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2.6 Controlling Vehicle Speed

Driving too fast is a major cause of fatal crashes. You must adjust your speed depending on driving conditions. These include traction, curves, visibility, traffic and hills.

Speed and Stopping Distance

Three things add up to total stopping distance:

Perception Distance
+ Reaction Distance
+ Braking Distance
--------------------------
= Total Stopping Distance

Effect of Speed on Stopping Distance

Whenever you double your speed, it takes about four times as much distance to stop, and your vehicle will have four times the destructive power if it crashes. High speeds increase stopping distances greatly. By slowing down a little, you can gain a lot in reduced braking distance.

Effect of Vehicle Weight on Stopping Distance

The heavier the vehicle, the more work the brakes must do to stop it and the more heat they absorb. But the brakes, tires, springs and shock absorbers on heavy vehicles are designed to work best when the vehicle is fully loaded. Empty trucks require greater stopping distances, because an empty vehicle has less traction. It can bounce and lock up its wheels, giving much poorer braking. (This is not usually the case with buses.)

Matching Speed to Road Surface

You cannot steer or brake a vehicle unless you have traction. Traction is friction between the tires and the road. There are some road conditions that reduce traction and call for lower speeds.

It is very important that you memorize the below stopping distance formula as well as memorize the the meaning, time, and distance of each term in the formula. This will very likely show up on your written exam!
Be sure you remember this as the written exam frequently asks how much longer it takes to stop when you double your speed.
Yes, it's true! Empty trucks take longer to stop than fully loaded trucks. The written exam often asks about this so make sure you remember that.

Review Questions - Click On The Picture To Begin...

Which statement is false?
  • Empty trucks require greater stopping distances, because an empty vehicle has less traction
  • A heavily loaded truck will take longer to stop than an empty truck
  • The heavier the vehicle, the more work the brakes must do to stop it and the more heat they absorb
  • All of these statements are false

Quote From The CDL Manual:

The heavier the vehicle, the more work the brakes must do to stop it and the more heat they absorb. But the brakes, tires, springs and shock absorbers on heavy vehicles are designed to work best when the vehicle is fully loaded. Empty trucks require greater stopping distances, because an empty vehicle has less traction. It can bounce and lock up its wheels, giving much poorer braking. (This is not usually the case with buses.)

TruckingTruth's Advice:

This type of question is frequently asked on written exams. Understand that truck braking systems are designed for when the truck is fully loaded. An empty or very light truck will actually take longer to stop than a fully loaded and heavy truck.

Next
At 55 mph on dry pavement with good brakes, it can take a heavy vehicle about 170 feet and about 4 1/2 seconds to stop. This describes which of the following:
  • Reaction Distance
  • Total Stopping Distance
  • Perception Distance
  • Braking Distance

Quote From The CDL Manual:

Braking Distance is the distance it takes to stop once the brakes are applied. At 55 mph on dry pavement with good brakes, it can take a heavy vehicle about 170 feet and about 4 1/2 seconds to stop.

TruckingTruth's Advice:

Remember, braking distance is only one of three parts of the total stopping distance formula. Be sure to memorize the entire stopping distance formula:

Perception Distance
+ Reaction Distance
+ Braking Distance
---------------------
= Total Stopping Distance

Prev
Next
What is the following phrase referring to: At 55 mph it will take about 6 seconds to stop and your vehicle will travel the distance of a football field (60 + 60 + 170 = 290 feet)
  • Reaction Distance
  • Perception Distance
  • Total Stopping Distance
  • Braking Distance

Quote From The CDL Manual:

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

TruckingTruth's Advice:

Remember, braking distance is only one of three parts of the total stopping distance formula. Be sure to memorize the entire stopping distance formula:

Perception Distance
+ Reaction Distance
+ Braking Distance
---------------------
= Total Stopping Distance

Prev
Next
The distance your vehicle travels from the time your eyes see a hazard until your brain recognizes it defines which of the following terms:
  • Braking Distance
  • Reaction Distance
  • Perception Distance
  • Total Stopping Distance

Quote From The CDL Manual:

Perception distance 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.

TruckingTruth's Advice:

Take a moment to memorize the definition for Perception Distance. This will very likely show up on your written exam.

