Calculating Stopping Distance in Semi-Trucks

For all of us that drive a sedan or SUV every day, we have a pretty good understanding of how long it takes our vehicle to stop. Obviously, certain things like speed and road conditions will affect how quickly we are able to bring the vehicle to a complete stop. As you might expect, stopping a fully loaded, 80,000-pound tractor-trailer is a completely different ballgame.

How much distance does a truck driver need to come to a complete stop?

New truck drivers can find guidance in their state’s CDL manual. For example. the Illinois 2020 CDL Manual uses the following formula to teach stopping distance to CDL applicants: Perception Distance + Reaction Distance + Braking Distance = Total Stopping Distance.

Assuming perfect conditions, perception distance is the distance your vehicle travels from the time your eyes see a hazard until your brain recognizes it. According to the CDL Manual, the average perception time for an alert driver is 1 3/4 seconds.

At 55 mph, this accounts for 142 feet traveled. Obviously, one of the key factors in stopping any vehicle in time is being alert and attentive. While attentive driving will not reduce perception time, it will greatly improve your stopping distance by identifying hazards quicker.

When your brain identifies a hazard, the message must be sent through the nervous system down to your foot telling it to move off the accelerator and press the brake. Your reaction distance is the distance your vehicle travels during that time.

The Illinois manual estimates that the average driver has a reaction time of a three-fourths second to one second. At 55 mph, this accounts for 61 feet traveled.

Finally, once you depress the brake, the car doesn’t stop immediately. The braking distance represents the distance your vehicle travels from the time you first depress the brake until the vehicle comes to a complete stop.

Add up all of those distances, and you will have your total stopping distance. Assuming perfect brakes and road conditions, the minimum stopping distance for a truck to be brought to a complete stop is 419 feet.

Other conditions may increase stopping distance

Outside of a truck driver’s own perception and reaction, there are many variables that will affect his ability to stop his truck.

Speed

In general, the faster you drive, the longer it will take you to stop. When you double your speed from 20 to 40 mph, the braking distance is four times longer. When you triple your speed from 20 mph to 60 mph, the braking distance is nine times longer.

Weight

The weight of the truck will also have a significant impact on braking distance. The heavier the vehicle, the more work the brakes must do to stop it, and the more heat they absorb. The brakes, tires, springs, and shock absorbers on heavy vehicles are designed to work best when the vehicle is fully loaded. Though it may seem counterintuitive, empty trucks require greater stopping distances because an empty vehicle has less traction.

Weather

Finally, road conditions can significantly increase braking distance. Slippery surfaces can cause skidding which can double braking distance.

Because stopping a fully loaded semi-tractor trailer is so different than your average sedan, it is essential that trucking companies utilize a robust training program. This program must include behind-the-wheel training in the type of vehicle the driver will be expected to drive to teach their drivers how to safely operate their trucks.

Before a driver is handed the keys to a large truck, the driver must be able to show that he or she understands safe following distances and braking factors.