CDL General Knowledge Study Guide 2026

How CDL licensing works

A commercial driver's license (CDL) follows one national standard. Federal rules in 49 CFR Part 383 set the license classes, endorsements, tests and penalties, and your state issues the license. Every state uses the same model CDL manual.

License classes

A higher class covers the lower ones. With a Class A CDL you may drive Class B and C vehicles, and with Class B you may drive Class C vehicles, as long as you have any endorsements those vehicles need.

Endorsements

The knowledge tests

Everyone takes the general knowledge test. You may also need the air brakes test, the combination vehicles test, and a test for each endorsement. Federal rules require a score of at least 80% on every knowledge test. The number of questions is set by each state. These counts come from state licensing agencies:

The knowledge test may be given in another language, but no interpreter may be used. The skills test is always done in English.

Restrictions

• If you fail the air brakes test or take your skills test in a vehicle without air brakes, you may not drive a CMV with air brakes.
• If you test in a vehicle with air-over-hydraulic brakes, you may not drive a vehicle with full air brakes.
• If you test in an automatic, you may not drive a CMV with a manual transmission.
• If your Class A test vehicle used a pintle hook instead of a fifth wheel, you may not drive fifth-wheel combinations.
• A V restriction means the state was told you have a medical variance.

The commercial learner's permit (CLP)

After you pass the general knowledge test (and any other knowledge tests you need), you can get a CLP so you can practice with a trainer.

• A CLP is valid for no more than one year. Some states issue it for less, such as 6 months in Utah or 180 days in Wisconsin.
• A CDL holder with the right class and endorsements must sit in the front seat next to you at all times.
• You must hold the CLP for at least 14 days before you take the skills test.
• Only the P, S and N endorsements can be on a CLP. With P or S you may not carry passengers (other than people such as trainers and examiners). With N you may drive only an empty tank.
• A CLP holder may never drive a vehicle carrying hazardous materials.

Entry-level driver training (ELDT)

Since February 7, 2022, you must complete entry-level driver training from a provider listed on FMCSA's Training Provider Registry before you take the skills test for your first Class A or Class B CDL. ELDT is also required to upgrade from Class B to Class A, and for a first passenger, school bus or hazmat endorsement. For hazmat, the training comes before the hazmat knowledge test. The rule is not retroactive: if you got your CDL or endorsement before February 7, 2022, you do not need ELDT for it.

Age and medical rules

• You must be at least 21 to drive a CMV across state lines. Some states let drivers aged 18 to 20 drive within the state only.
• Your DOT medical exam must be done by a medical examiner on FMCSA's National Registry. Most drivers are re-examined at least every 24 months; some conditions need an exam every 12 months.
• Vision: at least 20/40 in each eye (glasses allowed) and a field of vision of at least 70 degrees in each eye.
• Hearing: you must hear a forced whisper at 5 feet or more in your better ear.
• You must read and speak English well enough to talk with the public, read signs, answer officials and fill out reports.

Rules every CDL holder must follow

• Hold only one driver's license.
• Tell your employer within 30 days if you are convicted of a traffic violation (other than parking) in any vehicle.
• When you apply for a driving job, list all driving jobs from the past 10 years.
• Never text or hold a phone to talk while driving a CMV. You may use a hand-held phone only to call police or emergency services in an emergency.
• Wear your seat belt, and never have a radar detector in a CMV.

Alcohol and disqualification

In a CMV the alcohol limit is 0.04%. You may not use alcohol within 4 hours before going on duty, and any detectable alcohol while on duty puts you out of service for 24 hours. Refusing an alcohol test counts the same as driving under the influence.

Inspecting your vehicle

Federal and state laws require you to inspect your vehicle, and an inspector can put an unsafe vehicle out of service. The model manual teaches a seven-step method so you never miss anything. Use the same order every time.

1. Vehicle overview. Walk up to the vehicle and look for leaning, leaks, damage and hazards around it. Review the last inspection report. Sign it only if defects were certified as repaired or not needing repair.
2. Engine compartment. Set the parking brake and/or chock the wheels first. Check oil, coolant, power steering fluid, washer fluid, the battery, belts, hoses and wiring.
3. Start the engine and inspect inside the cab. Check gauges, warning lights, controls, mirrors, windshield, emergency equipment and the seat belt.
4. Turn off the engine and check the lights. Turn on the low beams and four-way flashers, then get out.
5. Walkaround. Check lights, tires, wheels, suspension, brakes, cargo securement and fuel tanks on every side, front and rear.
6. Check signal lights. Check the brake lights and turn signals.
7. Start the engine and check the brakes. Test the parking brake by gently pulling against it in a low gear. Test the service brakes by moving forward at about 5 mph and braking firmly. With hydraulic brakes, pump the pedal three times and hold it for five seconds; it should not move.

