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CDL practice questions

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Question 1General Knowledge

What is the main purpose of a pre-trip inspection?

  1. Find defects that could cause a crash Correct answer.
  2. Adjust the mirrors and seat
  3. Record the trip mileage in the logbook
  4. Complete the insurance paperwork

Explanation

A pre-trip inspection is a systematic check of the vehicle's critical systems and parts before driving. Its purpose is to catch defects that could cause a crash or breakdown. Logbooks and paperwork are handled separately from the inspection.

Question 2General Knowledge

What is the maximum blood alcohol concentration allowed for a CDL holder while operating a commercial motor vehicle?

  1. 0.02 percent
  2. 0.08 percent
  3. 0.04 percent Correct answer.
  4. 0.10 percent

Explanation

A CDL holder is considered under the influence at a blood alcohol concentration of 0.04 percent or higher. That is half the 0.08 percent limit that applies to non-commercial drivers. A DUI conviction in a personal vehicle can also disqualify the CDL.

Question 3General Knowledge

After 10 consecutive hours off duty, how many hours may a driver drive?

  1. 10 hours
  2. 14 hours
  3. 11 hours Correct answer.
  4. 8 hours

Explanation

A driver may drive up to 11 hours after taking 10 consecutive hours off duty. All driving must also be completed within 14 consecutive hours of coming on duty. A 30-minute break is required after 8 cumulative hours of driving.

Question 4General Knowledge

Which commercial vehicles must always stop at a railroad crossing?

  1. Trucks with air brakes
  2. Tankers hauling water
  3. Buses and hazmat vehicles Correct answer.
  4. Trucks over 26,000 pounds

Explanation

Commercial vehicles carrying passengers or hazardous materials must stop at every railroad crossing, even when no train is visible. Other commercial vehicles must stop only where signs or signals require it or when a train is approaching.

Question 5General Knowledge

Texting while driving a commercial motor vehicle is:

  1. Prohibited for all CMV drivers Correct answer.
  2. Allowed only while stopped at a light
  3. Allowed when traffic is light
  4. Allowed with a hands-free device

Explanation

Federal rules prohibit texting while operating a commercial motor vehicle, including reading or sending messages. A hands-free device does not make texting legal. A texting conviction also counts as a serious traffic violation for CDL holders.

Question 6General Knowledge

If your vehicle is disabled on the highway, when must your warning devices be placed?

  1. Within 10 minutes of stopping Correct answer.
  2. Only if it is dark outside
  3. Only after the police arrive
  4. After you finish your paperwork

Explanation

Warning devices must be placed within 10 minutes of stopping so other drivers have time to react. Placement distance depends on the road: on a two-way road, 10 feet behind and ahead plus 100 feet behind, and farther back on divided highways, curves, and hills.

Question 7General Knowledge

How far ahead should you scan the road while driving?

  1. About 60 seconds
  2. About 12 to 15 seconds Correct answer.
  3. About 2 seconds
  4. About 5 seconds

Explanation

Looking 12 to 15 seconds ahead gives you time to spot hazards and adjust speed or lane position. At highway speed that distance is roughly a quarter mile.

Question 8General Knowledge

What is the best way to deal with drowsiness on a long trip?

  1. Stop and take a rest break Correct answer.
  2. Drink coffee and continue to your destination
  3. Open a window and keep driving
  4. Turn up the radio

Explanation

Drowsiness impairs judgment and reaction time and cannot be reversed for long with fresh air, music, or caffeine. The safe remedy is to stop and rest. Continuing to drive while sleepy is a major crash risk.

Question 9General Knowledge

Under the one-second-per-10-feet rule, what following distance should you keep when driving a 50-foot vehicle at 50 mph?

  1. 6 seconds Correct answer.
  2. 5 seconds
  3. 4 seconds
  4. 3 seconds

Explanation

The rule calls for one second for every 10 feet of vehicle length, plus one extra second at speeds above 40 mph. A 50-foot vehicle therefore needs 5 seconds below 40 mph and 6 seconds above it.

Question 10General Knowledge

What is the safest way to back a large commercial vehicle?

  1. Back toward the passenger side so you can watch in the mirror
  2. Back as quickly as possible to spend less time in the roadway
  3. Back toward the driver's side with a helper guiding you Correct answer.
  4. Back using only your right mirror

Explanation

Backing toward the driver's side lets you see the rear of the vehicle more directly, and a helper on the ground can watch your blind spots. If you lose sight of the helper, stop until you can see them again.

