CDL General Knowledge — All Questions

Total stopping distance is made up of which three parts?

• a.Following distance, reaction distance, and skid distance
• b.Perception distance, reaction distance, and braking distance✓
• c.Braking distance, coasting distance, and cushion distance
• d.Sight distance, signal distance, and stopping distance

Total stopping distance equals the distance traveled while you perceive a hazard, plus the distance traveled while you move your foot to the brake, plus the distance the vehicle travels while braking. Following distance and cushion space are separate space-management ideas, and terms like coasting or signal distance are not part of the calculation.

You are traveling at 55 mph on dry pavement in a heavy vehicle. Roughly what total distance will you cover from the moment a hazard appears until you are stopped?

• a.About 1,000 feet
• b.About 60 feet
• c.About 300 feet, longer than a football field✓
• d.About 100 feet

At 55 mph perception is roughly 60 feet, reaction is roughly 60 feet, and braking adds about 170 feet or more, for a total of around 300 feet. One hundred feet or 60 feet would not even cover perception and reaction, and 1,000 feet overstates the distance on dry pavement.

If you double your speed from 20 mph to 40 mph, what happens to your braking distance?

• a.It roughly doubles
• b.It stays about the same
• c.It becomes about four times longer✓
• d.It becomes about ten times longer

Braking distance increases with the square of speed, so doubling speed multiplies braking distance by about four. Assuming it stays the same or merely doubles badly underestimates the energy the brakes must absorb, and ten times overstates it.

You are driving a 40-foot straight truck at 35 mph on a dry road. What is the minimum safe following distance?

• a.2 seconds
• b.6 seconds
• c.3 seconds
• d.4 seconds✓

Below 40 mph the rule is one second for every 10 feet of vehicle length, so a 40-foot vehicle needs 4 seconds. Two or three seconds is too little for that length, and 6 seconds would be the figure for a much longer vehicle or after adding time for higher speed.

The same 40-foot vehicle is now traveling at 55 mph on dry pavement. What following distance should you keep?

• a.4 seconds
• b.5 seconds✓
• c.3 seconds
• d.2 seconds

Above 40 mph you add one second to the length-based figure, so 4 seconds becomes 5 seconds. Keeping only 2, 3, or 4 seconds leaves too little room for the longer stopping distance at highway speed.

Rain has just begun after a long dry spell and the road surface is slick. How should you adjust your following distance and speed?

• a.Increase speed slightly to get off the wet road sooner
• b.Follow closer so the vehicle ahead clears water from your lane
• c.Keep the same following distance but brake earlier
• d.Add at least one second of following distance and reduce speed by about one third✓

On a wet road you should slow by roughly a third and add space, because traction and stopping ability are both reduced. Braking earlier alone does not create the extra cushion, and increasing speed or following closer directly increases the risk of a rear-end collision.

You are approaching a curve posted at 40 mph while driving a loaded tractor-trailer. What is the safest approach?

• a.Enter at 45 mph and coast
• b.Enter at 40 mph and brake through the curve
• c.Slow to about 10 mph below the posted speed before entering, then accelerate gently through the curve✓
• d.Brake in the middle of the curve to control speed

Posted curve speeds are set for passenger cars, so a high, heavy vehicle should be well below them and should complete braking before the curve begins. Braking inside a curve can break traction and cause a skid or rollover, and coasting through above the posted speed invites a rollover.

You are starting down a long, steep grade in a loaded truck. What is the correct braking technique?

• a.Shift into neutral and use the brakes only when needed
• b.Select a low gear before the descent, then apply the brakes firmly enough to slow about 5 mph below your safe speed, release, and repeat as speed builds back up✓
• c.Use the service brakes continuously and downshift only at the bottom
• d.Ride the brakes lightly the whole way down to hold a constant speed

Snub braking, combined with a gear selected before the descent, lets the brakes cool between applications and keeps the engine helping to hold speed. Riding or continuously applying the brakes overheats them and causes brake fade, and coasting in neutral removes engine braking entirely and is illegal in many states.

Which factors determine the safe speed for descending a mountain grade?

