20 free CDL practice questions

Correct answer: A · At least 8 seconds: 7 for length, plus 1 for going over 40 mph

The CDL manual's rule of thumb is at least one second for each 10 feet of vehicle length at speeds below 40 mph, plus one more second at higher speeds. A 70-foot rig needs 7 seconds for its length, and at 55 mph you add 1, for at least 8 seconds. Seven seconds gets the length part right but drops the extra second for speed above 40 mph. One second for every 10 mph of speed is not the rule the manual teaches, and it ignores length, the main reason a long combination needs a bigger gap. These are minimums for good conditions; a slippery road needs much more space.

Look for this next time: Allow one second per 10 feet of vehicle length, add one second above 40 mph, and leave more room on slippery roads.

Correct answer: C · Within 10 minutes: about 10 and 100 feet behind, and 100 feet ahead

Federal rule 49 CFR 392.22 covers this case. On an ordinary two-way road, place the warning devices as soon as possible and within 10 minutes: one on the traffic side about 10 feet from the truck toward approaching traffic, one about 100 feet behind, and one about 100 feet ahead, because traffic comes from both directions. Putting all three behind the truck at about 10, 100, and 200 feet is the pattern for a divided or one-way highway, where traffic comes from one side only. Turning on the flashers was the right first step, but the rule keeps them flashing until the triangles are out and allows them in addition to the triangles, never instead of them.

Look for this next time: On a two-way road, triangles go about 10 and 100 feet behind and 100 feet ahead within 10 minutes; flashers do not replace them.

Correct answer: B · Possible black ice, so slow down before you reach it and drive smoothly

Below freezing, a road that looks wet when no rain or snow has fallen is the warning sign the CDL manual gives for black ice: a thin layer clear enough that the dark road surface shows through, so the road looks wet instead of white. Shaded spots also stay icy longer than open pavement. The safe assumption is ice, so slow down before you reach it and keep your steering and braking smooth. Expecting ice to look white or frosty is the mistake that makes black ice dangerous. Nothing in the scenario says the road was salted, and even a treated road is no reason to hold your normal speed on a surface that may be ice.

Look for this next time: When it is below freezing and the road looks wet, treat it as black ice and slow down before you reach it.

Correct answer: A · Steer onto the clear shoulder without braking in the turn, then counter-steer

The stem rules out stopping in time, so braking hard in your lane still ends in a collision with the car. The CDL manual notes that you can almost always turn to miss an obstacle faster than you can stop, and with a car beside you on the left, the clear shoulder is the open path. Steer only as much as you need to clear the car, without braking during the turn, and be ready to counter-steer once you are past it. Braking hard while you turn can lock the wheels, and locked wheels in a turn can send the truck into a skid. Swerving carries risk, but here it is the option that avoids the crash.

Look for this next time: When you cannot stop in time, steer toward open space, do not brake during the turn, and be ready to counter-steer.

Correct answer: C · Turn slowly, keep the rear near the curb, and swing wide as you finish

Off-tracking makes the trailer's rear wheels follow a shorter path than the tractor's, so they cut inside the corner. The CDL manual's method for a right turn that needs extra room is to turn slowly, keep the rear of the vehicle close to the curb so no one can pass on your right, and turn wide as you complete the turn. Swinging left toward the oncoming lane as you start can make a following driver think you are turning left and try to pass on your right, and you may hit that vehicle as you finish. Turning at the point a car would ignores off-tracking: the rear wheels cut the corner and can climb the curb or strike objects there.

Look for this next time: For a tight right turn, keep the rear near the curb and turn wide as you finish, not as you start.

Correct answer: B · Slow down and shift into a low gear before the grade, then start down

The CDL manual says to shift into a low gear before you start down the grade and not to try to downshift once your speed has built up. At that point you may not be able to get into a lower gear, or back into any gear, and all engine braking would be lost; engine braking is meant to be the main way you control speed on the grade. Starting down in a higher gear and planning to downshift later invites exactly that problem. Using the gear that climbed the hill is an older rule of thumb: modern trucks with low-friction parts, streamlined shapes, and more powerful engines may need a lower gear going down than going up, so know what is right for your vehicle.

Look for this next time: Choose the low gear before the grade begins; downshifting after speed builds can leave you with no engine braking.

Correct answer: B · By about 9:45 a.m., three hours after the first check

The CDL manual's rule is to inspect cargo securement within the first 50 miles of a trip, then every 150 miles or every three hours, whichever comes first. The 6:45 a.m. check at mile 38 met the first-50-miles rule. At about 35 mph, 150 more miles would take over four hours, so the three-hour limit comes first, and the next check is due by about 9:45 a.m. Waiting for 150 more miles uses the limit that arrives later, around 11:00 a.m. The 50-mile figure applies only to the first check of the trip, so 50 more miles, near 8:10 a.m., is not when the next check falls due.

