This paper describes a field study conducted to evaluate drivers' actual headways in car-following situations, their relationship to drivers' brake reaction times, and their relationship to the drivers' ability to estimate headways using different metrics. Drivers were asked to maintain "minimum safe distance" and "comfortable, normal distance with no intention to pass" behind the car ahead. Results show that under both sets of instructions, drivers adjusted their distance headways in relation to speed, maintaining constant time headways. A significant portion of the drivers maintained time headways that are considered unsafe in relation to reaction times. There was no significant relationship between minimal headways maintained by drivers and their brake reaction times under conditions of maximum attention/readiness to apply brakes. Accuracy of spoken estimates of headways varied widely among the 3 measures used to report perceived headway; meters and car lengths yielded much lower estimates than did seconds. The results have implications for headway perception, driving safety and education, and smart cruise-control design.
Abstract