CDL Practice Test: 40 Free General Knowledge Questions and Study Guide

Your state's age rules, permit wait, fees and skills test sites are on its own page: CDL requirements by state.

Getting a commercial driver's license (CDL) has two stages. First you pass one or more knowledge tests and get a commercial learner's permit (CLP). Then you practice on the road with a licensed CDL driver and take the skills test.

Everyone takes the general knowledge test. The other knowledge tests depend on the license class and the endorsements you want, as the table shows. Your state may add its own rules test too. Texas, for example, starts with a Texas commercial rules test.

Federal rules say you must answer at least 80 percent of the questions right on each knowledge test. States build their tests from a pool of questions the Federal Motor Carrier Safety Administration (FMCSA) approves. Your state's licensing office can tell you how many questions its test has. Your state's CDL manual is the study book. Many states publish the same model manual, written by AAMVA.

If you fail the air brakes test, you can still get a CLP or CDL, but it will carry an air brake restriction: you may not drive a commercial vehicle with any kind of air brakes. If you fail the combination vehicles test, you cannot get a Class A.

The skills test has three parts: the vehicle inspection, basic vehicle control and the road test. Our CDL pre-trip inspection checklist walks through the inspection.

The general knowledge test checks the parts of the CDL manual every driver needs: inspecting the vehicle, controlling it, seeing and signaling, speed and space, hazards, bad weather, mountain driving, railroad crossings, emergencies and skids, accidents and fires, alcohol and drugs, and securing cargo.

Many questions depend on a number. These are the ones worth memorizing.

1. Take all 40 questions in one sitting, without notes. That shows where you stand today.
2. Read the reason under every answer, including the ones you got right. A lucky guess will not help you on test day.
3. Write down the topic of every question you missed. Each question shows its topic.
4. Read those parts of your state's CDL manual. Take the test again two days later.

Aim for 36 out of 40 or better. You need 80 percent on the real test, so a strong score here gives you room to spare.

We wrote every question on this page ourselves, from the CDL manual. They are not the questions on your state's test.

• Day 1: Take the practice test. List the topics you missed.
• Day 2: Vehicle inspection: tires, wheels, brakes, steering and the seven-step inspection.
• Day 3: Speed and space: how far to look ahead, following distance, stopping distance, curves and slippery roads.
• Day 4: Hazards: night driving, fog, winter and hot weather, mountain grades and railroad crossings.
• Day 5: Emergencies: steering to avoid a crash, controlled braking, ABS, skids, tire failure, accidents and fires.
• Day 6: Alcohol, drugs, phone rules and cargo. Then study the section for each endorsement you want, such as air brakes or combination vehicles. Retake the practice test.
• Day 7: Rest. Gather the documents your licensing office asks for.

Still looking for a school? WIOA, the federal job training law, can pay for CDL training. See how WIOA pays for CDL training.

Read every question and every choice before you answer. Many questions depend on a number, so watch for feet, seconds and percentages.

If two answers look right, pick the one that keeps the most space around the truck and the most control of it.

Rules after a failed test differ by state. In Texas, for example, the application stays open for 90 days, and three failed tries or the end of those 90 days voids it, so you start over with a new application and fees.

You get your CLP. In Texas you must hold it at least 14 days before the skills test, and you may only drive a commercial vehicle with a CDL holder of the same class, 21 or older, in the seat beside you.

If you are getting a Class A or Class B CDL for the first time, upgrading to Class B or Class A, or adding a passenger, school bus or hazmat endorsement for the first time, federal entry-level driver training (ELDT) rules apply. In Texas, the state checks your training record electronically before it issues the license. See FMCSA's Training Provider Registry.

Once you have your CDL, Capstone Careers shows driving jobs near you, writes your resume for each one and fills in the long application for you to check and send. Your first application each month is free.