• a.Only the size of the engine
• b.Only the type of brakes installed
• c.Total vehicle weight, length and steepness of the grade, road and weather conditions, and the amount of traffic✓
• d.Only the posted speed limit

Safe descent speed depends on how much weight the brakes must control, how long and steep the grade is, and the conditions around you. A posted limit is a maximum for ideal conditions, and neither engine size nor brake type alone determines a safe speed.

How far ahead should a commercial driver normally be looking while driving?

• a.As far as the headlights reach at all times
• b.Just past the front bumper
• c.About 5 seconds ahead
• d.About 12 to 15 seconds ahead, which is roughly a quarter mile at highway speed✓

Looking 12 to 15 seconds ahead gives you time to see and respond to developing hazards instead of reacting at the last moment. Looking just past the bumper or only 5 seconds ahead is far too short, and headlight range is a night-driving limit rather than a general rule.

How often should you check your mirrors while driving?

• a.Only when changing lanes
• b.Only when another vehicle sounds its horn
• c.Once every mile
• d.Regularly, about every 5 to 8 seconds, and before any lane change, turn, or speed change✓

Frequent scanning every few seconds keeps you aware of traffic entering your blind spots and of your own vehicle and cargo. Checking only at lane changes, at fixed mileage, or after a horn means you learn about hazards too late.

Where are the largest blind spots on a tractor-trailer?

• a.Only behind the trailer
• b.Directly ahead of the bumper and above the trailer
• c.Immediately in front of the cab, along both sides especially the right, and directly behind the trailer✓
• d.Only on the driver's side

Heavy vehicles have no-zones in front of the cab, along both sides with the right side much larger, and directly behind the trailer. Saying the blind spot is only behind or only on the left ignores the right-side no-zone where most sideswipe crashes occur.

You are driving at night on an unlit two-lane road. Why should you keep your speed low enough to stop within the distance your headlights illuminate?

• a.Because tire traction is always lower at night
• b.Because if you cannot stop within the lighted distance you are overdriving your headlights and cannot react to a hazard in time✓
• c.Because engine braking works poorly at night
• d.Because high beams are illegal on two-lane roads

Low beams light roughly 250 feet and high beams roughly 350 to 500 feet, so speed must be low enough to stop inside that distance. High beams are legal when no oncoming traffic is present, engine braking works the same at night, and traction depends on the surface rather than the hour.

You run into dense fog on the highway. What is the best course of action?

• a.Slow down, use low beams and fog lights, and if visibility is very poor pull completely off the road and turn on your four-way flashers✓
• b.Turn on your high beams so you can see farther
• c.Speed up to get through the fog quickly
• d.Follow closely behind another vehicle's tail lights

Low beams cut glare in fog while high beams reflect back and reduce visibility further, and getting fully off the roadway is safest when you can barely see. Speeding up and tailgating another vehicle's lights are two of the most common causes of fog chain-reaction crashes.

You have just driven through deep standing water and your brakes feel weak. What should you do?

• a.Drive slowly while applying light pressure to the brakes to generate heat and dry them out✓
• b.Stop immediately and wait for the brakes to air dry
• c.Pump the brake pedal rapidly while accelerating
• d.Set the parking brake while rolling

Light steady pressure while moving slowly creates the friction heat that dries wet linings and restores braking. Waiting can leave the brakes wet, rapid pumping while accelerating does not create sustained heat, and applying the parking brake while rolling can lock the drive wheels.

At what speed can hydroplaning begin when there is a large amount of water on the road?

• a.Only above 70 mph
• b.Only when tires are over-inflated
• c.Only above 55 mph
• d.As low as 30 mph✓

With enough standing water, hydroplaning can start around 30 mph, and low tread depth or under-inflation makes it more likely. Believing it only happens at 55 or 70 mph leads drivers to keep unsafe speeds in heavy rain, and it is under-inflation rather than over-inflation that raises the risk.

Your drive wheels begin to skid when you brake hard on a slick surface. What is the correct recovery?

