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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

  • Perception distance: The distance your vehicle travels, in ideal conditions; from the time your eyes see a hazard until your brain recognizes it. Keep in mind certain mental and physical conditions can affect your perception distance. It can be affected greatly depending on visibility and the hazard itself. The average perception time for an alert driver is 1¾ seconds. At 55 mph this accounts for 142 feet traveled.
  • Reaction distance: The distance you will continue to travel, in ideal conditions; before you physically hit the brakes, in response to a hazard seen ahead. The average driver has a reaction time of ¾ second to 1 second. At 55 mph this accounts for 61 feet traveled.
  • Braking distance: The distance your vehicle will travel, in ideal conditions; while you are braking. At 55 mph on dry pavement with good brakes, it can take about 216 feet.
  • Total stopping distance: The total minimum distance your vehicle has traveled, in ideal conditions; with everything considered, including perception distance, reaction distance and braking distance, until you can bring your vehicle to a complete stop. At 55 mph, your vehicle will travel a minimum of 419 feet.

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.

CDL:

Commercial Driver's License (CDL)

A CDL is required to drive any of the following vehicles:

  • Any combination of vehicles with a gross combined weight rating (GCWR) of 26,001 or more pounds, providing the gross vehicle weight rating (GVWR) of the vehicle being towed is in excess of 10,000 pounds.
  • Any single vehicle with a GVWR of 26,001 or more pounds, or any such vehicle towing another not in excess of 10,000 pounds.
  • Any vehicle, regardless of size, designed to transport 16 or more persons, including the driver.
  • Any vehicle required by federal regulations to be placarded while transporting hazardous materials.

OWI:

Operating While Intoxicated

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

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)?
  • 100 feet
  • 70 feet
  • 142 feet
  • 40 feet

Quote From The CDL Manual:

Perception distance is The distance your vehicle travels, in ideal conditions; from the time your eyes see a hazard until your brain recognizes it. Keep in mind certain mental and physical conditions can affect your perception distance. It can be affected greatly depending on visibility and the hazard itself. The average perception time for an alert driver is 1¾ seconds. At 55 mph this accounts for 142 feet traveled.

TruckingTruth's Advice:

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

Next
What definition best describes Reaction Distance?
  • The distance it takes to stop once the brakes are applied
  • All of these answers describe 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 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 you will continue to travel, in ideal conditions; before you physically hit the brakes, in response to a hazard seen ahead. The average driver has a reaction time of ¾ second to 1 second. At 55 mph this accounts for 61 feet traveled.

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

Prev
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
  • The distance your vehicle travels from the time your eyes see a hazard until your brain recognizes it
  • None of these are correct

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
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?
  • 80 feet
  • 75 feet
  • 100 feet
  • 61 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
All of the following add up to total stopping distance except:
  • All of these add up to total stopping distance
  • Perception distance
  • Braking distance
  • Reaction 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
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 165 feet
  • About 150 feet
  • About 216 feet
  • About 200 feet

Quote From The CDL Manual:

At 55 mph on dry pavement with good brakes, it can take a heavy vehicle about 216 feet 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
The distance traveled from the time your brain recognizes the hazard until your foot physically hits the brakes defines which of the following:
  • Reaction Distance
  • Braking Distance
  • None of these are correct
  • 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 traveled from the time your brain tells your foot to move from the accelerator until your foot is actually pushing the brake pedal
  • The distance it takes to stop once the brakes are applied

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

Quote From The CDL Manual:

At 55 mph, your vehicle will travel a minimum of 419 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)

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

Quote From The CDL Manual:

The average perception time for an alert driver is 1 3/4 seconds. At 55 mhp this accounts for 142 feet travelled.

TruckingTruth's Advice:

Perception time and distance must be memorized.

Prev
Next
When you double your speed, how much will you increase your distance to stop?
  • About one and a half times as much distance is needed to stop
  • About five times as much distance is needed to stop
  • About four times as much distance is needed to stop
  • About twice 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
Which statement is false?
  • The brakes, tires, springs and shock absorbers on heavy vehicles are designed to work best when the vehicle is fully loaded.
  • The heavier the vehicle, the more work the brakes must do to stop it and the more heat they absorb
  • 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.

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.

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

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
When will an empty and light truck stop faster than a heavy and loaded truck?
  • 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
  • Empty and light trucks will only stop faster on slick or wet roadway conditions
  • Empty and light trucks will only stop faster on dry roadway conditions

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
The average driver has a reaction time of:
  • 1/8 second
  • 3/4 second to 1 second
  • 1-2 seconds
  • 2 seconds

Quote From The CDL Manual:

The average driver has a reaction time of ¾ second to 1 second. At 55 mph this accounts for 61 feet traveled.

TruckingTruth's Advice:

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

Prev
Next
At 55 mph on dry pavement with good brakes, it can take a heavy vehicle about 216 feet stop. This describes which of the following:
  • Reaction Distance
  • Total Stopping Distance
  • Braking Distance
  • Perception 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 216 feet 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
Finish
Please select an option
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17

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