1. How much tread depth do the front tires need, at a minimum, in every major groove?
   Answer: 4/32 inch. Front tires need at least 4/32 inch of tread in every major groove. The other tires need at least 2/32 inch.
2. You are hauling cargo. When must you first check how it is tied down after you start the trip?
   Answer: Within the first 50 miles. Check the cargo and its tie-downs within the first 50 miles of the trip, then keep checking it as you go.
3. After that first check, how often must you check how the cargo is tied down?
   Answer: Every 150 miles or every 3 hours, whichever comes first. After the first 50 miles, check again every 150 miles or every 3 hours, whichever comes first, and after every break.
4. A tire on your truck was just changed. What should you do?
   Answer: Stop a short while later and check that the wheel nuts are still tight. Wheel nuts can loosen after a tire change. Stop a short while later and recheck that they are tight.
5. How far ahead do most good drivers look?
   Answer: 12 to 15 seconds ahead. Look 12 to 15 seconds ahead, so you have time to slow down or change lanes before you reach a problem.
6. At highway speed, about how far is 12 to 15 seconds ahead?
   Answer: About a quarter of a mile. At highway speeds, 12 to 15 seconds is about a quarter of a mile. In town it is about one block.
7. You drive a 40-foot truck at under 40 mph. How much following distance should you keep?
   Answer: 4 seconds. Keep 1 second for every 10 feet of vehicle length below 40 mph. A 40-foot truck needs 4 seconds.
8. You drive a 50-foot truck at over 40 mph. How much following distance should you keep?
   Answer: 6 seconds. One second per 10 feet gives 5 seconds. Above 40 mph, add 1 more second, for 6 seconds.
9. In ideal conditions at 55 mph, about how far will your truck travel before it stops, counting the time to see, react and brake?
   Answer: About 419 feet. The manual adds about 142 feet to recognize the hazard, 61 feet to react and 216 feet to brake, for at least 419 feet.
10. If you double your speed from 20 mph to 40 mph, what happens to your braking distance?
    Answer: It becomes about 4 times longer. Doubling your speed makes the braking distance and the force of a crash about 4 times greater.
11. How much should you slow down on a wet road?
    Answer: Slow down by about one-third. Wet roads can double your stopping distance. Slow down by about one-third, for example from 55 to about 35 mph.
12. How much should you slow down on packed snow?
    Answer: By half or more. On packed snow, cut your speed by half or more. On ice, slow to a crawl and stop as soon as it is safe.
13. When the temperature drops near freezing, which part of the road usually freezes first?
    Answer: Bridges. Bridges freeze before the rest of the road. Be extra careful on them when it is close to 32 degrees.
14. Your truck starts to hydroplane on standing water. What should you do?
    Answer: Take your foot off the accelerator and push in the clutch. Release the accelerator and push in the clutch so the wheels turn freely. Do not use the brakes to slow down.
15. What is the safe way to take a curve?
    Answer: Slow to a safe speed before you enter the curve. Slow down before the curve. Braking in a curve can lock the wheels and cause a skid, and a tall truck can roll over even at the posted curve speed.
16. You are about to go down a long, steep grade. When should you shift to a low gear?
    Answer: Before you start down the grade. Shift down before the grade. Once speed builds up you may not be able to get into a lower gear, and you can lose all engine braking.
17. Your safe speed on a downgrade is 40 mph. You are in the right low gear and reach 40 mph. What should you do?
    Answer: Brake firmly until you slow to about 35 mph, then let off the brakes. Brake hard enough to feel a clear slowdown to about 5 mph under your safe speed, then release. Repeat each time you reach the safe speed.
18. What can happen if some of your brakes are out of adjustment?
    Answer: The other brakes do more work, overheat and can fade. Brakes out of adjustment stop doing their share. The others overheat and fade, so check adjustment often.
19. Why does an empty truck need more distance to stop than a loaded one?
    Answer: An empty truck has less traction. Brakes, tires and springs work best when the truck is loaded. An empty truck has less traction, so it needs more room to stop.
20. You must stop on the shoulder of a road. How soon must your warning triangles be out?
    Answer: Within 10 minutes. Turn on your four-way flashers and put out your warning devices within 10 minutes of stopping.
21. You stop on a divided highway. Where do the three warning triangles go?
    Answer: 10, 100 and 200 feet toward the approaching traffic. On a one-way or divided highway, place them 10 feet, 100 feet and 200 feet toward the traffic coming at you.
22. You stop on a two-lane road with traffic in both directions. Where do the triangles go?
