Insufficient sight distance can adversely affect the safety or operations of a roadway or intersection.
[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).
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.