the crash was 34 percent, but 62 percent of drivers were restrained in fatal crashes that were not speed related. Road Characteristics Road characteristics contribute to the speeds at which drivers operate their vehicles. Warren (1982) reported the most significant characteristics to be curvature, grade, length of grade, number of lanes, surface condition, sight distance, lateral clearance, number of intersections, and built-up areas near the roadway. Tignor and Warren (1990) reported that the number of access points and nearby commercial development are the factors that have the greatest influence on vehicle speeds. In contrast, Fildes et al. (1987, 1989) found road width and number of lanes to have the greatest influence on speed choice. More recently, the European Transport Safety Council (1995) reported that width, gradient, alignment, and layout, and the consistency of these variables, are the determinants of speed choice on a particular stretch of road. Road characteristics determine what is physically possible for a vehicle, but they also influence "...what seems appropriate to a driver." In this regard, individual perceptions of appropriate speed are influenced by the maintenance condition of the road. For example, Cooper et al. (1980) found that average vehicle speeds increased by 1.6 mi/h (2 km/h) after resurfacing major roads in the United Kingdom; no change in traffic speed was found in locations where surface unevenness remained the same after resurfacing. Parker (1997) found no change in speeds on two rural highways and a 3 mi/h (5 kin/h) increase on two urban streets that were resurfaced and had the speed limit raised. It was not possible to determine if the speed change was due to the higher speed limit or the resurfacing. Roadway surroundings, especially proximity of tall objects to the road, also can influence the speeds at which motorists choose to drive. Designing roadway features to influence driver perceptions of appropriate speeds is a subject that will be addressed briefly in a subsequent section of this report. The theory of speed adaptation predicts that apparent vehicle speed is influenced by the speed and duration of recent travel in the vehicle. This adaptation to vehicular speed is the combined result of the visual, auditory, and proprioceptive feedback associated with various rates of travel. Speed adaptation is a commonly experienced phenomenon that results in an under estimation of speed after encountering a reduced-speed zone (Schmidt and Tiffin, 1969; Mathews, 1978). In short, according to the speed adaptation hypothesis, the perceived speed of one's own vehicle will be lower than the actual speed if the driver has recently been operating the vehicle at a higher speed. http://www.ntl.bts.gov/ntl/DOCS/speed/speed.htm 07/19/2000