The Commercial Drivers License (CDL) Study Guide

Speed and Stopping Distance



There are three things that add up to total stopping distance: Perception Distance + Reaction Distance + Braking Distance = Total Stopping Distance

Perception distance.

This is the distance your vehicle travels from the time your eyes see a hazard until your brain recognizes it. The perception time for an alert driver is about 3/4 second. At 55 mph, you travel 60 feet in 3/4 second.

Reaction distance.

The distance traveled from the time your brain tells your foot to move from the accelerator until your foot is actually pushing the brake pedal. The average driver has a reaction time of 3/4 second. This accounts for an additional 60 feet traveled at 55 mph.

Braking distance.

The distance it takes to stop once the brakes are put on. At 55 mph on dry pavement with good brakes it can take a heavy vehicle about 170 feet to stop. It takes about 4 1/2 seconds.

Total stopping distance.

At 55 mph it will take about 6 seconds to stop and your vehicle will travel about the distance of a football field. (60 + 60 + 170 = 290 feet).

The Effect of Speed on Stopping Distance.

Matching Speed to the Road Surface

Identifying Slippery Surfaces.


Speed and Curves

Speed and Distance Ahead

Speed and Traffic Flow

Speed on Downgrades



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