Question 11General Knowledge

Before starting down a long, steep downgrade, you should:

  1. Shift into a lower gear and stay in it Correct answer.
  2. Put the transmission in neutral to save fuel
  3. Rely on the service brakes alone to control speed
  4. Increase speed to carry momentum up the next hill

Explanation

You should descend a grade in the same low gear you would use to climb it, letting engine braking hold your speed. Coasting in neutral is dangerous and illegal in many states. Continuous reliance on the service brakes can cause them to overheat and fade.

Question 12General Knowledge

If a front tire blows out while you are driving, you should:

  1. Accelerate to stabilize the vehicle
  2. Brake as hard as you can right away
  3. Turn sharply onto the shoulder at once
  4. Hold the wheel and slow gradually Correct answer.

Explanation

A blowout pulls the vehicle toward the failed tire, so grip the wheel firmly and keep the vehicle straight. Ease off the accelerator and let the vehicle slow before braking gently and pulling off the road.

Question 13General Knowledge

If the rear of your vehicle starts to skid, you should:

  1. Brake harder until the skid stops
  2. Accelerate to pull the vehicle straight
  3. Steer quickly away from the direction of the skid
  4. Let off the brakes and steer with the skid Correct answer.

Explanation

In a rear-wheel skid you should stop braking so the wheels can roll again and steer in the direction the rear is sliding. That realigns the vehicle with the road. Braking harder or steering the opposite way makes the skid worse.

Question 14General Knowledge

Hydroplaning is most likely to happen when:

  1. Tires are underinflated and the road is wet Correct answer.
  2. Tire tread is deep and speed is low
  3. You are driving with the cruise control on a dry road
  4. The road is dry and temperatures are high

Explanation

Hydroplaning occurs when tires ride up on a film of water instead of touching the pavement. Low tread depth, underinflation, high speed, and standing water all make it more likely. Ease off the accelerator without braking hard until traction returns.

Question 15General Knowledge

When descending a steep grade, how should you use the service brakes?

  1. Apply them fully and hold them down the whole grade
  2. Use them only in an emergency
  3. Ride them lightly and continuously
  4. Apply them firmly for short periods, then release Correct answer.

Explanation

Firm, brief brake applications slow the vehicle and then let the brakes cool while engine braking holds the speed. Riding the brakes lightly or holding them down overheats them and causes fade.

Question 16General Knowledge

Driving faster than your headlights let you see and stop is called:

  1. Off-tracking on a sharp curve
  2. Following the vehicle ahead too closely
  3. Coasting downhill in neutral
  4. Overdriving your headlights Correct answer.

Explanation

At night you should drive slowly enough to stop within the distance your headlights light up. If a hazard appears beyond that distance, you cannot stop in time.

Question 17General Knowledge

What is the first thing you should do at the scene of a crash involving your vehicle?

  1. Wait in your cab for the police
  2. Protect the area by warning traffic Correct answer.
  3. Take photos for the insurance company
  4. Move the cargo off to the shoulder

Explanation

Protecting the scene with flashers and warning devices prevents further crashes, which is the greatest immediate danger. After the area is protected, check for injuries, call for help, and report the crash.

Question 18General Knowledge

If your service brakes fail while you are driving, you should:

  1. Downshift and apply the parking brake gradually Correct answer.
  2. Shift into neutral and coast
  3. Turn off the engine immediately
  4. Hold the brake pedal to the floor and keep it there

Explanation

Downshifting uses engine braking to slow the vehicle, and applying the parking brake gradually can add stopping power without locking the wheels. Look for an escape ramp and use it if one is available.

Question 19General Knowledge

Which cargo must be covered?

  1. Only cargo that weighs more than 1,100 pounds
  2. Only cargo carried on flatbed trailers
  3. Any cargo that could blow or fall off Correct answer.
  4. Only cargo taller than the cab

Explanation

Cargo that could blow, fall, or be blown off the vehicle must be covered, such as loose gravel, sand, or trash. Covers protect other road users from flying debris.

Question 20General Knowledge

As you approach an intersection, you should:

  1. Scan left, right, and left again before entering Correct answer.
  2. Watch only the traffic light
  3. Speed up to clear the intersection before the light changes
  4. Assume cross traffic will stop for you

Explanation

Intersections are high-risk because other drivers may run signals or misjudge gaps. Scanning both directions lets you spot vehicles that are not obeying the signals or signs.

Question 21General Knowledge

Off-tracking means that:

  1. The front wheels slip on wet pavement
  2. The speedometer reads higher than actual speed
  3. Rear wheels cut inside the path of the front wheels Correct answer.
  4. The trailer drifts sideways during hard braking on a curve

Explanation

Because the rear wheels are not steerable, they follow a shorter path and cut inside the front wheels in a turn. Drivers must swing wide enough that the rear wheels clear curbs, poles, and other vehicles.