Look for this next time: Check cargo within the first 50 miles, then every 150 miles or three hours, whichever comes first.

Correct answer: A · She may not go on duty or drive until 4:30 p.m.

Federal rule 49 CFR 392.5 bars a driver from using alcohol within 4 hours before going on duty or operating a commercial motor vehicle. Counting from when she finished the beer at 12:30 p.m., she may not go on duty or drive until 4:30 p.m., however sober she feels. An alcohol concentration of 0.04 is the level at which a commercial driver is treated as driving under the influence, not a permission threshold, and the same rule also bars any measured alcohol concentration while on duty. A rule of thumb about how fast the body handles one drink is not the test either: the four-hour clock applies regardless.

Look for this next time: No alcohol within four hours before going on duty or driving a CMV, whatever a breath test or your own sense of sobriety says.

Correct answer: C · The steer tire is below its 4/32-inch minimum; the drive tire passes

The CDL manual's minimums are at least 4/32 inch of tread in every major groove on front tires and 2/32 inch on other tires. At 3/32 inch, the steer tire is below its minimum and needs to be replaced before the trip, while the drive tire at 3/32 inch still meets the 2/32-inch minimum for other tires. Treating 2/32 inch as the limit for every tire misses the stricter rule for front tires and would send the truck out on a steer tire below its minimum. Treating 4/32 inch as the limit for every tire would wrongly fail the drive tire, which meets its own minimum.

Look for this next time: Front tires need at least 4/32 inch of tread in every major groove; other tires need at least 2/32 inch.

Correct answer: A · Get out and away from the tracks, then call the number posted there

The CDL manual's first step when a vehicle is stuck on the tracks is to get out and away from the tracks. Then look on the signposts or signal housing at the crossing for emergency contact information, call 911 or the posted number, and give the crossing's location, including the DOT crossing number if one is posted. Low-slung trailers such as lowboys are among the units the manual warns can hang up on raised crossings. Staying in the cab to keep rocking the rig keeps you on the tracks, and no train in sight now says nothing about the next minute. Calling from the cab gets the order wrong: leave first, then call.

Look for this next time: If you are stuck on a crossing, get out and away from the tracks first, then call the posted emergency number with the crossing location.

Correct answer: B · Pull over and stop safely right away, while the foot brake still has air

The CDL manual says to bring the vehicle to a safe stop as soon as the low-air warning first comes on, while you can still control the brakes; an air leak may be the cause. Controlled braking works only while enough air remains in the tanks. If you keep driving, the spring brakes will come on when pressure drops into the 20 to 45 psi range, and a heavily loaded truck can then take a long distance to stop because the spring brakes do not work on all axles. Pumping the pedal does not rebuild pressure: each application uses air, which is how drivers lower tank pressure on purpose during the air brake check.

Look for this next time: When the low-air warning comes on, stop safely right away instead of waiting for the spring brakes to stop you.

Correct answer: C · It fails: this rig should lose no more than 4 psi in that minute

With the brakes applied, the CDL manual's limit is a drop of no more than 3 psi in one minute for a single vehicle or 4 psi for a two-vehicle combination such as this one, counted over the minute after the first drop that comes when you apply the brakes. A further 5 psi drop is too much, so find and fix the leak before driving, or you could lose your brakes on the road. A 5 psi applied limit for this rig is not the manual's figure; it is one psi too generous. The brakes-released test, with the engine off and the parking brake released, has its own tighter limits, about 2 psi a minute for a single vehicle and 3 for a two-vehicle combination, so both tests have limits.

Look for this next time: With brakes applied, one-minute air loss should be no more than 3 psi for a single vehicle or 4 psi for a two-vehicle combination.

Correct answer: B · Brake lag: air needs about half a second or more to reach the brakes

The CDL manual counts total stopping distance as perception distance plus reaction distance plus braking distance, and with air brakes it adds brake lag: the time air takes to flow through the lines before the brakes work, about half a second or more. By the manual's description, hydraulic brakes, used on cars and light or medium trucks, work instantly. The manual puts air brake lag at about 32 extra feet at 55 mph on dry pavement. Reaction time is part of every driver's stopping distance, whatever the brake system, so it is not the added factor. Brake fade comes from excess heat after heavy use, not from each normal stop, and it is not unique to air brakes.

Look for this next time: Air brakes add brake lag, about half a second or more, to perception, reaction, and braking distance.

