The paradox of driving speed: two adverse effects on highway accident rate

Whereas speeding is known to be a substantial risk factor in driving, there is no unequivocal evidence that accident rate on limited-access motor highways is considerably affected by average speed or by speed limits meant to regulate it. It is suggested here that the seeming puzzle actually may have a straightforward explanation: accident-prone interactions (APIs) between cars occur when they pass each other--mostly moving in the same directions or in opposite ones. Such interactions are shown here to happen more frequently, the lower average speed is. To the extent that high speed limits contribute to increase in average speed, they serve to reduce the number of such interactions, thereby to moderate at least part of the negative effect of speed on the driver's ability to avoid an impending accident.