Question 22General Knowledge

If a fire breaks out in your engine compartment, you should:

  1. Start the engine and drive to the nearest shop
  2. Keep the hood closed and spray through the grille Correct answer.
  3. Open the hood fully to aim at the flames
  4. Throw water on the engine from a distance

Explanation

Opening the hood feeds the fire with oxygen and can make flames flare up. Shut off the engine, get out, and aim the extinguisher through the grille or louvers.

Question 23General Knowledge

If your front wheels skid, you should:

  1. Turn the wheel sharply to the side
  2. Hold the wheel steady and ease off Correct answer.
  3. Accelerate hard to regain traction
  4. Brake as hard as you possibly can right away

Explanation

A front-wheel skid means the tires have lost traction and steering no longer responds. Releasing the accelerator lets the wheels roll again so steering returns. Hard braking or sharp turning makes the skid worse.

Question 24General Knowledge

If your vehicle stalls on a railroad crossing and a train is coming, you should:

  1. Stay in the cab and try to restart the engine
  2. Get out and run toward the train at an angle Correct answer.
  3. Run straight away from the tracks along the rails
  4. Stand beside the vehicle and wave the train down

Explanation

Getting clear of the vehicle is the priority. Running at an angle toward the approaching train moves you out of the path of the train and the debris it will throw.

Question 25General Knowledge

When should you turn off a retarder?

  1. When carrying a heavy load
  2. When driving down a long grade
  3. When driving at highway speeds
  4. When the road is wet or icy Correct answer.

Explanation

A retarder slows the drive wheels, and on a slippery surface it can cause them to break traction and skid. It is useful on dry downgrades but must be switched off when traction is poor.

Question 26Air Brakes

What is the purpose of the air compressor in an air brake system?

  1. It pumps air into the storage tanks. Correct answer.
  2. It cools the air before it reaches the brake chambers.
  3. It filters moisture out of the air lines.
  4. It releases air from the tanks when pressure gets too high.

Explanation

The engine-driven compressor is the source of pressure for the whole system; it pumps air into the storage tanks, where it is stored for braking. Moisture removal is handled by the air dryer and by draining the tanks, and excess pressure is released by the safety valve.

Question 27Air Brakes

How often should you drain the air tanks on a vehicle with manual drain valves?

  1. Once a month, whether or not you drive.
  2. At the end of each day of driving. Correct answer.
  3. Only when the air dryer cartridge is replaced.
  4. Only when the low pressure warning sounds.

Explanation

Compressed air carries water and oil into the tanks, and in cold weather that moisture can freeze and block the lines or valves. Draining the tanks at the end of each day of driving removes it before it can cause trouble. Automatic drain valves do this on their own, but manual valves must be opened by the driver.

Question 28Air Brakes

The low air pressure warning device must activate before pressure falls below:

  1. 100 psi
  2. 60 psi Correct answer.
  3. 80 psi
  4. 20 psi

Explanation

The warning light and buzzer must come on before system pressure drops below 60 psi. At that point the driver has lost a large part of the air supply and must stop safely and find the leak. Spring brakes do not begin to set until pressure is much lower.

Question 29Air Brakes

The yellow diamond-shaped knob on the dash controls:

  1. The air horn.
  2. The trailer service brakes only.
  3. A front brake limiting valve.
  4. The spring (parking) brakes. Correct answer.

Explanation

Pulling out the yellow diamond knob releases air from the spring brake chambers so the heavy springs apply the parking brakes; pushing it in sends air back in to release them. It is the parking and emergency brake control, not a service brake control.

Question 30Air Brakes

A dual air brake system has:

  1. Two compressors that share the same storage tank.
  2. Two separate air circuits that work independently. Correct answer.
  3. A single tank with a pressure gauge on each end.
  4. One circuit for the front axle and none for the rear.

Explanation

A dual system is really two complete air systems, each with its own tank, valves, and lines, so a failure in one still leaves braking on at least part of the vehicle. In most vehicles the split is front axle versus rear axle.

Question 31Air Brakes

When you press the brake pedal in an air brake vehicle, the service brakes are applied by:

  1. Air pressure from the storage tanks. Correct answer.
  2. Mechanical linkage from the pedal.
  3. Hydraulic pressure from a master cylinder.
  4. Vacuum from the engine intake.

Explanation

The foot valve meters air from the storage tanks into the brake chambers, where the air pushes a diaphragm and pushrod to apply the foundation brakes. Spring brakes work the opposite way: they apply when air is released.

