All exams/CDL General Knowledge/Free questions

CDL General Knowledge practice questions and answers

20 free questions from our 300-question CDL General Knowledge bank, each with the correct answer, explanation, and source note. Official format: Multiple-choice knowledge test · 80% to pass (49 CFR 383.135) · item count set by your state.

Questions last updated September 22, 2026 · Questions written from the federal CDL knowledge-test requirements in 49 CFR Part 383 and the required subject areas; not official state test questions.

Take these as a quiz About the exam

  1. Safe driving

    1. What should you do first when you notice your vehicle starting to skid on a wet road?

    1. ATake your foot off the accelerator
    2. BApply the brakes firmly
    3. CTurn the steering wheel sharply
    4. DDownshift to a lower gear

    Answer: A. 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

  2. Vehicle systems

    2. What does the air compressor do in a CMV air brake system?

    1. ASupplies air to the brake chambers
    2. BBuilds and maintains air pressure in the system
    3. CConverts air pressure to mechanical force
    4. DFilters moisture from the air lines

    Answer: B. 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

  3. Cargo

    3. When must a truck driver recheck cargo securement during a trip?

    1. AOnly at the destination
    2. BEvery 3 hours or 150 miles, whichever comes first
    3. COnce after the first 100 miles
    4. DEvery time the engine is turned off

    Answer: B. 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

  4. Trip inspection

    4. What is the minimum air pressure at which the low air pressure warning device must activate?

    1. A60 psi
    2. B55 psi
    3. C45 psi
    4. D50 psi

    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

  5. Braking

    5. What is the purpose of the emergency brake valve on a tractor-trailer?

    1. ATo apply only the trailer brakes
    2. BTo provide parking brake function
    3. CTo apply all spring brakes in an emergency
    4. DTo reduce air pressure for a controlled stop

    Answer: C. 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

  6. Accident procedures

    6. After a minor collision with no injuries, what should a CMV driver do first?

    1. AMove the vehicle out of traffic if possible
    2. BCall the police immediately
    3. CExchange information with the other driver
    4. DWait for a supervisor to arrive

    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

  7. Railroad

    7. How close to the tracks should a bus transporting passengers stop at a railroad crossing?

    1. AAs close to the tracks as safely possible
    2. BWithin 50 feet but not closer than 15 feet
    3. CAt least 20 feet from the nearest rail
    4. DExactly at the stop line, regardless of distance

    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

  8. Alcohol drugs

    8. Under federal regulations, a CDL driver may not operate a CMV within how many hours after drinking alcohol?

    1. A2 hours
    2. B4 hours
    3. C6 hours
    4. D8 hours

    Answer: B. 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

  9. Safe driving

    9. When driving in fog, what lights should you use?

    1. AHigh beams to cut through the fog
    2. BEmergency flashers and high beams
    3. CLow beams or fog lights only
    4. DNo lights, since they reflect back

    Answer: C. 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

  10. Vehicle systems

    10. What color is the air line connected to the emergency (spring) brake relay valve?

    1. ABlue
    2. BGreen
    3. CYellow
    4. DRed

    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

  11. Safe driving

    11. You are approaching a railroad crossing and the gates are starting to come down. What should you do?

    1. ASpeed up to cross before the gates fully lower
    2. BPull around the gate on the shoulder
    3. CStop before the gates and wait until they rise
    4. DStop, then proceed slowly once you can see the tracks

    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

  12. Vehicle systems

    12. What happens to the spring brakes when air pressure in the system drops below 60 psi?

    1. AThey disengage to allow normal driving
    2. BNothing, until the engine is shut off
    3. CThey lock in the released position
    4. DThey begin to apply gradually

    Answer: D. As system air pressure falls below 60 psi, the spring brake parking brake valves begin to apply. At approximately 20 to 45 psi (depending on the system), the spring brakes will fully engage. Never drive with low air pressure warnings active.

    Source: 49 CFR 393.95 — Spring Brake Operation

  13. Safe driving

    13. A CMV's stopping distance is affected by four main factors. Which of the following is NOT one of them?

    1. ATotal vehicle weight
    2. BBrake adjustment
    3. CTire tread depth
    4. DRoad surface conditions

    Answer: C. 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

  14. Cargo

    14. What is the maximum number of tiedowns required to secure a 10-foot-long piece of cargo weighing 500 pounds?

    1. AOne
    2. BTwo
    3. CThree
    4. DFour

    Answer: B. 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

  15. Trip inspection

    15. During a pre-trip inspection, which of the following items should you check on the steering system?

    1. AFluid level, play, and mounting bolts
    2. BOnly the power steering pump belt
    3. CThe brake pedal return spring
    4. DThe gear selector linkage

    Answer: A. 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

  16. Safe driving

    16. What is the "four-second rule" used for in CMV driving?

    1. AMeasuring proper following distance behind another vehicle
    2. BTiming how long you can run the engine without a break
    3. CDetermining how long your turn signal should flash
    4. DEnsuring you check your mirrors every four seconds

    Answer: A. 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

  17. Vehicle systems

    17. What is the purpose of the tractor protection system on a tractor-trailer?

    1. ATo protect the engine from overheating
    2. BTo disconnect the trailer during a jackknife
    3. CTo limit the speed of the trailer on downhill grades
    4. DTo automatically apply trailer brakes if air supply is lost

    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

  18. Braking

    18. When you use the trailer hand valve, what happens?

    1. AIt applies the tractor service brakes only
    2. BIt engages the tractor parking brake
    3. CIt applies the trailer service brakes only
    4. DIt cuts the engine fuel supply

    Answer: C. 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

  19. Trip inspection

    19. What should you check when inspecting the coupling devices during a pre-trip?

    1. AOnly the kingpin lock jaw
    2. BJust the fifth wheel lubrication
    3. CThe trailer door hinges
    4. DKingpin, locking mechanism,Mounting bolts, and apron

    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

  20. Accident procedures

    20. When must a CMV driver file an accident report with FMCSA?

    1. AOnly if the accident results in a fatality, injury requiring medical treatment, or a vehicle tow
    2. BAfter every accident, regardless of severity
    3. COnly if the police are called
    4. DOnly if there is more than $1,000 in damage

    Answer: A. 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

That’s the free set. The full bank has 300.

More original practice questions across the listed topics, a 50-question timed benchmark, and saved mistake review. 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. One payment of $8.99, yours forever.

Practice the free set