Numbers to know

Emergency equipment

• A properly charged and rated fire extinguisher.
• Warning devices: three reflective triangles (or six fusees or three liquid-burning flares).
• Spare electrical fuses, unless the vehicle has circuit breakers.

Basic control, shifting and seeing

Starting, steering and backing

Accelerate smoothly. If the drive wheels start to spin, take your foot off the accelerator. Hold the steering wheel firmly with both hands on opposite sides.

Backing is always dangerous because you cannot see everything behind you. Back slowly in the lowest reverse gear. Back toward the driver's side when you can, use a helper, and G.O.A.L.: Get Out And Look when you are not sure. When backing a trailer, turn the steering wheel the opposite way from where you want the trailer to go.

Shifting gears

Unsynchronized manual transmissions usually need double clutching. Shift down before you start down a hill and before you enter a curve. Retarders help slow the vehicle, but turn them off on wet, icy or snowy roads, because they can make the drive wheels skid.

Seeing ahead and to the sides

Most good drivers look 12 to 15 seconds ahead. In town that is about one block. On the highway it is about a quarter of a mile. Looking far ahead gives you time to slow down or change lanes smoothly.

Check your mirrors often and adjust them before each trip. On a tractor-trailer you can only check them properly when the trailer is straight. Convex mirrors show a wider area, but they make things look smaller and farther away than they really are. During a lane change, check your mirrors before you change, after you signal, right after you start, and after you finish.

Avoid driving alongside other vehicles for long. Another driver may change lanes into you, or you may be trapped when you need to move over.

Communicating and warning devices

Let other road users know what you are going to do. Signal well before you turn and keep signaling until you finish. Change lanes slowly and smoothly. To warn drivers behind you that you are slowing, tap the brake pedal lightly to flash the brake lights. Use your four-way flashers when driving very slowly or stopped, where it is legal.

Do not wave other drivers past you. You could cause a crash. At dawn, dusk, and in rain or snow, turn on your low beam headlights so others can see you.

Placing warning devices

When you stop on or beside the road, turn on your four-way flashers and put out your warning devices as soon as possible, within 10 minutes. Hold the triangles between yourself and oncoming traffic as you walk.

Speed and stopping distance

Driving too fast is a major cause of fatal crashes. Your total stopping distance has three parts: perception distance (the time your brain needs to see the danger), reaction distance (moving your foot to the brake), and braking distance.

Vehicles with air brakes take even longer, because of brake lag. Speed has a huge effect: when you double your speed (20 to 40 mph), braking distance and the force of a crash become about four times greater. At three times the speed they are about nine times greater.

Weight matters too. Empty trucks often need more distance to stop than loaded trucks because they have less traction.

Matching speed to the road

Hydroplaning can happen at speeds as low as 30 mph when there is a lot of water. If it happens, release the accelerator and push in the clutch. Do not brake.

Slow down before a curve, because braking in a curve can lock the wheels. At night, drive slowly enough to stop within the distance your headlights let you see. In work zones, slow down: speeding traffic is the number one cause of injury and death there.

Downgrades

Choose a low gear before you start down a long grade, and let the engine's braking effect do most of the work. Do not try to downshift after your speed has built up. Use the brakes this way:

1. Apply the brakes just hard enough to feel a definite slowdown.
2. When you are about 5 mph below your safe speed, release the brakes. Each application should last about 3 seconds.
3. When your speed is back up to the safe speed, repeat.

For example, if your safe speed is 40 mph, brake down to about 35 mph, then release. Overusing the brakes makes them overheat and fade. Escape ramps are built to stop runaway vehicles; know where they are on your route.

Managing space

You need space all around your vehicle: ahead, behind, to the sides, overhead and below.

Following distance

Leave at least 1 second for every 10 feet of vehicle length at speeds below 40 mph. Above 40 mph, add 1 second. To measure, wait until the vehicle ahead passes a fixed object, then count "one thousand-and-one, one thousand-and-two" until you reach it.

