South Carolina General Knowledge CDL Practice Test

This is a free 20-question practice test for the General Knowledge portion of the South Carolina 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 South Carolina Department of Motor Vehicles 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 South Carolina.

Question 1 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.

Question 2 of 20

You are driving a 40-foot vehicle at 35 mph. The minimum following distance you should maintain is:

2 seconds
3 seconds
4 seconds
7 seconds

Correct. The rule of thumb: 1 second per 10 feet of vehicle length below 40 mph. A 40-foot vehicle therefore needs 4 seconds of following distance.

Question 3 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 4 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 5 of 20

Which of the following is NOT part of the seven-step pre-trip inspection method?

Approach the vehicle and overview
Check the engine compartment
Test the AM/FM radio
Walk around inspection

Correct. The radio is not part of the safety inspection. The seven-step method covers approach, engine compartment, in-cab checks, lights, walk-around, signal lights, and brake checks.

Question 6 of 20

Which of these is a sign of tire failure?

Heavy thumping or vibration
A loud bang
The vehicle pulling to one side
All of the above

Correct. Vibration, a loud bang from a blowout, and pulling to one side are all classic signs of tire failure.

Question 7 of 20

Maximum allowable steering-wheel play is approximately:

5 degrees
10 degrees
20 degrees
30 degrees

Correct. Steering wheel play should not exceed 10 degrees (roughly 2 inches on a 20-inch wheel). More than that, the vehicle is hard to steer.

Question 8 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 9 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 10 of 20

Which of these is a major cause of crashes in heavy vehicles?

Driver fatigue
Heavy traffic
Following too closely
All of the above

Correct. Fatigue, heavy traffic, and following too closely are all leading factors in commercial vehicle crashes.

Question 11 of 20

When merging onto a highway, you should:

Stop at the end of the on-ramp
Accelerate to the speed of traffic and merge into a gap
Slow down to give yourself time
Force traffic to make space

Correct. Use the on-ramp to accelerate to traffic speed and merge into an existing gap. Stopping at the end of an on-ramp is dangerous.

Question 12 of 20

You are driving a 60-foot tractor-trailer at 55 mph. The minimum following distance you should maintain is:

4 seconds
5 seconds
6 seconds
7 seconds

Correct. Above 40 mph, add one extra second to the per-10-foot rule. A 60-foot vehicle = 6 seconds + 1 = 7 seconds at 55 mph.

Question 13 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 14 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 15 of 20

Hydroplaning is most likely to occur when:

Driving at low speed in light rain
Tires lose contact with the road and ride on water
Brakes are applied gently
Tires are over-inflated

Correct. Hydroplaning occurs when a film of water comes between the tires and the road, causing the tires to lose contact with the pavement. Higher speeds, worn tires, and standing water increase the risk.

Question 16 of 20

Which of the following can cause the wheels of your vehicle to skid on a curve?

Driving below the posted speed limit
Driving too fast for the curve
Slowing down before entering the curve
Driving with a light load

Correct. Posted curve advisory speeds assume cars; a heavy commercial vehicle with a high center of gravity may roll over or skid even at the posted speed. Slow before the curve, accelerate gently through it.

Question 17 of 20

How can you tell if you are driving in mountains?

You see deer
Your engine works harder on grades, both up and down
Roads are paved
It is colder

Correct. Mountain driving demands attention to grade-related challenges: gear selection on upgrades, speed control on downgrades, escape ramp awareness.

Question 18 of 20

When a vehicle ahead of you brakes hard, you should:

Brake harder
Apply the brakes immediately, even if you don't need to
Look for a way to steer around the vehicle if necessary, while preparing to brake
Honk to warn the driver behind you

Correct. Plan your escape: identify a clear lane or shoulder. If you can steer to safety, you save reaction time. If not, brake firmly without locking the wheels.

Question 19 of 20

When should you use four-way flashers?

Only when parked
When the vehicle is stopped or moving slowly enough to be a hazard to other traffic
Only at night
Whenever you want

Correct. Four-way emergency flashers warn other drivers when your vehicle is stopped or moving slowly relative to traffic.

Question 20 of 20

You should test your brakes before going down a hill by:

Driving down to test them
Pumping the pedal at the top
Lightly applying them as you start the descent to confirm they are working
Doing nothing

Correct. Just before the descent, gently apply the brakes at slow speed to confirm response. Combined with selecting a low gear in advance, this is the safe practice.

About the South Carolina 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 South Carolina Department of Motor Vehicles 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 South Carolina CDL practice tests.