Stopping sight distance is the distance traveled during the two phases of stopping a vehicle: perception-reaction time (PRT), and maneuver time (MT). Perception-reaction time is the time it takes for a road user to realize that a reaction is needed due to a road condition, decide what maneuver is appropriate (in this case, stopping the vehicle), and start the maneuver (taking the foot off the accelerator and depressing the brake pedal). Maneuver time is the time it takes to complete the maneuver (decelerating and coming to a stop). The distance driven during perception-reaction time and maneuver time is the sight distance needed.
The design standards of the American Association of State Highway and Transportation Officials (AASHTO) allow 1.5 seconds for perception time and 1.0 second for reaction time.
The values of stopping sight distance used in design represent a near worst-case situation. For design, a conservative distance is needed to allow a vehicle traveling at design speed to stop before reaching a stationary object in its path. A generous amount of time is given for the perception-reaction process, and a fairly low rate of deceleration is used. The design sight distance allows a below-average driver to stop in time to avoid a collision in most cases.
Driver perception/reaction distance is calculated by:
Based on the results of many studies, 2.5 seconds has been chosen for a perception-reaction time. This time will accommodate approximately 90 percent of all drivers when confronted with simple to moderately complex highway situations. Greater reaction time should be allowed in situations that are more complex.
Braking distance is calculated by: