Iowa DOT CDL General Knowledge Practice Test 3
Test 3 of 5. Pick an answer for each question to see if it is right and why; your score appears at the end. You need 40 correct answers to pass, the same ratio as the real Iowa test.
- Questions50
- Correct to pass40
- Passing score80%
- Minimum age18 years
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1. When traction is poor, such as on packed snow, how should you speed up?
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Answer C: Very gradually, so the drive wheels do not spin
Gentle throttle keeps the drive wheels from breaking traction, because spinning wheels give up both grip and directional control.
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2. You are entering a freeway from a short ramp with a fully loaded trailer. What should you do?
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Answer A: Begin building speed early and wait for a large gap
A loaded vehicle accelerates slowly, so it needs both a running start and a much larger gap than a car would use.
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3. If an intersection has two lanes marked for left turns, which one should a large vehicle use?
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Answer D: The right hand of the two turning lanes
The outside turning lane leaves room for the rear of the vehicle to swing without striking a vehicle in the adjacent lane.
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4. A commercial vehicle stops on the shoulder of an undivided two-lane road that carries traffic in both directions. Where do the warning devices go?
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Answer B: Within 10 feet of the vehicle, plus 100 feet to the front and 100 feet to the rear
On a two-way road the devices must mark the vehicle itself and warn traffic approaching from each direction, and they must be in place within ten minutes.
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5. Who may remove an out-of-service sticker from a commercial vehicle?
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Answer B: Only an authorized person, and only after the defect that caused it has been repaired
The sticker stays on the vehicle until the repairs are made and an authorized person removes it, so nobody can simply peel it off and drive away.
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6. Why does a load of live animals need special attention?
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Answer D: The animals move and lean, so the center of gravity shifts and the vehicle can be pushed off balance in a turn
A live load will not stay where it is put, and that movement changes the balance of the vehicle exactly when a curve is loading it most.
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7. Why does the commercial driver manual tell you to look far down the road instead of only at the vehicle ahead?
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Answer D: So you can adjust smoothly instead of braking hard at the last moment
A heavy vehicle cannot react quickly, so early information lets you slow gradually rather than make a sudden stop or swerve.
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8. Within total stopping distance, what exactly is perception distance?
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Answer D: The distance traveled from the moment your eyes see a hazard until your brain identifies it
Perception distance covers only the time between seeing a hazard and recognizing it as a hazard, before any physical reaction begins.
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9. You hear air escaping from a trailer tire during the pre-trip. What should you do?
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Answer C: Have it repaired or replaced before the vehicle is driven
An audible leak means the tire is failing now, and federal rules do not allow a vehicle to be operated on a flat or leaking tire.
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10. You are about to back across a quiet street into a driveway. Which of these is good practice before you start to move?
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Answer B: Turn on the four-way flashers and sound the horn to alert anyone nearby
Warning other road users and pedestrians before a reverse maneuver gives them the chance to stop or move clear before the vehicle starts back.
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11. Why should a large vehicle be accelerated smoothly and gradually?
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Answer A: Rough acceleration can damage the drivetrain and, on a combination, the coupling
Jerky throttle inputs put shock loads through the driveline and the coupling, which causes mechanical damage over time.
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12. While double clutching, what goes wrong if you take too long between the two clutch movements?
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Answer A: Engine speed falls too far and the gear becomes hard or impossible to engage
Double clutching only works while engine speed and road speed still match; a slow shift lets rpm drop away and the gear will not go in.
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13. What must the motor carrier do about a defect listed on a driver vehicle inspection report?
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Answer D: Repair it, or certify that repair is not needed, before the vehicle is dispatched again
A reported defect must be corrected, or certified as not needing correction, and that certification is what the next driver reads before taking the vehicle.
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14. What is the purpose of blocking placed against cargo on a trailer deck?
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Answer B: To fill the space around the cargo so it cannot slide
Blocking is shaped to fit snugly against the cargo and fastened to the deck, removing the room the load would need in order to move.
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15. What is the purpose of the header board, or headache rack, that you inspect behind the cab?
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Answer A: It protects the driver from cargo that shifts forward during a hard stop
In a hard stop the load keeps moving forward, and the header board is the barrier between that load and the back of the cab.
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16. Your vehicle begins to hydroplane on a wet road. What should you do?
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Answer B: Release the accelerator and push in the clutch so the wheels can turn freely and regain grip
Hydroplaning tires are riding on water; slowing without braking lets them settle back onto the road and grip again.
