STOPPING SIGHT DISTANCE PARAMETERS.

A review of stopping sight distance parameters has recently been completed. AASHTO currently recommends a driver perception-response time of 2. 5 sec and currently uses braking distances based on locked-wheel skidding on poor-condition wet pavement surfaces. It was concluded that if a vehicle with minimum legal tire tread is to be stopped in its own lane on a wet pavement of this type, braking distances should be increased. Lowering the driver eye height to 40 in. from the current AASHTO value of 42 in. would accommodate more than 95 percent of the automobile driver-vehicle combinations expected to be in use late in this decade. Such a change was recommended because a 42-in. eye height would not accommodate 25 percent of the vehicles. No research on the appropriate height of the object was performed. Several horizontal- and vertical-curve geometric design aids based on derivations made in the research are presented.