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Next
All of the following add up to total stopping distance except:
  • Perception distance
  • All of these add up to total stopping distance
  • Reaction distance
  • Braking distance

Quote From The CDL Manual:

Three things add up to total stopping distance:

Perception Distance
+ Reaction Distance
+ Braking Distance
---------------------------
= Total Stopping Distance

TruckingTruth's Advice:

Be sure to memorize this formula as well as memorizing the definitions for:

Perception Distance Reaction Distance Braking Distance Total Stopping Distance

Prev
Next
When a driver sees a roadway hazard, how long will it take for the drivers brain to process the situation (perception time)?
  • 3/4 second
  • 1/2 second
  • 1 second
  • 1/8 second

Quote From The CDL Manual:

The perception time for an alert driver is about 3/4 second. At 55 mph, you travel 60 feet in 3/4 second.

TruckingTruth's Advice:

Perception time and distance must be memorized.

Prev
Next
At 55mph on dry pavement with good brakes, it can take a heavy vehicle about how many feet to stop once the brakes are applied?
  • About 200 feet
  • About 165 feet
  • About 150 feet
  • About 170 feet

Quote From The CDL Manual:

At 55 mph on dry pavement with good brakes, it can take a heavy vehicle about 170 feet and about 4 1/2 seconds to stop.

TruckingTruth's Advice:

Make sure you memorize the definition of Braking Distance as well as the time and distance it takes for a vehicle to stop once the brakes are fully applied.

Prev
Next
At 55mph in dry conditions and with good brakes, what distance will it take for a heavy vehicle to come to a complete stop?
  • 250 feet
  • 275 feet
  • 290 feet
  • 320 feet

Quote From The CDL Manual:

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

TruckingTruth's Advice:

Make sure you have the Total Stopping Distance Formula memorized and each term of the formula memorized as well.

Perception Distance (60 feet)
+ Reaction Distance (60 feet)
+ Braking Distance (170 feet)
-----------------------------
= Total Stopping Distance (290 feet, the length of a football field)

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Next
What is perception distance?
  • The distance it takes to stop once the brakes are applied
  • 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
  • None of these are correct
  • The distance your vehicle travels from the time your eyes see a hazard until your brain recognizes it

Quote From The CDL Manual:

Perception distance 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.

TruckingTruth's Advice:

You absolutely must memorize the definition of perception distance as it will very likely show up on your written exam. You also need to memorize "perception time" and how far you will travel at 55mph.
Prev
Next
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 defines which of the following:
  • None of these are correct
  • Reaction Distance
  • Braking Distance
  • Perception Distance

Quote From The CDL Manual:

Reaction distance is 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.

TruckingTruth's Advice:

Be sure to have the definition of Reaction Distance memorized.

Prev
Next
Which of the following best describes Braking Distance?
  • None of these accurately describe Braking Distance
  • The distance your vehicle travels from the time your eyes see a hazard until your brain recognizes it
  • The distance it takes to stop once the brakes are applied
  • 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

Quote From The CDL Manual:

Braking Distance is the distance it takes to stop once the brakes are applied. At 55 mph on dry pavement with good brakes, it can take a heavy vehicle about 170 feet and about 4 1/2 seconds to stop

TruckingTruth's Advice:

Make sure you memorize the definition of braking distance as this will likely show up on your written exam. You should also memorize braking time as well.

Prev
Next
What three things add up to stopping distance?
  • Perception Distance, Braking Distance, Pressure Distance
  • Perception Distance, Reaction Distance, and Braking Distance
  • Reaction Distance, Skid Distance, and Friction Distance
  • Braking Distance, Forward Momentum Distance, and Friction Resistance

Quote From The CDL Manual:

Three things add up to total stopping distance:

Perception Distance
+ Reaction Distance
+ Braking Distance
---------------------------------
= Total Stopping Distance

TruckingTruth's Advice:

This formula will almost definitely show up on your written exam so it's extremely important to not only memorize the formula, but memorize what each definition means:

  • Perception distance 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 is 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 is the distance it takes to stop once the brakes are applied. At 55 mph on dry pavement with good brakes, it can take a heavy vehicle about 170 feet and about 4 1/2 seconds to stop.
  • Total stopping distance - At 55 mph it will take about 6 seconds to stop and your vehicle will travel the distance of a football field (60 + 60 + 170 = 290 feet).
Prev
Next
The average driver has a reaction time of:
  • 1/2 second
  • 3/4 second
  • 1 second
  • 1/8 second

Quote From The CDL Manual:

The average driver has a reaction time of 3/4 second. This accounts for an additional 60 feet traveled at 55 mph.

