Many proponents of intelligent vehicle highway systems (IVHS) believe that the technology can reduce crashes by providing an early warning of imminent danger or by automatically executing crash avoiding action. It has been suggested by Enke in 1979 that such action taken a half-second to one second earlier could prevent roughly half of all vehicle-to-vehicle collisions. This paper reviews Enke's analysis showing that it is based on several implicit and explicit assumptions that raise doubts about the viability of IVHS in reducing motor vehicle crashes. The authors examine the half-second hypothesis which was arrived at as the results of a simple mathematical calculation applied to physical characteristics of cars and crashes. The average deceleration assumed is based on braking in typical road conditions and will not apply to downward slopes or in slippery conditions. It is also suggested that the speed has been reduced by half at the time of the crash, whereas there are often circumstances where the driver can be accelerating at the time of the crash. Often sight distances are very short, and collisions then develop too quickly to permit vehicles to avoid them.
Abstract