    Answer: One within 10 feet of a front or rear corner, one 100 feet behind and one 100 feet ahead. On a two-way road, mark the truck within 10 feet of a front or rear corner, then put one 100 feet behind and one 100 feet ahead.
23. You must stop at a railroad crossing that has no white stop line. Where do you stop?
    Answer: No closer than 15 feet and no more than 50 feet from the nearest rail. With no stop line, stop no closer than 15 feet and no farther than 50 feet from the nearest rail.
24. About how long does a typical tractor-trailer take to clear a single railroad track?
    Answer: At least 14 seconds. A typical tractor-trailer needs at least 14 seconds to clear one track and more than 15 seconds for two. Be sure you can get all the way across before you start.
25. Your truck gets stuck on the railroad tracks. What should you do?
    Answer: Get out, move away from the tracks, and call the number posted at the crossing or 911. Get out and away from the tracks. Call the emergency number on the crossing sign or 911, and give the crossing's DOT number if it is posted.
26. What is "controlled braking"?
    Answer: Braking as hard as you can without locking the wheels. In controlled braking you brake as hard as you can without locking the wheels, and keep steering movements very small.
27. You must steer quickly to miss a crash. What should you avoid?
    Answer: Braking while you are turning. Do not brake while you turn. It is easy to lock the wheels and skid out of control.
28. A front tire blows out. What should you do first?
    Answer: Hold the steering wheel firmly and stay off the brake until the truck slows down. Hold the wheel firmly with both hands. Stay off the brake until the truck slows, then brake gently and pull off the road.
29. The yellow ABS malfunction lamp stays on while you drive. What does it mean?
    Answer: You may have lost antilock braking, but you still have regular brakes. Drive normally and get it fixed soon. A lamp that stays on means ABS may not be working on one or more wheels. Your regular brakes still work, so drive normally and have it serviced soon.
30. The rear drive wheels lock up and start to skid. What should you do?
    Answer: Stop braking, then counter-steer. Let off the brakes so the rear wheels roll again, then counter-steer as the truck comes back on course.
31. What causes most front-wheel skids?
    Answer: Driving too fast for conditions. Most front-wheel skids come from driving too fast for conditions. Worn front tires and too little weight on the front axle also cause them.
32. You are at an accident scene. What is the first thing to do?
    Answer: Protect the area so another crash does not happen there. First keep another crash from happening: move your truck off the road if you can, turn on your flashers and put out triangles.
33. Your engine catches fire. What should you do?
    Answer: Turn off the engine and avoid opening the hood if you can. Turn off the engine and do not open the hood if you can avoid it. Spray through the vents (louvers), the radiator or from underneath, and never pull into a service station.
34. There is a fire in the cargo of your closed van trailer. What should you do with the doors?
    Answer: Keep them shut. Keep the doors shut. Opening them feeds the fire air and can make it burn very fast, especially with hazardous cargo.
35. Where do you aim a fire extinguisher?
    Answer: At the base or source of the fire. Aim at the base of the fire, stay as far back as you can, and stand upwind so the wind carries the spray to the fire.
36. It is illegal to drive a commercial vehicle with a blood alcohol concentration of:
    Answer: .04% or more. At .04% or more it is illegal, and a first offense costs your CDL for at least one year. Any detectable alcohol under .04% still puts you out of service for 24 hours.
37. What sobers a person up after drinking?
    Answer: Only time. The liver clears alcohol at a fixed rate, about one-third of an ounce an hour. Only time works.
38. Which use of a phone is allowed while you drive a commercial vehicle?
    Answer: Talking on a hands-free phone that is close to you. You may only use a hands-free phone that is close to you. Holding it, dialing with more than one button or reaching for it is not allowed, except to call for help in an emergency.
39. How many tie-downs does cargo need?
    Answer: At least one for every 10 feet of cargo, and never fewer than two. Use at least one tie-down for every 10 feet of cargo. Even small cargo needs at least two.
40. How should you load heavy cargo?
    Answer: As low as possible, with the heaviest parts under the lightest. A high center of gravity makes rollover more likely. Keep cargo low and put the heaviest parts under the lightest.

• Texas Department of Public Safety, Commercial Motor Vehicle Drivers Handbook (DL-7C, AAMVA model manual), revised March 2026
• 49 CFR Part 383, Subpart H: Tests (passing score, knowledge test pool)
• FMCSA Training Provider Registry (entry-level driver training)