CDL Greenlight

Study guide

CDL general knowledge: the numbers, worked out

The test every applicant takes. Learn the manual's figures for it, each with the arithmetic behind it.

Who takes it
Every CDL applicant, whatever the class.
To pass
80 percent.
Questions
50 in Michigan and Pennsylvania for Class B and C. Their Class A test has 70.
What it covers
Inspection, vehicle control, speed and space, hazards, emergencies and cargo.
On this page
  1. Stopping from 55 mph, piece by piece
  2. Worked example: following distance
  3. What speed and weather do to a stop
  4. Seeing and being seen
  5. Inspection, cargo and alcohol
  6. Five answers that look right and are wrong
  7. Three practice questions
  8. Sources

Stopping from 55 mph, piece by piece

A stop has three parts. Learn each one, and the total.

142 feet
Perception distance. An alert driver needs about 1¾ seconds to recognize a hazard, and the truck keeps moving the whole time. Manual 2.6
61 feet
Reaction distance. Getting your foot onto the brake takes ¾ second to 1 second. Manual 2.6
216 feet
Braking distance, on dry pavement with good brakes. Manual 2.6
419 feet
The three added up: 142 + 61 + 216. It is the least you need at 55 mph in ideal conditions. With air brakes, add about 32 feet of brake lag. Manual 2.6, 5.4

Worked example: following distance

At least one second for each 10 feet of vehicle length. Above 40 mph, one second more. Count from the moment the vehicle ahead passes a mark until you reach it. Manual 2.7

  • A 40-foot truck at 35 mph.

    4 seconds. 40 feet divided by 10 is 4. Below 40 mph nothing is added.

  • The same truck at 55 mph.

    5 seconds. 4 for the length, plus 1 for driving over 40 mph.

  • A 60-foot tractor-trailer at 55 mph.

    7 seconds. 6 for the length, plus 1. On a slippery road you need much more than the rule gives.

What speed and weather do to a stop

4 times
Double your speed and the braking distance is four times as long. So is the force of a crash. At three times the speed it is nine times. Manual 2.6
One-third
How much to slow down on a wet road: from 55 to about 35 mph. A wet road can double your stopping distance. Manual 2.6
Half
On packed snow, cut your speed by half or more. On ice, slow to a crawl and stop driving as soon as you safely can. Manual 2.6
30 mph
With a lot of water on the road, hydroplaning can begin at a speed this low. Release the accelerator and push in the clutch. Do not brake. Manual 2.6

Seeing and being seen

12 to 15 seconds
How far ahead to look: about one block at lower speeds, about a quarter of a mile on the highway. Manual 2.4
10, 100, 200 feet
Where the three warning devices go on a one-way or divided highway, all toward approaching traffic. On a two-way road: within 10 feet of the front or rear corner, then 100 feet behind and 100 feet ahead. Manual 2.5
10 minutes
How long you have to set them out after you stop on a road or its shoulder. Manual 2.5
500 feet
Dim your high beams within this distance of an oncoming vehicle, and when you follow another vehicle this closely. Manual 2.11

Inspection, cargo and alcohol

4/32 inch
Least tread depth in every major groove of a front tire. All other tires: 2/32 inch. Manual 2.1
10 degrees
Steering wheel play beyond this makes the vehicle hard to steer. On a 20-inch wheel it is about 2 inches at the rim. Manual 2.1
50 miles
Check the cargo and what holds it within the first 50 miles. After that, every 3 hours or 150 miles, whichever comes first, and after every break. Manual 2.1, 3.1
.04%
The blood alcohol concentration at which driving a commercial vehicle is illegal. Any detectable amount below it puts you out of service for 24 hours. Manual 1.3

Five answers that look right and are wrong

  • Common answerAn empty truck stops in a shorter distance.

    The manual 2.6It needs a longer one. An empty vehicle has less traction.

  • Common answerTwice the speed means twice the braking distance.

    The manual 2.6Four times. At three times the speed it is nine times.

  • Common answerOn a long downgrade, shift down once the speed gets too high.

    The manual 2.16Get into a low gear before you start down. Once speed has built up you may not get the lower gear in at all, and then you have no engine braking.

  • Common answerSwing wide to the left before a tight right turn.

    The manual 2.7If you cannot make the turn without swinging into another lane, turn wide as you complete the turn and keep the rear of your vehicle close to the curb. If you swing left first, a driver behind may try to pass on your right.

  • Common answerThe posted speed for a curve is safe for a truck.

    The manual 2.6A truck with a high center of gravity can roll over at the posted speed. Slow to a safe speed before the curve: braking in it can lock the wheels and cause a skid.

Three practice questions

Pick an answer. The reason and the manual's own sentence follow.

You are driving a 60-foot tractor-trailer at 50 mph on a dry highway. What is the least following distance you need?

Show the answer

The answer is C. One second for each 10 feet of length makes 6 seconds. You are above 40 mph, so add one more: 7 seconds.

Commercial driver manual, section 2.7At greater speeds, you must add 1 second for safety.

You speed up from 20 mph to 40 mph. What happens to your braking distance?

Show the answer

The answer is B. Doubling your speed multiplies the braking distance by four, and the force of an impact with it.

Commercial driver manual, section 2.6When you double your speed from 20 to 40 mph the impact is 4 times greater. The braking distance is also 4 times longer.

You leave the yard with a secured load. When must you first check the cargo and its securing devices again?

Show the answer

The answer is A. The first check comes early, while there is still little trip behind you. The 3-hour and 150-mile checks come after it.

Commercial driver manual, section 3.1Inspect the cargo and its securing devices again within the first 50 miles after beginning a trip.

Now answer 5, free.

5 questions from the course, no signup. Miss one and see exactly why.

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Sources

Reviewed by CDL Greenlight on October 13, 2026. CDL Greenlight writes and checks this page. Every figure is the one the commercial driver manual prints, with its section named beside it. Where your state has a rule of its own, your state's manual decides.

Found a mistake? Write to support@cdlgreenlight.com. Corrections we have published