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Synthesis of Safety Research Related to Speed and Speed Management - Turner-Fairbank.. Page 4 of' 25 If conflicts created by large differences in travel speeds were a major factor in the likelihood of crashes, then one might expect to find a large number of crashes involving two or more vehicles traveling in the same direction. Cerrilli (1997) found less than one-third of all crashes and 5 percent of all fatal crashes in 1996 involved two or more vehicle traveling in the same direction. Many of these likely occurred as a consequence of a vehicle slowing or stopping for cause (i.e., to make an intended maneuver or avoid striking a stopped vehicle or other hazard) and being struck from behind by a vehicle following too closely or going too fast for the driver to stop in time to avoid the collision. By far, the predominant crash type on rural roads is a single vehicle running off the road. In a review of the issues associated with speed and traffic safety, Fildes and Lee (1993) reported that little research was conducted concerning the relationship between speed and crash involvement during the 1970s and 1980s. Lave (1985) revived the issue of speed variance as a contributor to crashes, suggesting that raising the speed limit would result in fewer crashes in situations where variance was reduced by the higher limit. Lave concluded that "speed limits designed to reduce the fatality rate should concentrate on reducing variance. This means taking action against slow drivers as well as fast ones." Similarly, Garber and Gadiraju (1988) reported that crash rates increased with increasing variance on all types of roadways and that speeds were higher on roads with higher design speeds, irrespective of the posted speed limits. They reported minimal variance when the posted speed limit was fewer than 16 km/h (10 mi/h) below the design speed of the road. In the analysis, the researchers combined data from different road types (e.g., rural two-lane, urban freeway, and rural freeway) which could lead to spurious results. n Da¥($olormn, 1984)l-brkeyet al. (lggo) L ....... .............. · N0htCSolorron, 19§4) ~ ~standDJnn(1971)/ Ill~ilo (1968) Hauer (1971) ! · · -22.5 -17.5 -12.5 -7.5 -2.5 25 7.5 12.5 17.5 22.5 Deviation from mean speed, mi~h Figure 4. Crash involvement and over, king rates relative to average rate and speed. Speed And The Severity Of Crashes The relationship between vehicle speed and crash severity is unequivocal and based on the laws of physics. The kinetic energy of a moving vehicle is a function of its mass and velocity squared. Kinetic energy is dissipated in a collision by friction, heat, and the deformation of mass. Generally, the more kinetic energy to be dissipated in a collision, the greater the potential for injury to vehicle occupants. Because kinetic energy is determined by the square of the vehicle's speed, rather than by speed alone, the probability of injury, and the severity of injuries that occur in a crash, increase exponentially with vehicle speed. For example, a 30-percent increase in speed (e.g., from 50 to 65 mi/h [80 to 105 km/h]) results in a 69-percent increase in the kinetic energy of a vehicle. http://www.ntl.bts.gov/ntl/DOCS/speed/speed.htm 07/19/2000