CDL Test Questions & Answers: 15 Practice Questions Explained

The best way to know if you're ready for the CDL knowledge test isn't re-reading the manual — it's answering real, exam-style questions and checking not just whether you got them right, but why. Below are 15 practice questions spanning General Knowledge, Air Brakes, Combination Vehicles, Pre-Trip Inspection, and Road Signs & Safety, each with the correct answer and the reasoning behind it.

General Knowledge & safe driving

Total stopping distance is made up of which three parts?

Perception, reaction, and braking distance ✓
Speed, weight, and weather
Braking, steering, and signaling distance
Reaction, following, and parking distance

Why: Stopping distance = perception distance (spotting the hazard) + reaction distance (moving your foot to the brake) + braking distance (the truck actually slowing down).

If you double your speed, what happens to your braking distance?

It stays the same
It roughly doubles
It roughly quadruples ✓
It roughly triples

Why: Braking distance grows with the square of speed, so doubling your speed roughly quadruples the distance it takes to stop.

Under 40 mph, the recommended following distance is:

1 second for every 10 feet of vehicle length ✓
A fixed 4 seconds no matter the length
2 car lengths
1 second total, regardless of length

Why: The rule of thumb is 1 second per 10 feet of your vehicle's length at speeds under 40 mph, with an extra second added above 40 mph.

The CDL blood alcohol concentration (BAC) limit is:

0.08
0.04 ✓
0.02
There is no limit for CDL holders

Why: The CDL limit is 0.04 BAC — half the standard 0.08 limit for regular drivers.

Under the standard Hours-of-Service rule, how many hours can you drive after 10 consecutive hours off duty?

8 hours
10 hours
11 hours ✓
14 hours

Why: After 10 consecutive hours off duty, a driver may drive up to 11 hours, within a 14-hour on-duty window.

ABS (anti-lock braking system) mainly helps you:

Stop in a shorter distance every time
Keep your wheels from locking up so you can maintain steering control ✓
Skip your pre-trip air-brake check
Brake safely without ever touching the pedal

Why: ABS keeps your wheels from locking during hard braking so you can still steer — it may not actually shorten your stopping distance.

Air Brakes

In a typical air brake system, the governor's cut-out pressure is around:

55 psi
100 psi
125 psi ✓
150 psi

Why: The governor typically stops the compressor (cuts out) at around 125 psi, then lets it restart (cuts in) around 100 psi.

The low-air pressure warning light and buzzer must activate before pressure drops below:

25 psi
55 psi ✓
80 psi
100 psi

Why: The low-air warning must come on before the pressure in the tanks drops below 55 psi, giving you an early alert to stop safely.

Spring (parking) brakes typically begin to apply automatically somewhere in the range of:

0–10 psi
20–45 psi ✓
60–80 psi
100–120 psi

Why: As air pressure continues to drop, spring brakes apply automatically somewhere in the 20–45 psi range to prevent the vehicle from moving with no air.

In a typical dual air system, pressure should build from 85 to 100 psi within about:

10 seconds
45 seconds ✓
3 minutes
10 minutes

Why: Pressure should build from 85 to 100 psi within roughly 45 seconds with the engine at operating rpm — slower buildup can mean trouble keeping up with heavy braking.

Combination Vehicles, Pre-Trip, and Road Signs

When a combination vehicle turns, the rear wheels of the trailer:

Follow the exact same path as the front wheels
Track inside the path of the front wheels — this is called off-tracking ✓
Always track wider than the front wheels
Lift off the ground briefly

Why: Off-tracking means the rear wheels cut a shorter, tighter path than the front wheels during a turn — important to account for around curbs and corners.

Before uncoupling a trailer, you should always:

Lower the landing gear before disconnecting air and electrical lines ✓
Disconnect air lines before lowering the landing gear
Release the trailer brakes first
Pull away slowly while still connected to test it

Why: Lower the landing gear to support the trailer's weight before disconnecting the air and electrical lines and pulling away from the fifth wheel.

During a pre-trip inspection, examiners primarily want to hear you say:

Nothing — the inspection is silent
"Checking for [specific thing]" as you point to and touch each item ✓
Only pass/fail for each item
A memorized poem about truck safety

Why: The spoken script format — pointing at an item and saying what you're checking it for — is what examiners are trained to listen for during the pre-trip.

An octagon-shaped sign always means:

Yield
Stop ✓
No entry
Construction zone

Why: The eight-sided octagon shape is reserved exclusively for STOP signs in the U.S., recognizable by shape alone even without the text.

Handheld device use (like texting) while driving a CMV is:

Allowed at speeds under 15 mph
Prohibited by federal regulation for CMV drivers ✓
Only prohibited in some states
Allowed if using a hands-free stand

Why: Federal regulations prohibit handheld device use, including texting, while operating a commercial motor vehicle — it takes your hands, eyes, and attention off the driving task.

Want more like this?

These 15 are a sample. The free practice test pulls a fresh, shuffled set of 20 questions from a much larger bank every time you take it, covering the same five categories — with instant explanations for every answer, right or wrong.