Explain the Perception Reaction Time

A significant variable in the successful processing and use of information is the speed with which this is done. Perception-Reaction Time (PRT) is a human factor often cited by traffic engineers concerned with safety. PRT is "the interval between the appearance of some object or condition in the driver's field of view and the initiation of a response" such as braking or changing course. Note that PRT involves the initiation of a response (e.g. pressing the brake), not the completion of the vehicle maneuver (stopping).

PRT depends on the situation. Response time is generally quickest when there is one specific response to be made to a single stimulus (brake lights of vehicle ahead). In the case of "choice reaction time," in which there is more than one stimulus and/or more than one possible response (e.g. toll plaza), reaction time increases as a function of the number of possibilities. A driver may, for example, have to decide whether to steer or brake, or both, to avoid a pedestrian. The PRT used for design standards by AASHTO includes 1.5 sec for perception and decision, 1.0 sec for making a response, for a total of 2.5 sec, which is generally considered adequate for all but the most complex driving situations.