How far your vehicle travels from when the eyes see a hazard until the brain recognizes it

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.

Speed and Stopping Distances

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 moves from the time your eyes see a hazard until your brain knows it. The perception time for an alert driver is about 3/4 of a second. At 55 mph you travel 60 feet in 3/4 of a 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 of a 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. (About 4 and 3/4 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).

 By the time your foot hits the brake, your vehicle has traveled about as many feet forward as the number of miles per hour you are traveling.

REACTION TIME: Use your foot to “cover the brake” as you proceed through an intersection. By covering the brake pedal, you significantly reduce the reaction time necessary to respond to a hazard and also allow the vehicle to slow by removing your foot from the accelerator. The “covering the brake” technique can be used effectively in the following situations: (1) When driving next to parked cars, (2) when you see the brake lights of other cars, and (3) when approaching intersections or signal lights.

If you are traveling at 50 miles per hour, it will take 50 feet just for you to react and put your foot on the brake. Of course, the stopping distance varies depending on the surface of the road, the condition of the brakes, and the skill of the driver in recognizing and reacting to hazards.

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Stopping a vehicle

Concepts and Research by Dale O. Ritzel, Ph.D., Safety Center, Southern Illinois University, Carbondale, IL 62901-6731

9 September 2003

Driving too fast is a major cause of crashes, injuries, and fatalities.  You must adjust your speed to suit weather conditions, the road (such as hills and curves), visibility and traffic.  Many persons drive in a false belief that if the car in front suddenly started braking, they would react and brake and end up stopped the same distance apart.

The total stopping distance of a vehicle is made up of 4 components.

·         Human Perception Time/Distance

·         Human Reaction Time/Distance

·         Vehicle Reaction Time/Distance

·         Vehicle Braking Time/Distance

• Human Perception time/distance is the distance your vehicle travels from the time your eyes see a hazard until your brain recognizes it. Perception time for an alert driver is about ¾ second. At 55 mph, you travel 60 feet in ¾ second (A good way to calculate this is to take 1.1 times the speed [in miles per hour] = perception distance in feet).
• Human Reaction time/distance is the distance traveled from the time your brain tells your foot to move from the accelerator until the time your foot pushes the brake pedal. An average driver reacts within ¾ second. This adds an additional 60 feet to the distance traveled at 55 mph (A good way to calculate this is to take 1.1 times the speed [in miles per hour] = reaction time in feet).
• Vehicle Reaction time/distance.  Once the brake pedal is applied there is the vehicles reaction time which depends on the brake pedal free-play, hydraulic properties of the brake fluid and working order of the braking system.  This time usually is from 0-¼ second (for purposes of this discussion, the vehicle reaction time/distance will be calculated as zero, however, it can be up to ¼ second). This is why the tailgating car usually cannot stop, when the brake light came on in the car in front, this driver had already completed the perception, human and vehicle reaction periods.
• Vehicle Braking distance is the distance it takes the vehicle to stop once you hit the brakes. At 55 mph on dry pavement, it takes a vehicle with good brakes about 4 ½ seconds to stop. Within that time, the vehicle will travel another 182 feet (braking distance = 0.06 times the speed squared).  This last factor is relating to the vehicle’s braking capability which depends on factors such as;
• the type of braking system,
• brake pad material,
• brake alignment,
• tire pressures,
• tire tread and grip,
• vehicle weight,
• suspension system,
• the co-efficient of friction of the road surface,
• wind speed,
• slope of road,
• surface smoothness
• braking technique applied by the driver.
• Total stopping distance; traveling at 55 mph, it will take about 6 seconds to stop your vehicle. The vehicle will travel approximately 302 feet before coming to a stop. That is longer than the length of a football field.

Rules of Thumb

• When you double your speed, it takes four times as much distance to stop your vehicle.
• Your vehicle will have four times the destructive power in a crash.
• You can not steer or brake a vehicle unless you have traction. Traction is the friction between the tires and the road. Reduce your speed on wet and slippery roads.
• Wet roads can double stopping distance. Reduce your speed by about 1/3 on a wet road. For example slow down from 55 mph to 35 mph.
• On packed snow, reduce your speed by ½ or more.
• If the road is icy, reduce your speed to a crawl. Stop driving as soon as you can.
• Empty trucks require greater stopping distance. An empty vehicle has less traction. The brakes are designed to control the maximum weight of the unit; therefore, the brakes lock up more readily when the trailer is empty or lightly loaded. This can cause skidding and loss of control.

Speed and Curves

If you take a curve too fast, your tires can lose traction with the road. This could cause your vehicle to skid off the road or roll over. Tests show that trucks with a high center of gravity can roll over at the posted speed limit for a curve.

• Slow to a safe speed before you enter a curve.
• Braking in a curve is dangerous because you can lock the wheels and cause a skid.
• Never exceed the posted speed limit for a curve.

Speed and Distance Ahead

• You should always be able to stop within the distance you can see ahead.
• Fog, rain or other conditions may require you to slow down.
• At night, you can't see as far with low beams as you can with high beams. When you use low beams, slow down.

Speed on Downgrades

• As you go downhill, your vehicle's speed increases.
• Never exceed the maximum safe speed on a downgrade.
• Downshift to a low gear before staring down a grade.
• You must use the braking effect of the engine to control your speed on downgrades. The engine's braking effect is greatest when it is near the governed RPMs and the transmission is in a low gear.
• Save your brakes so that you can slow or stop as required by road and traffic conditions.

Assignment:  For the following speeds indicated in the table, determine the human perception distance in feet, the human reaction distance in feet, the vehicle reaction distance in feet (use 0 seconds), and the vehicle braking distance in feet.  Finally, calculate the total stopping distance for 20 mph, 30 mph, 55 mph, and 70 mph.  Send the results of your calculations to your instructor at .