Stopping Distance Formula

Perception Distance
+ Reaction Distance
+ Brake lag Distance (with air brakes)
+ Braking Distance

Perception distance is the distance your truck travels from the time you spot a hazard to the time you realize it may be a problem.

Reaction distance is the distance your truck travels as you move your foot from the accelerator and depress the brake pedal. Average reaction time is 3/4 of a second. In that time, driving at 30mph will result in 33 feet of distance traveled.

Brake lag distance is the distance your truck travels after you apply the brakes but before they actuate. The brake lag can take 1/2 second or more.

Braking distance is the distance your truck travels after the brakes take hold. Good brakes and tires are essential to your braking distance. This distance changes on gravel roads, inclined roads, and icy or wet roads.

This formula, devised by traffic specialists show the factors that contributes to the total stopping distance. Results of this formula will vary from driver to driver. Age, alertness, road conditions, and speed are just some of the variables that influence the outcome of the total stopping distance. A vehicle traveling 65mph on the interstate travels 95 feet a second.
Lets say that a driver named Fred is traveling at 65mph on the interstate when an accident occurs ½ a mile on up the road. If he takes ½ a second for his perception time, ¾ of a second on his reaction time, ½ a second on the brake lag, and had a braking distance of 300 feet, what is his total stopping distance?

Perception(47.5ft) + Reaction(71.3ft) + Brake lag(47.5ft) + Brake distance(300ft) =

466.3 ft total stopping distance, or 1.3 football fields.

For every second added to Fred’s reaction time, 95 feet are added to his total stopping distance. This should help illustrate why it is so important to pay attention at all times. One second could be the difference between a near-miss and a collision.