Question 32Air Brakes

The couplers used to connect the trailer air lines to the tractor are called:

  1. Quick release valves.
  2. Glad hands. Correct answer.
  3. Relay valves.
  4. Shut-off cocks.

Explanation

Glad hands are the interlocking couplers at the ends of the tractor and trailer hoses, and they must be locked together with good seals. The lines must also be mated to the correct coupler, with red for the emergency line and blue for the service line.

Question 33Air Brakes

The safety valve on an air tank is designed to:

  1. Warn the driver when pressure is low.
  2. Drain water from the tank automatically.
  3. Release air if tank pressure climbs too high. Correct answer.
  4. Maintain pressure between cut-in and cut-out.

Explanation

The safety valve is a spring-loaded relief valve set near 150 psi that vents air if the governor or compressor malfunctions and pressure keeps rising. It protects the tank and lines from overpressure and is not part of normal pressure regulation.

Question 34Air Brakes

The governor's job in an air brake system is to:

  1. Turn the compressor on and off to hold pressure. Correct answer.
  2. Limit how much air reaches each brake chamber.
  3. Release air from the system when it overpressurizes.
  4. Keep the tractor and trailer pressures equal.

Explanation

The governor senses tank pressure and cuts the compressor out at the cut-out pressure, then restarts it at the cut-in pressure. That keeps the system in a safe operating band instead of letting pressure climb or fall without limit.

Question 35Air Brakes

In a typical air brake system, the governor cuts the compressor out at about:

  1. 125 psi Correct answer.
  2. 150 psi
  3. 200 psi
  4. 80 psi

Explanation

The governor stops the compressor at roughly 125 psi and restarts it at about 100 psi. The safety valve is set near 150 psi, so cut-out must occur well before that point to leave a margin.

Question 36Air Brakes

On a vehicle with spring brakes, at what pressure range will the parking brakes begin to apply by themselves?

  1. 5 to 10 psi
  2. 20 to 45 psi Correct answer.
  3. 60 to 75 psi
  4. 80 to 90 psi

Explanation

Spring brakes are held off by air pressure, so when pressure falls to roughly 20 to 45 psi the springs overcome the remaining air and start to set the brakes. This is why a severe air leak leaves a vehicle with its brakes locked rather than free to roll.

Question 37Air Brakes

The tractor protection valve:

  1. keeps the tractor's brakes from applying before the trailer's.
  2. prevents the trailer from rolling when it is parked.
  3. meters air to the trailer chambers in proportion to load.
  4. closes the trailer lines if they break or leak badly. Correct answer.

Explanation

The tractor protection valve automatically shuts off the tractor-to-trailer air lines when pressure falls too low or a line separates. That keeps the tractor's remaining air from escaping through a broken trailer line so the tractor can still be stopped.

Question 38Air Brakes

The air dryer in a brake system removes:

  1. oil vapor from the brake chambers.
  2. moisture before the air reaches the tanks. Correct answer.
  3. carbon monoxide from the compressor intake.
  4. excess pressure above the governor cut-out point.

Explanation

Compressed air carries water vapor that condenses in the tanks and lines and can freeze in cold weather. The dryer's desiccant cartridge strips that moisture out before the air enters the tanks. It does not regulate system pressure.

Question 39Air Brakes

The alcohol evaporator is used to:

  1. raise the cut-out pressure in cold weather.
  2. lubricate the compressor's internal parts.
  3. clean the brake shoes and drums.
  4. help keep moisture in the system from freezing. Correct answer.

Explanation

Alcohol vapor added to the air lowers the freezing point of any water in the lines and valves, which prevents ice from blocking them in cold weather. It is not a cleaner or a lubricant, and it does not change governor settings.

Question 40Air Brakes

In a two-line trailer air brake system, the emergency (supply) line is identified by which color?

  1. Green
  2. Black
  3. Red Correct answer.
  4. Blue

Explanation

The emergency or supply line is red and the service line is blue, and the glad hands should be color-matched when you couple up. Crossed lines can leave the trailer unable to apply its brakes in an emergency.

Question 41Air Brakes

What is the main advantage of a relay valve on a trailer or a long wheelbase vehicle?

  1. It sends air to the brake chambers faster, reducing brake lag. Correct answer.
  2. It keeps the trailer from pushing the tractor in a hard stop.
  3. It limits the pressure delivered to the front brake chambers.
  4. It lets the driver release the parking brakes from outside the cab.

Explanation

A relay valve takes a small signal pressure from the foot valve and uses air stored nearby to apply the brakes directly, shortening the path the air must travel. That reduces lag on long vehicles with large air volumes.

