Truck Accident News

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. The relationship between travel speed and the severity of injuries sustained in a crash was examined by Solomon (1964), who reported an increase in crash severity with increasing vehicle speeds on rural roads. From an analysis of 10,000 crashes, Solomon concluded that crash severity increased rapidly at speeds in excess of 60 mi/h (96 km/h), and the probability of fatal injuries increased sharply above 70 mi/h (112 km/h).

Bowie and Waltz (1994), in an analysis of tow-away crashes reported in the National Accident Sampling System over a 7-year period, found that the chance of being injured in a crash depended on the change in speed at impact (delta V) . As shown in table 1, the risk of a moderate or more serious injury was less than 5 percent when delta V was less than 10 mi/h (16 km/h) and increased to more than 50 percent when delta V exceeds 30 mi/h (48 km/h). The risk of a fatality begins to rise when the change in speed at moment of impact exceeds 30 mi/h (48 km/h) and is more than 50 percent likely to be fatal when the change exceeds 60 mi/h (96 km/h). The probability of death from an impact speed of 50 mi/h (80 km/h) is 15 times the probability of death from an impact speed of 25 mi/h (40 km/h). The relationship between impact speed and crash severity is particularly critical for pedestrians, the most vulnerable road users. In a recent review of the issues, the European Transport Safety Council (1995) report that only 5 percent of pedestrians died when struck by a vehicle traveling at 20 mi/h (32 km/h); however, the proportion of fatalities increased to 45 percent at 30 mi/h (48 km/h) and to 85 percent at 40 mi/h (64 km/h).

Kloeden et al. (1997) compared the estimated speeds of over 150 cars involved in non-alcohol related injury crashes in 60 km/h speed zones in Australia with the free speed of cars measured at the same location at the same time of day and day of week. The pre-crash traveling speeds were based on detailed investigations of each crash scene and computer-aided crash reconstruction. The average and median speed of traffic was about 60 km/h ( 37 mi/h). Vehicles exceeding the 90th percentile speed or traveling more than 7 km/h faster (4 mi/h) than the speed limit and median speed had above average injury crash involvement rates. Nearly 25 percent of the cars involved in injury crashes were traveling faster than 72 km/h (45 mi/h) compared to only 2 percent of free flow traffic. Clearly, a research or engineering approach to speed management that ignores the injury consequences of vehicle speed could lead to unintended results.

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	Did You Know?
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