TruckingTruth's Advice:

Be sure to have reaction time and distance memorized. This will likely show up on your written exam.

Prev
Next
At 55mph, how far will you travel in the time it takes your foot to move from the accelerator to the brake pedal after perceiving a hazard?
  • 100 feet
  • 60 feet
  • 80 feet
  • 75 feet

Quote From The CDL Manual:

The average driver has a reaction time of 3/4 second. This accounts for an additional 60 feet traveled at 55 mph.

TruckingTruth's Advice:

Be sure you memorize the definition or reaction time, including the distance traveled at 55mph.

Prev
Next
When will an empty and light truck stop faster than a heavy and loaded truck?
  • Empty and light trucks will only stop faster on dry roadway conditions
  • Empty and light trucks will only stop faster on slick or wet roadway conditions
  • Empty and light trucks always stop quicker than a heavy and loaded truck
  • Empty and light trucks never stop as quickly as a loaded truck

Quote From The CDL Manual:

The heavier the vehicle, the more work the brakes must do to stop it and the more heat they absorb. But the brakes, tires, springs and shock absorbers on heavy vehicles are designed to work best when the vehicle is fully loaded. Empty trucks require greater stopping distances, because an empty vehicle has less traction. It can bounce and lock up its wheels, giving much poorer braking. (This is not usually the case with buses.)

TruckingTruth's Advice:

A question regarding stopping distance with a loaded or empty truck will likely be included on your written exam. Be sure you understand why an empty truck takes longer to stop.

Prev
Next
At 55mph, what distance will you travel from the time you perceive a hazard until your brain is able to process the information (perception distance)?
  • 70 feet
  • 40 feet
  • 60 feet
  • 100 feet

Quote From The CDL Manual:

Perception distance 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.

TruckingTruth's Advice:

Perception time and distance should be memorized as it will very likely show up on your written exam.

Prev
Next
When you double your speed, how much will you increase your distance to stop?
  • About five times as much distance is needed to stop
  • About twice as much distance is needed to stop
  • About one and a half times as much distance is needed to stop
  • About four times as much distance is needed to stop

Quote From The CDL Manual:

Whenever you double your speed, it takes about four times as much distance to stop, and your vehicle will have four times the destructive power if it crashes. High speeds increase stopping distances greatly. By slowing down a little, you can gain a lot in reduced braking distance.

Prev
Next
What definition best describes Reaction Distance?
  • The distance it takes to stop once the brakes are applied
  • 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
  • All of these answers describe Reaction Distance
  • The distance your vehicle travels from the time your eyes see a hazard until your brain recognizes it

Quote From The CDL Manual:

Reaction distance is 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.

TruckingTruth's Advice:

It is very important to memorize the definition of Reaction Distance as well as memorizing Reaction Time and the distance traveled at 55mph. Nearly all written exams have questions about either Perception Distance, Reaction Distance, Braking Distance, or Total Stopping Distance. Make sure you know the definitions and distances for each.

Prev
Next
At 55mph, how long will it take to stop a heavy vehicle in dry conditions and with good brakes from the time a hazard is seen until the time the vehicle comes to a complete stop?
  • 7 seconds
  • 6 seconds
  • 4 seconds
  • 5 seconds

Quote From The CDL Manual:

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

TruckingTruth's Advice:

Be sure to memorize the Total Stopping Distance Formula:

Perception Distance (3/4 second)
+ Reaction Distance (3/4 second)
+ Braking Distance (4 1/2 seconds)
----------------------------------------
= Total Stopping Distance (about 6 seconds)

Prev
Next
At 55mph on dry pavement with good brakes, how many seconds will it take a heavy vehicle to stop once the brakes are applied?
  • 5 seconds
  • 4 3/4 seconds
  • 4 1/2 seconds
  • 3 3/4 seconds

Quote From The CDL Manual:

Braking distance is the distance it takes to stop once the brakes are applied. At 55 mph on dry pavement with good brakes, it can take a heavy vehicle about 170 feet and about 4 1/2 seconds to stop.

TruckingTruth's Advice:

Make sure you have the definition of Braking Distance memorized, including the time and distance it takes to stop once the brakes are fully applied.

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