Perception-Response Time to Unexpected Roadway Hazards

Perception-response (PR) time, the time from the first sighting of an obstacle until the driver applies the brakes, is an important component of stopping sight distance. The purpose of this study was to measure the PR time of unalerted subjects to an obstacle in their lane encountered while cresting a hill. Data were obtained from 64 subjects, of whom 49 were young and 15 older. Measures were made of the time from first sighting of the obstacle until the accelerator was released, as well as accelerator-to-brake time. The results indicate a 95th percentile PR time of about 1.6 s for both age groups.