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Synthesis of Safety Research Related to Speed and Speed Management - Turner-Fairbank.. Page 8 of' 25 Several studies have explored the speed adaptation hypothesis. For example, Denton (1976) found that drivers who had traveled at 70 mi/h (113 kin/h) for three minutes tended to drive 5 to 15 mi/h (8 to 24 kin/h) faster in a 30 mi/h (48 km/h) zone than drivers who had not previously driven at the faster speed. Casey and Lund (1967) found a lesser, but more persistent, effect when drivers made the transition from 55 mi/h to 35 mi/h zones (88.5 to 56.3 kin/h). Vehicle speeds on streets and roadways leading from highways and freeways were greater than the speeds approaching the highways and freeways, even though the posted speed limits are the same. The review of speed-related issues prepared by Fildes and Lee (1993) for the Australian Federal Office of Road Safety describes the cognitive aspects of speed perception. In particular, the authors summarize how the visual pattern that is presented to a moving observer creates a blur of increasing magnitude at greater deviations from the fixation point. This "retinal streaming" provides cues that are used to help estimate speed. Human capabilities, however, are limited in this regard. Most research on the topic has found that drivers underestimate their speeds, especially at the medium and high speed ranges. Further, research has found perceptual limitations that contribute to drivers underestimating the curvature of an approaching bend (Shinar, 1977). Brummelaar (1983) and Fildes (1986) identified road curve features that influence a driver's perception of curvature. Environmental Conditions Weather conditions influence the vehicle speed selected by most drivers. For example, reduced visibility due to fog caused a 6 mi/h (10 km/h) decline in mean speeds on a freeway in Minnesota (CRC, 1995). Greater reductions in speed can be observed under extreme conditions (Schwab, 1992). Although drivers reduce their speeds during poor environmental conditions, this reduction is often accompanied by higher variation in speeds. Liang et al. (1998) in an analysis of speeds on a rural freeway in Idaho found the standard deviation of speed doubles during fog events and triples during snow. The researchers also found that drivers reduce their speeds an average of 0.7 mi/h for every mi/h that the wind speed exceeds 25 mi/h or 0.4 km/h for every 1 km/h that wind speed exceeds 40 km/h. Although wet road surfaces will affect traction when attempting to stop, pass, or negotiate a curve or turn, most drivers do not reduce their speeds very much when traveling on wet roads. Olson et al. (1984) compared speed data collected during daylight hours on wet and dry days at 22 sites in Illinois and found no practical differences. The maximum difference in speed was less than 2.5 mi/h (4km/h). Similarly, Lamm et al. (1990) found no differences in operating speeds on dry and wet pavements for 11 curves studied on two-lane rural roads in New York. Although light rain had little effect on speeds, Ibrahim and Hall (1994) observed 3 to 6 mi/h (5 to 10 km/h) reductions during periods of heavy rain. SPEED LIMITS AND SPEEDS In a survey of speed zoning practices, Parker (1985) found that all states and most local agencies consider the speed of traffic in setting speed limits. The primary factors considered in engineering studies to set speed limits were, in order of their importance: · 85th percentile speed. · Type and amount of roadside development. · Accident experience. · Adjacent Limits. · 10 mi/h pace (i.e., speed range that contains the largest percentage of vehicles). · Horizontal and vertical alignment. · Design speed. · Average test run speed. · Pedestrians. http://www.ntl.bts.gov/ntl/DOCS/speed/speed.htm 07/19/2000