Illinois General Knowledge CDL Practice Test

This is a free 20-question practice test for the General Knowledge portion of the Illinois Commercial Driver's License knowledge exam. Questions are pulled from a pool of 97 drawn from the AAMVA CDL Manual, which is the source document the Illinois Secretary of State Commercial Driver Licensing uses to write its actual exam.

How it works: Click an answer. The correct choice highlights in green, and you'll see a short explanation. Aim for 85% or better before you sit for the real test in Illinois.

Question 1 of 20

How many red reflective triangles must you carry in your commercial vehicle for emergency warning?

One
Two
Three
Six

Correct. You must carry three red reflective triangles, six fusees, or three liquid-burning flares for emergency warning use.

Question 2 of 20

If your vehicle hydroplanes, what should you do?

Brake hard immediately
Steer sharply to recover
Release the accelerator and push in the clutch
Accelerate to push through

Correct. Release the accelerator and push in the clutch. Do not brake or steer hard until the tires regain contact.

Question 3 of 20

What is the minimum tread depth required for the front (steering) tires of a commercial vehicle?

1/32 inch
2/32 inch
4/32 inch
6/32 inch

Correct. Steer-axle tires must have at least 4/32-inch tread depth in every major groove. Other tires require at least 2/32 inch.

Question 4 of 20

Most drivers who fall asleep at the wheel:

Are aware they are tired and pull over
Do not realize they are about to fall asleep
Have been drinking
Have been on the road less than an hour

Correct. Most drivers who fall asleep do not realize how drowsy they are. The only fix is to stop and rest before fatigue reaches that point.

Question 5 of 20

The four-way flashers on a commercial vehicle should be used:

Anytime you are stopped or moving slowly enough that other drivers might not expect
Only when parked
Only at night
Only on the interstate

Correct. Use four-way flashers anytime your vehicle is stopped on or near the roadway, or moving slowly enough that approaching traffic might be surprised.

Question 6 of 20

A fully loaded tractor-trailer traveling at 55 mph on dry pavement requires roughly what total stopping distance (perception + reaction + braking)?

100 feet
200 feet
300 feet (about the length of a football field)
600 feet

Correct. Total stopping distance for a heavy commercial vehicle at 55 mph on dry pavement is approximately 300 feet, or the length of a football field including end zones.

Question 7 of 20

When approaching a downgrade, you should select:

A higher gear than you would for an upgrade
A lower gear before starting down, never after
Neutral, and let gravity slow you
Whatever gear you are in

Correct. Select your gear before the downgrade — modern transmissions can be hard or impossible to shift into a lower gear once you are descending and engine RPM has dropped.

Question 8 of 20

When you exit a highway in a heavy commercial vehicle, you should:

Maintain highway speed onto the ramp
Slow down before reaching the ramp curve, since posted advisory speeds assume cars
Speed up to clear the ramp
Brake only at the bottom

Correct. Posted ramp speeds assume passenger cars. A loaded heavy vehicle, especially with high CG, can roll over at the posted speed. Slow before the curve.

Question 9 of 20

On a 6-tire single vehicle, the maximum allowable air loss rate (engine off, brakes released) is:

1 psi per minute
2 psi per minute
3 psi per minute
4 psi per minute

Correct. For a single vehicle, no more than 2 psi per minute leakage with the engine off and brakes released. Combinations are limited to 3 psi/min.

Question 10 of 20

You are about to drive a vehicle that you have not driven before. What should you do?

Get in and drive — every commercial vehicle handles the same
Inspect the vehicle to make sure it is safe and the cargo is properly secured
Drive briefly without cargo to learn the controls
Skip the inspection if the previous driver inspected it

Correct. Federal regulations require the driver to be satisfied that the vehicle is in safe operating condition before driving. The previous driver's inspection does not relieve you of that responsibility.

Question 11 of 20

When should you use your high beams?

