[3] 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).
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: Where: 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.