CDL Practice Test (General Knowledge)

10 random questions on the CDL general knowledge test topics, with an explanation for every answer.

Tap an answer to see right away if you got it, and why. Questions don't repeat until you've seen them all.

How this practice test works

The CDL general knowledge test is the first written test every commercial driver takes. These questions follow the topics in the federal model CDL driver's manual (section 2, Driving Safely): vehicle inspection, space management and following distance, seeing ahead, braking on grades, skids and emergencies, cargo securement, railroad crossings, alcohol rules and emergency equipment. We wrote them in our own words.

Each round picks 10 at random. After each answer you see the correct choice and why. At the end you get your score and a list of what you missed.

Your state's CDL manual is the final word, and it may add state rules. Air brakes, combination vehicles and endorsements are separate tests.

Study all the questions

Show all 41 questions with answers
  1. What is the minimum tread depth for the front (steering) tires on a commercial vehicle?
    Answer: 4/32 inch in every major groove. Steering tires need at least 4/32 inch of tread in every major groove. Other tires need at least 2/32 inch.
  2. What is the minimum tread depth for tires other than the front tires?
    Answer: 2/32 inch. All tires other than the steering tires need at least 2/32 inch of tread.
  3. How much following distance do you need, below 40 mph?
    Answer: At least 1 second for each 10 feet of vehicle length. The CDL manual's rule: at least one second for every 10 feet of vehicle length at speeds under 40 mph.
  4. You drive a 40-foot vehicle at 50 mph. What following distance do you need, at minimum?
    Answer: 5 seconds. 40 feet means 4 seconds. Above 40 mph, add 1 second: 5 seconds in good conditions. Add more in bad weather.
  5. How far ahead should you look while driving?
    Answer: 12 to 15 seconds ahead. Look 12 to 15 seconds ahead. At highway speed that is about a quarter of a mile, so you can spot problems early.
  6. After you stop on the road for an emergency, how soon must you put out your warning devices?
    Answer: Within 10 minutes. Turn on your hazard flashers right away and place the warning triangles within 10 minutes of stopping.
  7. On a two-lane road with traffic both ways, where do you place your three warning triangles?
    Answer: One about 10 feet from the vehicle on the traffic side, one 100 feet behind and one 100 feet ahead. Place one about 10 feet from the front or rear on the traffic side, one about 100 feet behind and one about 100 feet ahead, so drivers from both directions are warned.
  8. On a one-way or divided highway, where do you place your warning triangles?
    Answer: 10, 100 and 200 feet behind the vehicle, toward approaching traffic. All traffic comes from behind, so place the triangles 10 feet, 100 feet and 200 feet back toward approaching traffic.
  9. You stop just over a hill or around a curve. How far back may you place the rear warning triangle?
    Answer: Up to 500 feet back. Move the rear triangle back as far as needed, up to 500 feet, so drivers see it before they reach the hill or curve.
  10. When should you first check your cargo and its securement after starting a trip?
    Answer: Within the first 50 miles. Check cargo within the first 50 miles, then every 150 miles or 3 hours (whichever comes first), and after every break.
  11. After the first check, how often should you re-check cargo securement?
    Answer: Every 150 miles or every 3 hours, whichever comes first. Loads shift and straps loosen. Re-check every 150 miles or every 3 hours, and after every break you take.
  12. How many tie-downs should secure cargo, as a general rule?
    Answer: At least one for every 10 feet of cargo, and never fewer than two. The manual's rule of thumb: at least one tie-down for every 10 feet of cargo, and at least two tie-downs no matter how small the load.
  13. Going down a long grade, your safe speed is 40 mph. How do you use snub braking?
    Answer: Brake firmly until you are 5 mph below 40 (35 mph), release, and repeat when you reach 40. Brake hard enough to feel a slowdown until you're 5 mph under your safe speed, then release. Repeat when you're back at your safe speed. Light, constant braking overheats brakes and causes brake fade.
  14. How should you go down a long, steep downgrade?
    Answer: Shift into a low gear before starting down and let the engine slow you. Pick a low gear before the grade starts. Engine braking does most of the work, so the brakes stay cool for when you need them. Never coast in neutral.
  15. What causes most serious skids?
    Answer: Driving too fast for road conditions. The manual says most serious skids come from driving too fast for the conditions. Over-braking, over-steering and over-accelerating also cause skids.
  16. To avoid a hazard, what is usually faster than stopping?
    Answer: Steering around it. You can almost always turn to miss an obstacle faster than you can stop. Top-heavy vehicles may tip, so don't brake while turning.
  17. An oncoming driver drifts into your lane. In general, which way should you steer?
    Answer: To the right. In general, steer to the right. If you steer left, the other driver may correct back into the same lane you swerved into.
  18. You must leave the road to avoid a crash. What should you do?
    Answer: Avoid braking until you slow to about 20 mph, keep one set of wheels on the pavement if you can. Hard braking on a loose shoulder can cause a skid. Let the vehicle slow down, keep one set of wheels on the pavement if possible, then brake gently.
  19. How should you make a right turn in a large vehicle?
    Answer: Turn wide as you complete the turn, keeping the rear close to the curb. Keep the rear of the vehicle near the curb so cars can't squeeze in on your right, then swing wide as you finish. Swinging left first invites drivers to pass you on the right.
  20. When backing, what is the safest choice?
    Answer: Back toward the driver's side and use a helper. Backing toward the driver's side lets you see better. Get out and look first, back slowly, and use a helper when you can.