• a.Brake harder to force the wheels to grip
• b.Accelerate to pull the trailer straight
• c.Release the brakes to let the wheels roll again, push in the clutch, and steer in the direction you want the vehicle to go✓
• d.Set the parking brake to straighten the vehicle

A locked wheel cannot steer, so you must let it turn again before steering will do anything, and countersteering brings the rear back in line. Braking harder deepens the skid, the parking brake locks the drive axle, and accelerating in a skid worsens the loss of traction.

You are steering into a corner and the front end pushes straight ahead instead of turning. What is the most likely cause of this front-wheel skid?

• a.The trailer brakes are dragging
• b.Driving too fast for conditions, worn or under-inflated front tires, or a very light load on the front axle✓
• c.The steering wheel is turned too little
• d.The engine brake is engaged

Front-wheel skids come from asking more of the steer tires than the available traction allows, usually because of excess speed, poor tire condition, or too little weight on the front axle. Dragging trailer brakes and an engaged engine brake affect the rear, and turning the wheel more only increases the skid.

You are driving a placarded hazardous materials load and approach an unguarded railroad crossing. What must you do?

• a.Sound the horn and cross at normal speed
• b.Stop directly on the first rail to get the best view
• c.Proceed without stopping if no train is visible
• d.Stop no closer than 15 feet and no farther than 50 feet from the nearest rail, look and listen, then cross without shifting gears✓

Placarded hazmat vehicles, most buses, and cargo tanks must stop between 15 and 50 feet from the nearest rail, check both directions, and cross in a gear that will not require shifting. Rolling through, relying on the horn, or stopping on the tracks all defeat the purpose of the required stop.49 CFR §392.10

Why should you never start across a railroad crossing unless you are certain you can clear it completely?

• a.Because the crossing surface damages tires
• b.Because crossing gates close automatically after 10 seconds
• c.Because state law requires a permit
• d.Because a long, heavy vehicle can become trapped on the tracks with no room to move forward or back✓

Traffic backed up on the far side can leave a long vehicle stopped on the rails with nowhere to go, so you must have room on the other side before you start. Gates do not operate on a fixed 10-second timer, no permit is required for a normal crossing, and tire damage is not the reason for the rule.

You are entering a highway work zone with reduced lanes and workers present. What is the correct driving behavior?

• a.Change lanes as often as needed to keep moving
• b.Use the shoulder to bypass slow traffic
• c.Slow to the posted work-zone speed, increase following distance, avoid unnecessary lane changes, and watch for workers and equipment✓
• d.Maintain highway speed until you see a worker in the lane

Work zones combine narrowed lanes, sudden stops, and people on foot, so lower speed and extra space are essential, and fines are often doubled. Maintaining highway speed, weaving between lanes, and using the shoulder all increase the risk to workers and to you.

You are about to change lanes on a busy interstate. What is the correct communicating procedure?

• a.Signal only if another vehicle is close behind
• b.Change lanes first and signal once you are established
• c.Signal early, check mirrors and the blind spot, change lanes gradually, and cancel the signal after completing the move✓
• d.Flash your headlights instead of signaling

Signaling early gives other drivers time to react, and canceling afterward prevents confusion about your intentions. Signaling after the move, substituting headlight flashes, or signaling only sometimes leaves other drivers guessing.

You must stop on the shoulder of a divided highway because of a mechanical problem. Where do you place your warning devices?

• a.One within 10 feet of the rear, one about 100 feet back, and one about 200 feet back toward approaching traffic✓
• b.One at 50 feet and two at 500 feet
• c.All three within 10 feet of the vehicle
• d.Only one, directly behind the vehicle

On a divided or one-way roadway the triangles go at roughly 10, 100, and 200 feet behind the vehicle facing approaching traffic, and they must be placed within 10 minutes. Bunching them at the vehicle, using only one, or the 50 and 500 foot spacing does not give approaching drivers proper warning.

You are driving an empty flatbed trailer in strong crosswinds on an open stretch of highway. What is the greatest concern?

• a.Fuel consumption will drop
• b.The engine will overheat
• c.The air brakes will lose pressure faster
• d.The light trailer can be pushed sideways or even blown over, so speed must be reduced✓

An empty or lightly loaded high-sided trailer has little weight to resist wind force, so a strong gust can push it out of the lane or tip it. Wind does not cause overheating or air loss, and fuel savings are irrelevant to the safety issue.

