All exams/Driving & Licenses/CDL General Knowledge/Free questions
Tap an answer to see if you are right, and why. 20 questions across 8 topics, mostly Safe driving (5).
Safe driving1 of 20
What should you do first when you notice your vehicle starting to skid on a wet road?
The answer
Answer: B. When a skid begins, your first response should be to remove your foot from the accelerator. This lets the tires regain traction naturally. Slamming the brakes or yanking the steering wheel can make the skid worse or cause you to lose control entirely.
Source: AAMVA CDL Manual — Chapter 2: Safe Driving
Vehicle systems2 of 20
What does the air compressor do in a CMV air brake system?
The answer
Answer: A. The air compressor is the heart of the air brake system. It pumps air into the system's storage tanks, building pressure to the normal operating range of 100 to 125 psi. Without a functioning compressor, the system cannot maintain the pressure needed for braking.
Source: AAMVA CDL Manual — Chapter 6: Air Brakes
Cargo3 of 20
When must a truck driver recheck cargo securement during a trip?
The answer
Answer: D. Federal regulations require drivers to reexamine cargo and load securement devices whenever they change duty status, or after every 3 hours of driving, or every 150 miles—whichever comes first. This rule exists because shifting loads are a leading cause of highway accidents. Even if the load looks fine at the start, vibrations and road conditions can loosen straps and chains over time.
Source: 49 CFR 392.9(b)(3) — Cargo Inspection
Trip inspection4 of 20
What is the minimum air pressure at which the low air pressure warning device must activate?
The answer
Answer: D. The low air pressure warning must come on before air pressure falls below 55 psi (or one half the compressor cut-out pressure, whichever is greater). At or below 50 psi, the spring brakes should automatically apply. Never drive with the warning light or buzzer on.
Source: 49 CFR 393.95 — Low Air Pressure Warning
Braking5 of 20
What is the purpose of the emergency brake valve on a tractor-trailer?
The answer
Answer: B. The emergency brake valve (often the red button on the dash) is designed to apply the spring brakes on both the tractor and trailer in the event of a total air loss. Pulling this valve exhausts all air from the system and lets the spring brakes engage.
Source: AAMVA CDL Manual — Chapter 6: Air Brakes
Accident procedures6 of 20
After a minor collision with no injuries, what should a CMV driver do first?
The answer
Answer: A. If the accident is minor and there are no injuries, move your vehicle out of the traffic lane to prevent further collisions. After securing the scene, exchange information, take photos, and file a report if required by your carrier or state law.
Source: AAMVA CDL Manual — Chapter 2: Accidents
Railroad7 of 20
How close to the tracks should a bus transporting passengers stop at a railroad crossing?
The answer
Answer: B. Buses carrying passengers must stop within 50 feet of the tracks but not closer than 15 feet. After stopping, the driver must look and listen in both directions before crossing. This rule exists because buses carry vulnerable passengers and need adequate clearance.
Source: 49 CFR 392.10(a)(1) — Railroad Grade Crossings
Alcohol and drugs8 of 20
Under federal regulations, a CDL driver may not operate a CMV within how many hours after drinking alcohol?
The answer
Answer: D. Federal law prohibits CMV drivers from using alcohol or being under the influence of alcohol within 4 hours before going on duty or operating a commercial motor vehicle. This is a zero-tolerance rule that applies to any measurable amount of alcohol. A violation results in an immediate 24-hour out-of-service order.
Source: 49 CFR 392.5(a)(1) — Alcohol Prohibition
Safe driving9 of 20
When driving in fog, what lights should you use?
The answer
Answer: A. In fog, use low beams or fog lights. High beams bounce light off the water droplets in the fog and reflect it back at you, actually making visibility worse. Emergency flashers can confuse other drivers about whether you are moving or stopped, and should only be used when you are completely stopped.
Source: AAMVA CDL Manual — Chapter 2: Adverse Conditions
Vehicle systems10 of 20
What color is the air line connected to the emergency (spring) brake relay valve?
The answer
Answer: D. The emergency (supply) line is color-coded red. If air pressure is lost in this line, the spring brakes on both the tractor and trailer will apply automatically. The service line, which is controlled by the brake pedal, is color-coded blue. Never mix up these connections.
Source: AAMVA CDL Manual — Chapter 6: Air Brake System Components
Safe driving11 of 20
You are approaching a railroad crossing and the gates are starting to come down. What should you do?
The answer
Answer: C. When railroad crossing gates begin to lower, you must stop and wait until the gates fully rise and the signal stops before crossing. Never drive around lowered gates—this is both illegal and extremely dangerous. Trains travel faster than they appear and cannot stop quickly.