  21. What blood alcohol concentration (BAC) disqualifies a commercial driver while driving a CMV?
    Answer: 0.04% or higher. For drivers of commercial motor vehicles the limit is 0.04%, half the usual limit for cars.
  22. A CMV driver has any detectable alcohol, but less than 0.04%. What happens?
    Answer: Placed out of service for 24 hours. Any detectable amount of alcohol under 0.04% means 24 hours out of service.
  23. Which fire extinguisher is right for an electrical fire?
    Answer: A B:C type extinguisher. Water on an electrical fire can shock you. Use a B:C (or A:B:C) extinguisher, which handles electrical and burning-liquid fires.
  24. At what speed can hydroplaning begin if there is a lot of water on the road?
    Answer: As low as 30 mph. With enough water, tires can ride up on it at speeds as low as 30 mph. Lightly loaded vehicles are more likely to hydroplane.
  25. Your brakes are wet after driving through deep water. What should you do?
    Answer: Put on the brakes lightly while accelerating gently to dry them. Wet brakes can grab or fail. Apply light brake pressure while driving slowly for a short distance to dry them out.
  26. What usually causes brake fade?
    Answer: Overheating from using the brakes too much. Heavy, constant braking (especially downhill) overheats the brakes until they stop working well. Use low gears and snub braking.
  27. What should you never do while crossing railroad tracks?
    Answer: Shift gears. If you stall on the tracks, you could be hit. Pick a gear that gets you all the way across without shifting.
  28. Buses and placarded hazmat vehicles must stop before a railroad crossing. How far from the nearest rail?
    Answer: Within 50 feet, but not closer than 15 feet. Stop no more than 50 feet and no less than 15 feet from the nearest rail, then look and listen before crossing.
  29. Which statement about overhead clearance is true?
    Answer: An empty van sits higher than a loaded one. Weight lowers a trailer, so an empty one is taller. Repaving or packed snow can make posted clearances wrong, so don't assume.
  30. What is the only real cure for driver fatigue?
    Answer: Sleep. Coffee and fresh air only mask tiredness for a short time. Stop and sleep.
  31. During a pre-trip inspection, how much play in the steering wheel is too much?
    Answer: More than 10 degrees (about 2 inches at the rim of a 20-inch wheel). More than about 10 degrees of free play, roughly 2 inches at the rim of a 20-inch wheel, can make the vehicle hard to steer.
  32. On which vehicles are recapped or regrooved tires not allowed on the front wheels?
    Answer: Buses. Recapped or regrooved tires are banned on the front wheels of a bus.
  33. Which emergency equipment must a commercial vehicle carry?
    Answer: A fire extinguisher, spare fuses (unless it has circuit breakers) and warning devices. The manual lists a fire extinguisher, spare electrical fuses (if the vehicle uses them) and warning devices such as three reflective triangles.
  34. When does rain make roads most slippery?
    Answer: Right after the rain begins. Oil and dirt float up with the first rain and make the surface slick until rain washes them away.
  35. What is Gross Vehicle Weight Rating (GVWR)?
    Answer: The maximum weight the manufacturer rates for a single vehicle plus its load. GVWR is the manufacturer's maximum for the vehicle and its load. Exceeding it hurts braking and steering.
  36. Which phone use is allowed for a CMV driver while driving?
    Answer: Hands-free calling that takes no more than a single touch. Federal rules ban texting and holding a mobile phone while driving a CMV. Fines can reach thousands of dollars, and repeat violations can cost you your CDL.
  37. What are the three parts of total stopping distance?
    Answer: Perception distance + reaction distance + braking distance. Total stopping distance is the distance you travel while noticing the hazard, the distance while moving your foot to the brake, and the distance while braking.
  38. If you double your speed, how much more distance does it take to stop?
    Answer: About four times as much. Braking distance grows with the square of speed. Double the speed and it takes about four times the distance to stop.
  39. Why can an empty truck need more distance to stop than a loaded one?
    Answer: It has less traction. With less weight pressing the tires down, an empty truck has less grip, so the wheels lock up more easily and stopping takes longer.
  40. You are involved in a crash. What should you do first?
    Answer: Protect the area to prevent another crash (hazard lights, warning devices). First protect the area so no one else crashes into the scene, then notify authorities and help the injured.
  41. How should you take a curve in a large vehicle?
    Answer: Slow down before the curve, not in it. Posted curve speeds can be too fast for a tall, heavy vehicle. Slow down before entering and don't brake while in the curve.

Frequently asked questions

How many questions are on the CDL general knowledge test?

Most states use 50 questions and require 80% (40 correct) to pass. Your state's DMV can tell you its exact format.

What does the general knowledge test cover?

Safe driving: vehicle inspection, basic control, shifting, seeing, communicating, speed and space management, hazards, night and winter driving, emergencies, skids, crashes, fires, alcohol and drugs, and transporting cargo safely.

Do I need other tests too?

Probably. Most trucks need the air brakes test, and tractor-trailers need combination vehicles. Endorsements like hazardous materials, tanker, doubles/triples, passenger and school bus each have their own test.

Is this an official CDL test?

No. CalcSunny is independent and not affiliated with the FMCSA or any state DMV. These are practice questions we wrote from the model CDL manual's topics.

How long is a CDL learner's permit (CLP) valid?

Under federal rules a CLP is valid for up to one year, and you must hold it at least 14 days before taking the CDL skills test.

More free practice tests

Practice questions for study only. CalcSunny is not affiliated with USCIS, the DMV or the FMCSA.