Idaho General Knowledge practice test
50 questions · 80% to pass · Required for every CDL applicant.
How to use this practice test
Read each question, click an answer, and the correct choice is highlighted with a short explanation referencing the underlying CDL Manual concept. Your live score appears at the top of the page. Refresh to reset.
This test runs entirely in your browser. Nothing is uploaded; no account is required. Pages refresh to a new randomized cut from the bank.
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1. When operating a CMV in interstate commerce, black ice is dangerous because:
Explanation. Black ice is a thin layer of ice clear enough that the road below shows through, making it look merely wet. (Reinforcement variant — same underlying CDL Manual concept.)
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2. A vehicle is most likely to roll over when:
Explanation. High center of gravity plus sudden steering or braking is the recipe for rollovers.
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3. Controlled braking means:
Explanation. Controlled braking is firm pressure that does not lock the wheels, allowing you to continue steering.
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4. When operating a CMV in interstate commerce, when driving in the mountains, gravity will:
Explanation. Going downhill, gravity adds to your speed and the distance needed to stop. (Reinforcement variant — same underlying CDL Manual concept.)
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5. You should signal continuously when turning because:
Explanation. Continuous signaling keeps other drivers informed and prevents the signal from cancelling before the turn is complete.
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6. Per federal regulations, convex (curved) mirrors:
Explanation. Convex mirrors widen the view but make objects appear smaller and farther away than they actually are. (Reinforcement variant — same underlying CDL Manual concept.)
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7. When taking the CDL knowledge exam, what does the term "perception distance" mean?
Explanation. Perception distance is the distance traveled while your brain identifies a hazard, typically about 1.5 seconds at highway speed. (Reinforcement variant — same underlying CDL Manual concept.)
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8. Which of these is NOT a sign of a tired driver?
Explanation. A "second wind" is a brief, deceptive boost. All of the others, including the second wind, are warning signs of fatigue.
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9. In real-world commercial driving, you should NOT drive when taking which of these?
Explanation. Drowsiness-causing medications, even legal ones, can disqualify you from driving. (Reinforcement variant — same underlying CDL Manual concept.)
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10. Brake fade is caused by:
Explanation. Brakes lose effectiveness when overheated; this is why low gears are essential on grades.
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11. When operating a CMV in interstate commerce, a vehicle is most likely to roll over when:
Explanation. High center of gravity plus sudden steering or braking is the recipe for rollovers. (Reinforcement variant — same underlying CDL Manual concept.)
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12. Carbon monoxide poisoning may be a hazard if:
Explanation. A cracked exhaust manifold or rusted-through pipe can leak CO into the cab, causing drowsiness or death.
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13. In real-world commercial driving, when driving in fog, you should:
Explanation. High beams reflect off fog and reduce vision; low beams and slower speed are correct. (Reinforcement variant — same underlying CDL Manual concept.)
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14. Per federal regulations, the 14-hour rule means a driver may not drive after the:
Explanation. After 14 consecutive hours on duty following 10 hours off, no further driving is allowed even if some of those 14 hours were not driving. (Reinforcement variant — same underlying CDL Manual concept.)
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15. When taking the CDL knowledge exam, hydroplaning is most likely when:
Explanation. Hydroplaning can occur at speeds as low as 30 mph if there is enough water and the tires cannot channel it away. (Reinforcement variant — same underlying CDL Manual concept.)
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16. When checking your vehicle, the engine should be off and the wheels chocked because:
Explanation. Chocking wheels and shutting down the engine prevents the vehicle from rolling away while you are underneath or alongside it.
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17. When taking the CDL knowledge exam, a safe following distance for a 40-foot vehicle at 50 mph is at least:
Explanation. Use 1 second per 10 feet of vehicle length under 40 mph, plus 1 extra second above 40 mph: 4 + 1 = 5 seconds for a 40-foot truck at 50 mph. (Reinforcement variant — same underlying CDL Manual concept.)
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18. When backing a commercial vehicle, you should:
Explanation. Backing toward the driver side gives the driver a direct line of sight, which is much safer than blind-side backing.
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19. When merging onto a highway, you should:
Explanation. Match traffic speed and yield; stopping is dangerous and can cause rear-end collisions.
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20. Black ice is dangerous because:
Explanation. Black ice is a thin layer of ice clear enough that the road below shows through, making it look merely wet.
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21. A safe following distance for a 40-foot vehicle at 50 mph is at least:
Explanation. Use 1 second per 10 feet of vehicle length under 40 mph, plus 1 extra second above 40 mph: 4 + 1 = 5 seconds for a 40-foot truck at 50 mph.
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22. Most state DMV CDL handbooks state that you should check your mirrors:
Explanation. Frequent mirror checks (every 5-8 seconds) are part of safe scanning. (Reinforcement variant — same underlying CDL Manual concept.)
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23. During a pre-trip inspection, a driver may use a hand-held mobile phone while driving a CMV:
Explanation. Federal rules prohibit hand-held mobile phone use by CMV drivers except in emergencies. (Reinforcement variant — same underlying CDL Manual concept.)
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24. Experienced commercial drivers know that when backing a commercial vehicle, you should:
Explanation. Backing toward the driver side gives the driver a direct line of sight, which is much safer than blind-side backing. (Reinforcement variant — same underlying CDL Manual concept.)
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25. Experienced commercial drivers know that when merging onto a highway, you should:
Explanation. Match traffic speed and yield; stopping is dangerous and can cause rear-end collisions. (Reinforcement variant — same underlying CDL Manual concept.)
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26. In real-world commercial driving, a safe following distance for a 40-foot vehicle at 50 mph is at least:
Explanation. Use 1 second per 10 feet of vehicle length under 40 mph, plus 1 extra second above 40 mph: 4 + 1 = 5 seconds for a 40-foot truck at 50 mph. (Reinforcement variant — same underlying CDL Manual concept.)
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27. When you double your speed, your stopping distance:
Explanation. Stopping distance increases roughly with the square of speed, so doubling speed nearly quadruples the distance.
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28. Convex (curved) mirrors:
Explanation. Convex mirrors widen the view but make objects appear smaller and farther away than they actually are.
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29. When making a right turn in a large vehicle, you should:
Explanation. Stay in the right lane and watch the trailer; swinging left first invites a same-direction driver to squeeze in on your right.
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30. A driver loses their CDL for life if convicted of:
Explanation. Using a CMV to commit a controlled-substance felony is grounds for a lifetime CDL disqualification.
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31. In real-world commercial driving, hours-of-service property-carrying drivers may drive a maximum of how many hours after 10 consecutive hours off duty?
Explanation. The federal HOS rule allows up to 11 hours of driving after 10 consecutive hours off duty. (Reinforcement variant — same underlying CDL Manual concept.)
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32. During a pre-trip inspection, the best way to recover from a drive-wheel skid is to:
Explanation. Releasing the accelerator and disengaging the clutch removes power from the drive wheels so they can regain traction. (Reinforcement variant — same underlying CDL Manual concept.)
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33. During a pre-trip inspection, the most common cause of serious skids is:
Explanation. Driving too fast for road and weather conditions is the leading cause of skids. (Reinforcement variant — same underlying CDL Manual concept.)
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34. Per federal regulations, when making a right turn in a large vehicle, you should:
Explanation. Stay in the right lane and watch the trailer; swinging left first invites a same-direction driver to squeeze in on your right. (Reinforcement variant — same underlying CDL Manual concept.)
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35. A driver who is convicted of DUI in any vehicle while holding a CDL faces:
Explanation. A first DUI conviction in any vehicle disqualifies a CDL for at least one year.
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36. You should warm up the engine before driving because:
Explanation. Allowing the engine to warm circulates oil and reduces wear before load is applied.
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37. The most common cause of serious skids is:
Explanation. Driving too fast for road and weather conditions is the leading cause of skids.
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38. Empty trucks need:
Explanation. Empty trucks have less traction and brakes designed for the loaded weight, so they can actually take longer to stop and lock up more easily.
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39. According to the FMCSA CDL Manual, texting while driving a CMV is:
Explanation. Texting is banned for all CMV drivers under federal law. (Reinforcement variant — same underlying CDL Manual concept.)
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40. Most state DMV CDL handbooks state that when driving in fog, you should:
Explanation. High beams reflect off fog and reduce vision; low beams and slower speed are correct. (Reinforcement variant — same underlying CDL Manual concept.)
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41. Which is the best way to handle a tire failure?
Explanation. Hard braking after a tire failure can cause loss of control. Hold the wheel firmly, ease off the throttle, and brake gently after slowing.
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42. Experienced commercial drivers know that aggressive driving by other motorists is best handled by:
Explanation. The safest response is to disengage and report when safe; never engage an aggressive driver. (Reinforcement variant — same underlying CDL Manual concept.)
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43. When driving down a long, steep grade, you should:
Explanation. Always select a low gear before descending so engine compression helps hold speed and brakes do not overheat.
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44. When operating a CMV in interstate commerce, stab braking is used:
Explanation. Stab braking is an emergency technique for non-ABS vehicles: brake hard until wheels lock, release, brake again. (Reinforcement variant — same underlying CDL Manual concept.)
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45. Experienced commercial drivers know that the eye-lead time recommended for highway driving is at least:
Explanation. Look 12-15 seconds ahead so you can identify hazards in time to react smoothly. (Reinforcement variant — same underlying CDL Manual concept.)
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46. A driver may use a hand-held mobile phone while driving a CMV:
Explanation. Federal rules prohibit hand-held mobile phone use by CMV drivers except in emergencies.
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47. When operating a CMV in interstate commerce, which is the best way to handle a tire failure?
Explanation. Hard braking after a tire failure can cause loss of control. Hold the wheel firmly, ease off the throttle, and brake gently after slowing. (Reinforcement variant — same underlying CDL Manual concept.)
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48. Hours-of-service property-carrying drivers may drive a maximum of how many hours after 10 consecutive hours off duty?
Explanation. The federal HOS rule allows up to 11 hours of driving after 10 consecutive hours off duty.
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49. During a pre-trip inspection, when driving in the mountains, gravity will:
Explanation. Going downhill, gravity adds to your speed and the distance needed to stop. (Reinforcement variant — same underlying CDL Manual concept.)
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50. When operating a CMV in interstate commerce, when checking lights during the pre-trip, you should turn the key on and:
Explanation. Have someone else watch, or turn lights on and off and walk around to verify each lamp is working. (Reinforcement variant — same underlying CDL Manual concept.)
About the General Knowledge exam
The General Knowledge exam is required for every Class A, B, and C CDL applicant. It covers vehicle inspection, basic control, shifting, backing, communicating with other drivers, distracted driving, fatigue, hazard perception, emergency maneuvers, skid control, accident procedures, hazardous materials awareness, and federal hours-of-service rules.
This Idaho-specific edition uses the same federal source material every U.S. state adopts. The Idaho ITD administers the actual exam at its service centers. Bring your CDL handbook for any last-minute reference; it’s free at every Idaho ITD office.
Once you’re consistently scoring above 90% on this practice set, you’re well above the 80% required to pass the official Idaho exam. Drill the questions you miss most often, then take the test cold once a day for a week leading up to your appointment.
Tips that actually work
- Read every answer choice before clicking. CDL questions are famous for "best answer" wording where two choices look right.
- Don’t memorize question text — learn the underlying rule. The DMV reshuffles wording constantly.
- Keep a list of the questions you miss. Re-drill them in isolation until you can’t miss them.
- Practice in short, frequent sessions. Two 20-minute sessions per day beats one two-hour cram.
What happens after I pass?
Passing the knowledge test earns you a Commercial Learner’s Permit (CLP). You must hold the CLP for at least 14 days before you can take the road skills test, and you must drive with a CDL-holding instructor in the cab during that period. Once you pass the road skills test, your CDL is issued.