Which surface is most likely to be icy first when temperatures drop toward freezing?

• a.The center of an urban intersection
• b.Bridges, overpasses, and shaded areas✓
• c.A long straight stretch of dry asphalt
• d.A recently paved uphill lane

Bridges and overpasses lose heat from both the top and the underside and freeze before the surrounding road, and shaded spots stay cold longer. Sun-exposed straight roads, busy intersections, and fresh pavement generally retain heat longer.

You are following a car at a safe distance when another driver cuts into the gap in front of you. What should you do?

• a.Speed up and stay close so no one else can merge
• b.Sound your horn continuously
• c.Flash your high beams to make the driver move over
• d.Ease off the accelerator and drop back to re-establish a safe following distance✓

The only useful response is to rebuild the space cushion you need to stop safely. Flashing lights, tailgating, and prolonged horn use escalate the situation without restoring the stopping distance you just lost.

You are driving slower than the flow of traffic because of a heavy load on a long upgrade. What is the correct action?

• a.Stay in the right lane and turn on your four-way flashers to warn faster traffic✓
• b.Speed up beyond a safe speed to match traffic
• c.Drive on the shoulder
• d.Move to the left lane so others can pass on the right

Staying right and using hazard flashers warns approaching drivers that you are moving well below traffic speed. Blocking the left lane, driving on the shoulder, and forcing an unsafe speed all create additional hazards.

Why does an empty truck often take longer to stop than the same truck when loaded?

• a.The brakes are disconnected when empty
• b.Empty trucks always travel at higher speeds
• c.With less weight pressing the tires down there is less traction, and the stiff suspension and brakes sized for a full load cause wheels to lock up more easily✓
• d.Empty trucks have smaller brake chambers

Braking systems and suspensions on heavy vehicles are designed for a loaded weight, so an empty vehicle has reduced traction and lock-up occurs sooner. Speed is a driver choice, brakes are never disconnected, and chamber size does not change with the load.

A driver behind you is tailgating your trailer on a two-lane road. What is the safest response?

• a.Increase your own following distance, stay steady, avoid sudden moves, and make it easy for the driver to pass✓
• b.Turn on your hazard lights and stop in the lane
• c.Speed up until the driver falls back
• d.Brake sharply to send a message

Extra space ahead gives you room to slow gradually instead of braking hard in front of a tailgater. Brake-checking, racing away, and stopping in a travel lane all raise the chance of the rear-end collision you are trying to avoid.

On a mountain grade you see a sign for a runaway vehicle ramp ahead. What is its purpose?

• a.It is a parking area for chaining up in winter
• b.It is a mandatory rest stop for descending vehicles
• c.It is a required weigh station for loaded trucks
• d.It is an escape route designed to stop a vehicle that has lost braking ability✓

Escape ramps use gravel beds or upgrades to stop a vehicle whose brakes have faded or failed, and using one is far better than continuing downhill. They are not weigh stations, chain-up areas, or required rest stops.

You are driving in heavy snow and your windshield keeps icing over despite the wipers. What should you do?

• a.Open the side window and steer by watching the fog line
• b.Reach out the window and clear the glass while driving slowly
• c.Pull off the road in a safe place, clear the ice, and check that the defroster and wipers work before continuing✓
• d.Follow closely behind a plow so its spray keeps the glass clear

Driving without a clear view is unsafe at any speed, so the correct response is to stop somewhere safe and restore visibility. Clearing glass while moving, tailgating a plow, and steering by the fog line all substitute for the vision you no longer have.

Why should you manage space to the sides as well as ahead of your vehicle?

• a.A heavy vehicle needs room to maneuver, and staying centered keeps you from being trapped when another driver drifts or a hazard appears✓
• b.Side space only matters in city driving
• c.Wide vehicles are exempt from lane position rules
• d.Side clearance is regulated only for oversize loads

Keeping a cushion on both sides preserves an escape path when a hazard develops and prevents being boxed in by other traffic. Side space matters at highway speed as much as in town, and no exemption relieves a wide vehicle of staying in its lane.