Misperception of time-to-collision by drivers in pedestrian accidents.

The approach of an object can be monitored from its optic flow. More specifically, it has been postulated that time-to-collision at constant velocity is perceived by relating visual angle theta to its rate of change theta, time-to-collision being theta/theta. This hypothesis is reappraised, and an alternative based on the parameters theta and angular acceleration theta is proposed. The expression 2 theta/theta also specifies time-to-collision, with the benefits that it removes reliance on theta and permits time-to-collision to be determined from even momentary perception of an approaching point. This is supported by tests in which subjects responded to computer simulations of approaching objects. A further benefit is that if the object is accelerating rather than at constant velocity, time-to-collision is adjusted by 2 theta/theta, but not by theta/theta. As time-to-collision increases, however, its cognitive derivation should transfer from optic flow to separate perceptions of distance and speed. It is proposed that when drivers of road vehicles are in potential collision with pedestrians their perception of distance is based primarily on familiar size, resulting in overestimation of size and therefore of time-to-collision with child pedestrians. This is supported by further computer simulations and is corroborated by predicting the effect of that overestimation on certain types of accidents, then testing from national accident statistics. These analyses indicate that drivers' misperception of time-to-collision has a dramatic effect on the accident rate of child pedestrians. It is proposed that this could be greatly reduced by the provision of remedial measures.