Whenever it is dark
When other drivers are within 500 feet ahead
When other drivers are not within 500 feet of you and you are not following another vehicle within 500 feet
In fog

Correct. Federal and state law requires you to dim high beams when within 500 feet of an oncoming vehicle, or when following within 500 feet of another vehicle.

Question 12 of 20

A vehicle's "no zone":

Refers to the lane next to the cab
Includes the front, sides, and rear blind spots where other vehicles disappear from view
Is the area in front of the truck only
Does not exist for cars

Correct. The "no zone" is the set of blind spots around a truck — front, both sides (especially right), and rear — where other vehicles cannot be seen.

Question 13 of 20

To recover from a drive-wheel skid, you should:

Brake harder to slow down
Steer in the opposite direction of the skid
Stop accelerating, push in the clutch, and counter-steer
Apply the parking brake

Correct. Stop applying power, push in the clutch (or shift to neutral) to let the wheels turn freely, and counter-steer in the direction you want the front of the vehicle to go.

Question 14 of 20

The most important reason for doing a vehicle inspection is to:

Comply with the law
Save fuel
Find safety problems before they cause a crash
Document mileage

Correct. A pre-trip inspection is meant to find problems that could cause a crash or breakdown. Compliance is a side effect, not the purpose.

Question 15 of 20

A driver who has been disqualified is:

Allowed to drive a commercial vehicle
Not allowed to drive a commercial vehicle
Allowed to drive only with another driver in the cab
Allowed to drive only on private property

Correct. A disqualified driver may not operate a commercial motor vehicle on public roads, regardless of supervision.

Question 16 of 20

What does it mean when a driver is "hydroplaning"?

The truck is overheating
The tires lose contact with the road and ride on a film of water
The brakes have failed
The driver is dehydrated

Correct. Hydroplaning means tires have lost contact with the pavement and are riding on standing water. Steering and braking become ineffective.

Question 17 of 20

Empty trucks:

Always stop in less distance than fully loaded ones
Require longer stopping distances than loaded ones because of lighter brake force per axle
Have the same stopping distance as loaded trucks
Cannot use the engine brake

Correct. An empty truck has lighter axle weight, so the brakes generate less friction relative to vehicle weight — the truck can take longer to stop than the same vehicle loaded.

Question 18 of 20

When you arrive at a customer to deliver:

Just open the trailer
Sign in, follow site safety rules (PPE, speed limits, designated routes)
Skip site rules
Park anywhere

Correct. Customer sites have their own safety rules — sign in, wear required PPE, follow speed limits and designated routes.

Question 19 of 20

Which of the following is true about the front-end shock absorbers?

They have nothing to do with handling
Worn shock absorbers can cause poor handling and uneven tire wear
They are part of the brake system
They affect only ride comfort, never safety

Correct. Shock absorbers control suspension oscillation. Worn shocks contribute to poor handling, uneven tire wear, and reduced control on rough roads.

Question 20 of 20

When should you place reflective triangles on a divided highway after stopping due to a breakdown?

10, 50, and 100 feet behind the vehicle
10 feet to the rear, 100 feet to the rear, and 200 feet to the rear
10, 100, and 200 feet to the rear, on the traffic side
100, 200, and 500 feet to the rear

Correct. On a divided highway with traffic from one direction, place triangles 10, 100, and 200 feet behind the vehicle, on the shoulder or in the lane you stopped in.

About the Illinois General Knowledge exam

Most state DMVs administer 50 General Knowledge questions and require 80% (40 correct) to pass. Questions are pulled from a much larger pool, so two applicants taking the test on the same day will see different mixes. The questions on this site are drawn from the AAMVA CDL Manual chapters that every state's exam is built from.

The Illinois Secretary of State Commercial Driver Licensing follows the federal CDL standards established by FMCSA. To earn the General Knowledge credential, you must answer at least 80% of the questions correctly. Many candidates score lower the first time because the test pulls from a large pool — refreshing this page will give you a different mix of questions, drawn from the same authoritative source.

Want more practice? Try the full General Knowledge question bank or browse all Illinois CDL practice tests.