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17. How long is an Iowa commercial driver disqualified for committing two serious traffic offenses within any three-year period?
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Answer A: Sixty days
Iowa Code section 321.208 sets the disqualification at sixty days for two of the listed offenses within any three-year period and one hundred twenty days for three of them.
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18. Why does a heavy vehicle need a much larger gap than a car in order to cross or enter traffic?
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Answer C: Because it accelerates slowly and needs far more road and time to clear the intersection
Slow acceleration combined with the length of the vehicle means a gap that would feel comfortable in a car is nowhere near enough.
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19. May a commercial driver use a hand-held phone while stopped at a red light?
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Answer B: No, a stop in traffic still counts as driving
Hand-held use is allowed only when the vehicle has been moved to the side of or off the highway and halted where it can safely remain stopped.
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20. You drop your phone on the floor of the cab while driving. What do federal rules require?
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Answer A: Leave it, because reaching for it while driving is banned
Federal rules forbid reaching for a mobile device in any way that takes the driver out of a seated, belted driving position.
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21. What must be checked on a sliding fifth wheel during the inspection?
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Answer B: That it is locked in position and not so far forward that the trailer can hit the cab
A slider that is unlocked can move under braking, and one set too far forward lets the trailer strike the cab when the tractor turns sharply.
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22. According to the Iowa DOT, how many questions are on the air brake knowledge test?
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Answer B: 25 questions
The Iowa DOT CDL Testing page lists the air brake test as 25 questions with 5 allowed misses, while combination, passenger, school bus, doubles and triples, and tank tests are 20 questions each.
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23. Why does a load of hanging meat in a refrigerated trailer require special care in curves?
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Answer D: The meat swings and has a high center of gravity, which makes the trailer unstable
Hanging meat is heavy, sits high and swings, so the trailer can tip in a curve or on a ramp taken at normal speed.
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24. What information does an inspection decal on a commercial vehicle carry?
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Answer B: The date of the inspection, where the inspection report is kept, and information identifying the vehicle
The decal shows when the inspection was done, the name and address of the entity that holds the inspection report, and which vehicle it belongs to, so an officer can confirm the inspection at a glance.
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25. On the Interstate System, what is the federal maximum weight allowed on a single axle?
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Answer A: 20,000 pounds
Federal limits for the Interstate System cap one single axle at 20,000 pounds.
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26. The temperature is around freezing and the road looks wet. What is a practical way to tell whether it is icy?
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Answer C: Check for ice building up on the mirrors, antenna or the front of the vehicle
Ice that forms on the mirror, antenna or windshield is a warning that the road surface is freezing too, and black ice can look like a wet road.
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27. Which exhaust system condition must be reported as a defect?
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Answer B: A section repaired by wrapping tape or patching material around the pipe
Exhaust parts may not be patched with wrap or tape, because the repair does not contain the gas or the heat and it fails quickly in service.
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28. At what time of day do drowsy driving crashes occur most often?
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Answer A: Between midnight and 6 a.m.
The body clock makes people sleepiest in the early morning hours, which is when most fatigue-related crashes happen.
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29. What is checked about the clutch and the gearshift during the in-cab inspection?
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Answer D: That the clutch free play is within the range the manufacturer specifies
Too little or too much clutch free play causes slipping or dragging, either of which can leave the driver unable to select the gear a hill demands.
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30. How should the steering wheel be held when driving a large vehicle?
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Answer D: Firmly with both hands, placed on opposite sides of the wheel
Both hands on opposite sides give the leverage and control needed if a tire drops off the pavement or the wheel is jolted.
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31. How is the fluid level in an automatic transmission usually checked?
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Answer A: With the engine running, following the procedure in the operator manual
Most automatic transmissions are designed to be checked with the engine running, and the operator manual gives the exact conditions for that vehicle.
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32. What is the most common cause of a front-wheel skid, where the vehicle keeps going straight even though you turn the wheel?
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Answer B: Driving too fast for the conditions
A front-wheel skid usually comes from too much speed for the traction available; the only cure is to let the vehicle slow down and stop turning or braking so hard.
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33. Your windshield washer reservoir is empty and rain and road spray are expected. What should you do?
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Answer D: Fill it before the trip, because you need the washers to keep the windshield clear
Wipers alone smear the film of oil and grit thrown up by traffic, so the washers are part of keeping a usable view of the road.
