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Braking distance refers to the distance a vehicle will travel from the point when its brakes are fully applied to when it comes to a complete stop. It is primarily affected by the original speed of the vehicle and the coefficient of friction between the tires and the road surface, [Note 1] and negligibly by the tires' rolling resistance and vehicle's air drag.
A vehicle accelerating from a stop travels this distance in time t i = √ 2d i ⁄ a i while through traffic travels a distance equal to their speed multiplied by that time. The time t pc , for the stopped motorist, is the sum of perception time and the time required to actuate an automatic transmission or shift to first gear which is usually ...
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: d PRT = 0.278 Vt (metric) d PRT = 1.47 Vt (US customary) Where:
Extra time should be added for wet, rainy, slippery, foggy or other weather situations accordingly. [8] For heavy duty commercial vehicles it is recommended 4-6 seconds following distance for speeds under 30 mi/h (48 km/h), and 6-8 seconds following distance for speeds over 30 mi/h (48 km/h). [ 9 ]
The three second rule is a time for the defensive driver to judge the minimum safe trailing distance to help avoid collisions under ideal driving conditions. The red car's driver picks a tree to judge a two-second safety buffer. The two-second rule is a rule of thumb by which a driver may maintain a safe trailing distance at any speed.
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