Question 42Air Brakes

On an S-cam drum brake, what happens when air enters the brake chamber?

  1. The S-cam rotates the brake drum to slow the wheel.
  2. Air pressure squeezes a disc between two pads.
  3. The pushrod pushes both brake shoes straight outward into the drum.
  4. The pushrod works the slack adjuster, which turns the S-cam. Correct answer.

Explanation

The chamber diaphragm pushes the pushrod out, and the pushrod lever turns the slack adjuster, which is splined to the S-cam shaft. As the S-cam rotates, its shape forces the shoes outward against the drum.

Question 43Air Brakes

Brake fade is best described as:

  1. air leaking past a worn brake chamber diaphragm.
  2. loss of braking power from overheated drums and linings. Correct answer.
  3. the delay between pressing the pedal and the brakes applying.
  4. the compressor cycling on and off as you brake.

Explanation

Repeated or continuous braking overheats the drums and linings, and hot brakes grab less, so stopping power drops. Controlling speed with a low gear and engine braking instead of riding the brakes is the main defense.

Question 44Air Brakes

When the ABS light stays on:

  1. the spring brakes will not release until it goes out.
  2. the trailer brakes are automatically disconnected.
  3. the vehicle must be taken out of service because all braking is lost.
  4. you have normal brakes but the wheels may lock in a hard stop. Correct answer.

Explanation

A steady ABS light means the anti-lock function is disabled, but the foundation brakes still work through the normal air system. Drive on and get the system repaired, allowing for the chance of wheel lockup during hard braking.

Question 45Air Brakes

Stab braking on a vehicle without ABS means:

  1. holding the pedal down and steering hard against a skid.
  2. applying light, steady pressure to keep the shoes cool.
  3. pumping the pedal as fast as you can until you stop.
  4. brake hard, release, then brake hard again. Correct answer.

Explanation

Stab braking means applying the brakes fully, holding until the wheels are about to lock or the vehicle slows enough, releasing, and repeating. It is for vehicles without ABS on dry pavement; on slippery roads or with ABS, steady pressure is correct.

Question 46Air Brakes

With the engine off, the system fully charged, and the brakes released, a single-unit vehicle should lose no more than:

  1. 3 psi in one minute. Correct answer.
  2. 10 psi in one minute.
  3. 20 psi in one minute.
  4. 3 psi in five minutes.

Explanation

The static leak test allows a loss of no more than 3 psi per minute on a single vehicle, or 4 psi on a combination, with the brakes released. Then you hold the brakes fully applied and check again, allowing no more than 4 psi per minute on a single vehicle.

Question 47Air Brakes

With the engine at operating speed and the system at 85 psi, pressure should build to 100 psi within about:

  1. 5 minutes.
  2. 2 minutes.
  3. 10 seconds.
  4. 45 seconds. Correct answer.

Explanation

The compressor should raise pressure from 85 to 100 psi in roughly 45 seconds. Much longer points to a weak compressor, a slipping belt, or a leak, and it also means the spring brakes will take longer to release.

Question 48Air Brakes

If the emergency (supply) line separates while you are driving:

  1. the trailer brakes stay released and only the tractor can stop.
  2. the trailer air supply valve pops out and the trailer brakes set. Correct answer.
  3. nothing happens until you shut the engine off.
  4. the trailer service brakes apply only when you press the pedal.

Explanation

Losing the supply line drops trailer pressure, so the trailer's spring brakes apply automatically and the trailer air supply knob pops out. This protects against a runaway trailer, which is why breakaway protection must be checked before a trip.

Question 49Air Brakes

Before you measure pushrod travel on a vehicle with spring brakes, you must:

  1. disconnect the slack adjuster from the pushrod.
  2. build pressure to the cut-out point and hold it there.
  3. drain the tanks so the springs are fully applied.
  4. chock the wheels and release the parking brakes. Correct answer.

Explanation

The spring brakes must be released, or they will hold the pushrods out and give a false reading. Chock the wheels first so the vehicle cannot move once the brakes are released, then have someone apply the brakes while you measure.

Question 50Air Brakes

A front brake limiting valve on an older vehicle, when set to the slippery position:

  1. adds air to the rear chambers to balance the tractor.
  2. blocks the exhaust port so the front brakes release slowly.
  3. shuts off the front brakes completely until traction returns.
  4. reduces air pressure to the front brake chambers by about half. Correct answer.

Explanation

The limiting valve halves the pressure delivered to the front chambers in slippery conditions to reduce the chance of front wheel lockup and loss of steering. It does not shut the front brakes off entirely, and the normal setting should be used on dry pavement.