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34. You normally pull an empty trailer, but today the same tractor is loaded to its legal maximum. What changes about getting under way?
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Answer A: You may need to start in a lower gear, since the load takes more pulling power
The heavier the load, the lower the gear needed to get the vehicle rolling without lugging or stalling the engine.
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35. Why does the manual insist on a firm grip with both hands on opposite sides of the steering wheel?
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Answer A: A hard bump can pull the wheel out of a loose grip
Hitting a curb or a deep pothole can jerk the wheel violently, and only a firm two-handed grip keeps the vehicle under control when that happens.
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36. What is the purpose of an escape ramp on a mountain grade?
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Answer C: To stop a runaway vehicle without injuring the driver
Escape ramps use soft material or a steep upgrade to bring a vehicle with failed brakes to a controlled stop.
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37. Why does a cracked frame rail or cross member make a vehicle unsafe?
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Answer B: The frame carries the whole load, and a crack can spread until the structure fails
The frame is the backbone that supports the body, the load and the running gear, so a crack in it is a structural failure that grows with every mile.
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38. What is the minimum tread depth required in every major groove of a steering axle tire?
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Answer D: 4/32 inch
Federal rules set a higher tread minimum for front steering tires than for other positions because a steering tire failure is far harder to control.
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39. What does the federal bridge formula do?
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Answer C: It reduces the weight allowed on a group of axles as those axles get closer together
Axles bunched together concentrate the load on a short span of a bridge, so the formula lowers the allowable weight as the spacing shrinks.
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40. Why does a load need to be balanced from side to side?
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Answer B: Because a heavy side raises the rollover risk and can overload the tires and springs on that side
An uneven load tips the balance of the vehicle in curves and pushes the tires and suspension on the loaded side past their rating.
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41. Why can an empty truck need a longer stopping distance than a loaded one?
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Answer C: An empty vehicle has less traction, so its wheels lock up more easily
Brakes and tires are designed for a loaded vehicle, and with little weight on the axles the tires grip less and lock sooner.
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42. You are given a ratchet strap with a deep cut along one edge. What should you do?
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Answer C: Do not use it; a damaged tiedown cannot be relied on to hold its rated load
Cuts, burns, knots and broken stitching weaken a strap far below its marked working load limit, so it must be taken out of service.
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43. Why should high beams not be used in fog?
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Answer A: The light reflects back off the droplets and blinds you
Fog is made of water droplets that scatter light straight back at the driver, so a stronger beam makes the wall of glare worse rather than better.
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44. You are stopped in a line of traffic behind a car. How much space should you leave in front of you?
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Answer C: Enough to see where the rear tires of that car meet the road
That amount of space lets you pull around the vehicle ahead if it stalls and absorbs some of the shock if you are struck from behind.
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45. What must an Iowa applicant already hold before a commercial learner's permit can be issued?
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Answer A: A valid Iowa class A, B, C, or D driver's license, held at eighteen years of age or older
Iowa Code section 321.180 requires an applicant for a commercial learner's permit to hold a valid class A, B, C, or D driver's license issued in Iowa and to be at least eighteen years of age.
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46. You pick up a trailer that another driver dropped in the yard. What must you do before pulling it?
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Answer B: Inspect it yourself, because you are responsible for the whole vehicle you drive
Responsibility follows the driver who moves the vehicle, and a trailer that has stood in a yard may have lost air, developed a flat tire or been damaged.
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47. A driver snores heavily, stops breathing briefly during the night and is tired all day despite long hours in bed. What should be suspected?
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Answer B: Sleep apnea
Sleep apnea repeatedly interrupts breathing during sleep, so the driver never gets true rest and stays dangerously drowsy during the day.
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48. In a vehicle with a manual transmission, when should the clutch be pushed in during a normal stop?
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Answer A: Just before the engine would stall, when the vehicle is nearly stopped
Staying in gear until the last moment keeps engine braking and steady control, so the clutch goes down only when the engine is about to drop below idle.
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49. On the Interstate System, what are the federal maximums for a tandem axle group and for gross vehicle weight?
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Answer B: 34,000 pounds and 80,000 pounds
The federal Interstate limits allow 34,000 pounds on a tandem axle group and 80,000 pounds gross.
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50. At a stop on a hot day you find a tire that is too hot to touch. What should you do?
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Answer C: Wait until the tire cools before driving on, and do not let air out of it
A tire that is too hot to touch may be about to fail or catch fire; releasing air only leaves it underinflated once it cools, which makes things worse.