If someone tailgates you, open up more space ahead so you will not have to stop suddenly. Do not speed up or flash your lights at them.

Sides, overhead and below

Keep your vehicle centered in the lane. An empty van rides higher than a loaded one, so it is more likely to hit a low bridge, and posted clearance heights may be wrong. Raised railroad tracks and drainage channels can catch low trailers underneath.

Turns

Never swing left before a right turn: a driver behind you may try to pass on your right. For a left turn, drive to the center of the intersection before turning. If there are two left-turn lanes, use the right-hand one. When you cross or enter traffic, you need a much bigger gap than a car, and more when you are loaded.

Hazards, night, fog and weather

Watch for clues that a road or a driver is a hazard: work zones, drop-offs, objects in the road, delivery trucks, parked cars, and drivers who seem confused, distracted or impaired. Slow-moving vehicles show an orange triangle with a red border. Always have a plan for what you would do in an emergency.

Distraction and aggressive drivers

If a distraction makes you react just half a second slower, your chance of a crash doubles. Pull off the road safely before making or taking a call. If an aggressive driver confronts you, get out of their way, avoid eye contact and report them when it is safe.

Night driving

Do not wear sunglasses at night. If you get sleepy, the only safe cure is to get off the road and sleep.

Fog, winter and hot weather

• Fog: the best advice is not to drive. If you must, slow down before you enter it, use low beams and fog lights, and turn on your flashers.
• Winter: carry the right number of chains plus extra cross-links, and learn to put them on before you need them. An exhaust leak can let deadly carbon monoxide into the cab. Dry wet brakes by braking lightly in a low gear.
• Hot weather: check tires every 2 hours or every 100 miles. If a tire is too hot to touch, stay stopped until it cools, and never let air out of a hot tire. Never open a hot cooling system. Tar that bleeds onto the road surface is very slippery.

Railroad crossings and emergencies

Railroad crossings

A round, yellow, black-lettered sign warns that a railroad crossing is ahead. A typical tractor-trailer needs at least 14 seconds to clear a single track and more than 15 seconds to clear a double track. Never race a train, and never shift gears while crossing the tracks.

Buses carrying passengers and vehicles carrying certain hazardous materials must stop between 15 and 50 feet from the nearest rail. If your low trailer gets stuck on a raised crossing, get out and away from the tracks, then call 911 and give the crossing's location and DOT number if it is posted.

Steering and braking in an emergency

You can almost always steer around a problem faster than you can stop. Keep both hands on the wheel, do not brake while turning, and turn only as much as you need. If an oncoming driver drifts into your lane, move to your right. Leaving the road onto the shoulder may be safer than a collision.

• Controlled braking: brake as hard as you can without locking the wheels.
• Stab braking: brake fully, release when the wheels lock, and brake fully again when they roll.
• Antilock brakes (ABS): keep the wheels from locking so you can steer. ABS does not always shorten stopping distance. A yellow lamp warns of an ABS problem; your normal brakes still work.

Transporting cargo safely

You are responsible for your cargo even if someone else loaded it. Check that it is balanced, secured, not overloaded, and does not block your view or your emergency equipment.

Cargo inspections

Check the cargo and its securement within the first 50 miles of a trip. After that, re-check after every 3 hours or 150 miles, whichever comes first, when your duty status changes, and after every break.

Weight terms

States set limits on gross and axle weights. Many use a bridge formula, which allows less weight on axles that are closer together. Overloading hurts steering, braking and speed control.

Loading and securing

• Keep the load low, with the heaviest items under the lightest, to avoid a high center of gravity.
• Too little weight on the drive axles gives poor traction. Too much on the steering axle makes steering hard.
• Use at least one tiedown for every 10 feet of cargo, and never fewer than two.
• The total working load limit of the tiedowns must be at least half the weight of the cargo.
• A header board (headache rack) protects you from cargo sliding forward.
• Cover cargo to protect people from spills and the cargo from weather.
• With sealed loads you cannot inspect the cargo, but you must still stay within weight limits.

Cargo that needs extra care

Dry bulk, hanging meat and livestock all raise the center of gravity or can shift, so go slowly on curves and ramps. Use false bulkheads to keep a partial load of livestock together. Oversized loads need special permits and are often limited to certain times; some need escorts.

If any of your cargo is hazardous material that needs placards, you must have a hazmat endorsement.