Correct answer: A · Water and oil build up, and the water can freeze and cause brake failure

The CDL manual explains that compressed air usually carries some water and compressor oil, both bad for the air brake system, and that they collect at the bottom of the tanks. In cold weather the water can freeze and cause brake failure, which is why tanks with manual drains are drained completely at the end of each day of driving. An alcohol evaporator helps reduce ice in valves and other parts, but the manual says daily draining is still needed unless the tanks have automatic drains. Treating the oil as only a compressor problem is also wrong: the manual names both water and oil as harmful to the brake system. Normal pressure build-up does not show that the tanks are dry.

Look for this next time: Drain manual air tanks daily; water and oil collect there, and the water can freeze and cause brake failure.

Correct answer: C · The governor, which is meant to stop the compressor at cut-out pressure

The governor controls when the compressor pumps. At its cut-out pressure, around 125 psi in the CDL manual's example (check the manufacturer's specification), it stops the compressor, and at cut-in, around 100 psi, it lets the compressor start again. The manual's in-cab check puts cut-out between about 120 and 140 psi, so pressure that keeps rising past 140 psi points to the governor, which may need to be fixed before the truck is driven. The safety valve does not control the compressor; it protects the system from too much pressure and is usually set to open at about 150 psi. The low-air warning is a signal for the driver that pressure is too low; it does not tell the compressor when to stop.

Look for this next time: The governor stops the compressor at cut-out and restarts it at cut-in; pressure that keeps climbing past cut-out points to the governor.

Correct answer: B · At 35 mph, brake just hard enough to slow to about 30, then release and repeat

The CDL manual's method, once you are in the proper low gear, is to let the speed reach your “safe” speed, then brake just hard enough to feel a definite slowdown until you are about 5 mph below it, which takes about three seconds, then release and repeat each time you are back at 35 mph. The brakes only supplement engine braking on the grade. Light, steady pressure all the way down keeps the brakes working without a break, the kind of heavy use that builds heat and leads to brake fade. Letting the speed climb to 45 mph means running above the speed you chose as safe, and braking hard from that higher speed adds even more heat.

Look for this next time: On a downgrade, brake from your safe speed to about 5 mph below it, release, and repeat.

Correct answer: A · Plan it early and steer gently, since a quick move can roll the trailer

Rearward amplification causes the crack-the-whip effect: in a quick lane change, the sideways swing grows toward the rear of the rig, and the CDL manual notes many crashes in which only the trailer overturned. The defense is to look far enough ahead to plan the move early, then steer gently and smoothly. Making the change quickly to spend less time across the lane line is the sudden steering that sets off the effect. The trailer hand valve should not be used while driving at all; the manual warns that it can make the trailer skid, while the foot brake, which works all the brakes, is much less likely to cause a skid or jackknife.

Look for this next time: Plan lane changes early and steer gently, because a quick move can overturn the trailer even when the tractor stays upright.

Correct answer: C · The tractor protection valve shuts, and the trailer's emergency brakes come on

The emergency line supplies the trailer's air tanks and controls the trailer emergency brakes. When it loses pressure, the trailer emergency brakes come on, and the tractor protection valve closes, so the red trailer air supply knob pops out. The tractor protection valve exists to keep air in the tractor's brake system if the trailer breaks away or develops a bad leak, so the tractor's air does not drain out through the broken line. Expecting the trailer brakes to release gets the fail-safe backward: losing emergency-line pressure applies them. The CDL manual warns of possible loss of control when this happens, so be ready to keep the rig straight.

Look for this next time: Losing the red emergency line closes the tractor protection valve and puts on the trailer emergency brakes.

Correct answer: B · Pull gently forward with the trailer brakes still locked, as a tug test

In the CDL manual's coupling sequence, the next step is to check the connection: raise the trailer landing gear slightly off the ground and pull the tractor gently forward while the trailer brakes are still locked, to confirm that the trailer is locked onto the tractor. You then secure the tractor and inspect: no space between the upper and lower fifth wheel, jaws closed around the kingpin shank, the locking lever in the lock position, and the safety latch in place. Releasing the trailer brakes removes the resistance the tug test needs. Raising the landing gear all the way comes later, after the inspection and line checks, and a test drive is not a coupling check.

Look for this next time: After backing under, tug gently forward against the locked trailer brakes before you secure the tractor and inspect the coupling.

Correct answer: A · Air can leak off and release the trailer brakes; use parking brakes and chocks

The trailer hand valve, also called the trolley valve or Johnson bar, works the trailer brakes, and the manual's trailer-brake section says to use it only to test them. It must never be used for parking: on a trailer without spring brakes, the air can leak out and release the brakes, letting the rig roll. Always use the parking brakes when parking, and chock the wheels of a trailer that has no spring brakes. The manual makes no exception for a short stop or a running engine. Thinking the valve works only the tractor brakes gets its job backward; it works the trailer brakes, which is why an air leak matters here.

Look for this next time: Never park or hold a rig with the trailer hand valve; use the parking brakes, and chock a trailer without spring brakes.