Source: 49 CFR 392.10 — Railroad Grade Crossings
Vehicle systems12 of 20
The low air pressure warning comes on while you are driving. What should you do?
The answer
Answer: B. Stop and safely park as soon as possible: there might be an air leak, and controlled braking works only while enough air remains in the tanks. If pressure keeps falling into the 20 to 45 psi range the spring brakes come on, which can take a long distance to stop a loaded vehicle or skid a light one. Pumping the pedal uses up air faster.
Source: AAMVA CDL Manual — Section 5.4.7: Low Air Pressure (p. 5-9)
Safe driving13 of 20
A CMV's stopping distance is affected by four main factors. Which of the following is NOT one of them?
The answer
Answer: D. While tire condition matters for overall safety, the four key stopping distance factors are perception distance, reaction distance, brake lag distance, and brake effectiveness. Weight, brake adjustment, road surface, and speed are critical. Tire tread depth is not among the four primary factors.
Source: AAMVA CDL Manual — Chapter 5: Stopping Distance
Cargo14 of 20
What is the maximum number of tiedowns required to secure a 10-foot-long piece of cargo weighing 500 pounds?
The answer
Answer: A. For cargo 5 feet or longer, you generally need a minimum of two tiedowns. Shorter pieces of cargo under 5 feet may need only one. The tiedowns must be properly tensioned and rated for the cargo weight. Always check the specific requirements for your cargo type.
Source: 49 CFR 393.110 — Number of Tiedowns Required
Trip inspection15 of 20
During a pre-trip inspection, which of the following items should you check on the steering system?
The answer
Answer: B. A thorough steering system inspection includes checking power steering fluid level, ensuring no more than 2 inches of play (10 inches at the steering wheel), and verifying that all mounting bolts are secure. A loose steering system can lead to loss of vehicle control.
Source: 49 CFR 392.7(a) — Equipment, Inspection and Use
Safe driving16 of 20
What is the "four-second rule" used for in CMV driving?
The answer
Answer: C. The four-second rule helps CMV drivers maintain a safe following distance. Pick a fixed point ahead; if you pass it less than four seconds after the vehicle in front of you, you are following too closely. Add more time for wet or icy roads.
Source: AAMVA CDL Manual — Chapter 2: Following Distance
Vehicle systems17 of 20
What is the purpose of the tractor protection system on a tractor-trailer?
The answer
Answer: D. The tractor protection system is a safety device that closes the trailer air supply valve when air pressure drops too low, applying the trailer spring brakes. This ensures the trailer brakes even if the tractor-trailer air connection fails completely. Without it, a runaway trailer could result.
Source: AAMVA CDL Manual — Chapter 6: Tractor Protection System
Braking18 of 20
When you use the trailer hand valve, what happens?
The answer
Answer: D. The trailer hand valve (trolley valve) applies only the trailer service brakes. It is useful for checking the trailer brakes before a trip but should never be used for normal braking because it can cause a jackknife. Use the foot brake for all routine stops.
Source: AAMVA CDL Manual — Chapter 6: Using the Brakes
Trip inspection19 of 20
What should you check when inspecting the coupling devices during a pre-trip?
The answer
Answer: D. Coupling device inspection includes verifying the kingpin is locked in the fifth wheel jaws, checking that locking mechanism is engaged, inspecting mounting bolts for tightness, and ensuring there is no gap between the apron and the fifth wheel plate. A failed coupling can result in a dropped trailer.
Source: 49 CFR 392.7(a) — Coupling Devices
Accident procedures20 of 20
When must a CMV driver file an accident report with FMCSA?
The answer
Answer: D. A driver must file a written report within 30 days when an accident results in a fatality, bodily injury requiring immediate medical treatment away from the scene, or disabling damage to any vehicle requiring it to be towed or transported. Property damage alone does not require a federal report.
Source: 49 CFR 390.15 — Accidents Involving CMVs
The official CDL General Knowledge knowledge examination covers Multiple-choice knowledge test with a passing threshold of 80%. Questions written from the federal CDL knowledge-test requirements in 49 CFR Part 383 and the required subject areas; not official state test questions. Every question on this page is original, with an explanation and a note citing its source.
Now try them under test conditions.
Practice the free set as a quiz with instant scoring, drill one topic at a time, or sit the free timed mock exam. 80% is the federal threshold in 49 CFR 383.135. The regulation sets the percentage only — your state driver licensing agency sets how many questions the test has.
Part of the bank cover alcohol and drugs. One is free here, with the answer and where it comes from: