Commercial driver’s license (CDL) knowledge tests · California DMV
Controlling your speed
Questions and answers
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Which three distances add up to total stopping distance?
Perception distance, reaction distance, and braking distance
Total stopping distance is perception distance plus reaction distance plus braking distance.
Section 2 · 2.6.1 – Stopping Distance ↗ -
What is perception distance?
The distance traveled from when your eyes see a hazard until your brain recognizes it
Perception distance covers the time between seeing a hazard and recognizing it.
Section 2 · 2.6.1 – Stopping Distance ↗ -
What is the average perception time for an alert driver?
1 3/4 seconds
An alert driver takes about 1 3/4 seconds to perceive a hazard.
Section 2 · 2.6.1 – Stopping Distance ↗ -
At 55 mph, how far does the vehicle travel during the average perception time?
142 feet
Perception alone accounts for 142 feet at 55 mph.
Section 2 · 2.6.1 – Stopping Distance ↗ -
What is reaction distance?
The distance you keep traveling before you physically hit the brakes
Reaction distance is covered between recognizing the hazard and getting on the brakes.
Section 2 · 2.6.1 – Stopping Distance ↗ -
What is the average driver’s reaction time?
3/4 second to 1 second
The average reaction time is 3/4 second to 1 second.
Section 2 · 2.6.1 – Stopping Distance ↗ -
At 55 mph, how far does the vehicle travel during the average reaction time?
61 feet
Reaction accounts for 61 feet at 55 mph.
Section 2 · 2.6.1 – Stopping Distance ↗ -
At 55 mph on dry pavement with good brakes, about how long is the braking distance?
About 216 feet
Braking itself takes about 216 feet at 55 mph in ideal conditions.
Section 2 · 2.6.1 – Stopping Distance ↗ -
At 55 mph in ideal conditions, what is the minimum total stopping distance?
419 feet
Adding 142, 61, and 216 feet gives a minimum of 419 feet.
Section 2 · 2.6.1 – Stopping Distance ↗ -
You double your speed from 20 to 40 mph. How does the braking distance change?
It is 4 times longer
Doubling speed makes the impact 4 times greater and the braking distance 4 times longer.
Section 2 · 2.6.1 – Stopping Distance ↗ -
You triple your speed from 20 to 60 mph. How much greater are impact and braking distance?
9 times greater
Tripling the speed multiplies impact and braking distance by 9.
Section 2 · 2.6.1 – Stopping Distance ↗ -
Compared with 20 mph, how much greater are impact and braking distance at 80 mph?
16 times greater
At four times the speed, impact and braking distance are 16 times greater.
Section 2 · 2.6.1 – Stopping Distance ↗ -
At 60 mph, how does your stopping distance compare with an American football field?
It is greater than the length of the field
At 60 mph the stopping distance is longer than a football field.
Section 2 · 2.6.1 – Stopping Distance ↗ -
How does a heavier vehicle affect the work of the brakes?
The brakes must do more work and absorb more heat
More weight means more braking work and more heat for the brakes to absorb.
Section 2 · 2.6.1 – Stopping Distance ↗ -
Why do empty trucks need greater stopping distances?
An empty vehicle has less traction
Brakes, tires, springs, and shock absorbers on heavy vehicles work best fully loaded. Empty, the vehicle has less traction.
Section 2 · 2.6.1 – Stopping Distance ↗ -
When are the brakes, tires, springs, and shock absorbers on heavy vehicles designed to work best?
When the vehicle is fully loaded
These parts are designed for a fully loaded vehicle.
Section 2 · 2.6.1 – Stopping Distance ↗ -
From 20 mph, within what distance must the service brakes stop a single vehicle of 10,000 pounds GVWR or more, or any bus?
40 feet
The maximum stopping distance from 20 mph is 40 feet for a single vehicle of 10,000 pounds or more and for any bus.
Section 2 · 2.6.1 – Stopping Distance ↗ -
From 20 mph, what is the maximum stopping distance for combinations that fall under “all other combinations of vehicles”?
50 feet
Combinations not otherwise listed must stop within 50 feet from 20 mph.
Section 2 · 2.6.1 – Stopping Distance ↗ -
On which grades must the service brake be able to hold the vehicle stationary?
On any grade where it is operated, loaded or unloaded
The service brake must hold the vehicle or combination on any grade where it is operated, under all conditions of loading.
Section 2 · 2.6.1 – Stopping Distance ↗ -
What is traction?
Friction between the tires and the road
Without traction, which is tire-to-road friction, you cannot steer or brake.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
How can a wet road affect stopping distance?
It can double it
Wet roads can double the stopping distance.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
By about how much should you reduce your speed on a wet road?
By about 1/3
Slow by about a third on a wet road.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
You are traveling at 55 mph on a dry road and it becomes wet. To about what speed should you slow?
About 35 mph
The handbook’s example is slowing from 55 to about 35 mph.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
By how much should you reduce speed on packed snow?
By a half or more
On packed snow, cut speed by half or more.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
The road surface is icy. What should you do?
Reduce speed to a crawl and stop driving as soon as you safely can
On ice, slow to a crawl and stop driving as soon as it is safe to do so.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
Why are shaded parts of the road a hazard in cold weather?
They stay icy and slippery long after open areas have melted
Shade keeps ice on the road after sunny stretches have cleared.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
When the temperature drops, which freezes first?
Bridges, before the road
Bridges freeze before the road does.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
Near what temperature should you be especially careful on bridges?
Close to 32 degrees Fahrenheit
Take special care when the temperature is near freezing, 32 degrees Fahrenheit.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
How does slight melting change ice?
Wet ice is much more slippery than ice that is not wet
A little melting leaves the ice wet, and wet ice is far more slippery.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
What is black ice?
A thin clear layer of ice that makes the road look wet
Black ice is thin and clear enough to show the road underneath, so the road just looks wet.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
The temperature is below freezing and the road ahead looks wet. What should you watch out for?
Black ice
A wet-looking road in freezing temperatures is the sign of black ice.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
What is an easy way to check whether the road is starting to ice up?
Open the window and feel the front of the mirror, mirror support, or antenna
Ice on the mirror front, its support, or the antenna suggests the road is icing too.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
Why is the road very slippery just after rain begins?
The water mixes with oil left on the road by vehicles
Fresh rain mixes with oil on the surface. If the rain continues, the oil washes away.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
What happens when a vehicle hydroplanes?
The tires lose contact with the road and have little or no traction
Hydroplaning is like water skiing. You may not be able to steer or brake.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
Your vehicle starts to hydroplane. How do you regain control?
Release the accelerator and push in the clutch
Releasing the accelerator and pushing in the clutch slows the vehicle and lets the wheels turn freely. Do not brake.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
What should you NOT do to slow down while the vehicle is hydroplaning?
Use the brakes
Braking is the wrong response to hydroplaning.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
With a lot of water on the road, hydroplaning can happen at speeds as low as what?
30 mph
It does not take much water. With a lot of it, hydroplaning can begin at 30 mph.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
What makes hydroplaning more likely?
Low tire pressure or worn tread
Shallow grooves cannot carry the water away, and low pressure adds to the risk.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
Which signs on the road point to standing water?
Clear reflections, tire splashes, and raindrops on the road
Reflections, splashes, and raindrops on the surface indicate standing water.
Section 2 · 2.6.2 – Matching Speed to the Road Surface ↗ -
What two things can happen if you take a curve too fast?
The vehicle skids off the road or it rolls over
Either the tires lose traction and you go straight on, or they keep traction and the vehicle rolls.
Section 2 · 2.6.3 – Speed and Curves ↗ -
When should you slow to a safe speed for a curve?
Before you enter the curve
Slow down before the curve. Braking in a curve can lock the wheels and cause a skid.
Section 2 · 2.6.3 – Speed and Curves ↗ -
Why is braking in a curve dangerous?
It is easier to lock the wheels and cause a skid
In a curve the wheels lock more easily under braking.
Section 2 · 2.6.3 – Speed and Curves ↗ -
What gear should you be in while going through a curve?
One that lets you accelerate slightly in the curve
A gear that allows slight acceleration through the curve helps you keep control.
Section 2 · 2.6.3 – Speed and Curves ↗ -
How far ahead should you always be able to stop?
Within the distance you can see ahead
Fog, rain, or darkness may mean slowing down so you can stop in the distance you can see.
Section 2 · 2.6.4 – Speed and Distance Ahead ↗ -
At night you must switch from high beams to low beams. What should you do with your speed?
Slow down, because you cannot see as far
Low beams show less of the road, so slow down when you must use them.
Section 2 · 2.6.4 – Speed and Distance Ahead ↗ -
In heavy traffic, what is the safest speed?
The speed of the other vehicles
Vehicles going the same way at the same speed are unlikely to run into each other.
Section 2 · 2.6.5 – Speed and Traffic Flow ↗ -
In California, by how much can truck and bus speed limits differ from those for cars?
By as much as 15 mph
Limits for trucks and buses are lower and can differ by up to 15 mph, so take extra care changing lanes or passing.
Section 2 · 2.6.5 – Speed and Traffic Flow ↗ -
What does driving faster than the flow of traffic achieve?
Little time saved, with more passing, more risk, and more fatigue
You will not save much time, and constant passing raises the chance of an accident and tires you.
Section 2 · 2.6.5 – Speed and Traffic Flow ↗ -
On a downgrade, what must be your principal way of controlling speed?
The braking effect of the engine
Use engine braking as the main control and save the brakes for slowing or stopping when traffic requires it.
Section 2 · 2.6.6 – Speed on Downgrades ↗ -
When is the braking effect of the engine greatest?
Near the governed rpms with the transmission in the lower gears
Engine braking is strongest near governed rpm in the lower gears.
Section 2 · 2.6.6 – Speed on Downgrades ↗ -
When should you shift to a low gear for a downgrade?
Before starting down the grade
Shift to a low gear before you start down.
Section 2 · 2.6.6 – Speed on Downgrades ↗ -
Which of these should guide your choice of speed on a downgrade?
Total weight, length and steepness of the grade, road conditions, and weather
Pick a speed that is not too fast for the weight of vehicle and cargo, the grade, the road, and the weather.
Section 2 · 2.6.6 – Speed on Downgrades ↗ -
A sign on a downgrade shows “Maximum Safe Speed”. What does it mean for you?
Never exceed the speed shown
Never go faster than a posted limit or a Maximum Safe Speed sign on a downgrade.
Section 2 · 2.6.6 – Speed on Downgrades ↗ -
What is the number one cause of injury and death in roadway work zones?
Speeding traffic
Speeding traffic is the leading cause, so observe posted limits at all times in work zones.
Section 2 · 2.6.7 – Roadway Work Zones ↗ -
In a work zone, a worker is close to the roadway. What should you do with your speed?
Decrease it even further
Slow for adverse weather or road conditions, and slow even more when a worker is close to the roadway.
Section 2 · 2.6.7 – Roadway Work Zones ↗ -
On a grade outside a business or residential district, you want to pass a vehicle moving at less than 20 mph. What must be true?
You can pass at least 10 mph faster than it and finish within 1/4 mile
The pass is allowed only if you go at least 10 mph faster than the slow vehicle and complete it within a quarter mile.
Section 2 · 2.6.8 – Overtaking or Following Another Vehicle ↗ -
Within what distance must you complete a pass of a slow vehicle on a grade?
1/4 mile
The pass must be completed within 1/4 mile.
Section 2 · 2.6.8 – Overtaking or Following Another Vehicle ↗ -
How close may you follow a motor truck with 3 or more axles on a two-lane road outside a business or residential district?
No closer than 300 feet
You must stay at least 300 feet behind the listed vehicles.
Section 2 · 2.6.8 – Overtaking or Following Another Vehicle ↗ -
When does the 300-foot following rule NOT apply?
When there are 2 or more lanes for traffic in each direction
The rule does not apply during overtaking and passing, with 2 or more lanes each way, or in a business or residential district.
Section 2 · 2.6.8 – Overtaking or Following Another Vehicle ↗ -
Which vehicle is on the list you may not follow closer than 300 feet?
A school bus transporting any school pupil
The list includes large trucks, vehicles towing, school buses with pupils, farm labor vehicles with passengers, vehicles with explosives, and trailer buses.
Section 2 · 2.6.8 – Overtaking or Following Another Vehicle ↗ -
Large vehicles are traveling in caravan on the open highway. How much space must be left between them?
At least 100 feet
A gap of at least 100 feet lets other vehicles overtake and pass.
Section 2 · 2.6.8 – Overtaking or Following Another Vehicle ↗ -
Why must vehicles in a caravan leave a gap between them?
So other vehicles can overtake and pass them
The gap exists to let other vehicles overtake and pass.
Section 2 · 2.6.8 – Overtaking or Following Another Vehicle ↗
Questions and answers by topic
- Licenses, endorsements and tests105
- Disqualifications and state rules110
- Vehicle inspection80
- Basic control and shifting gears47
- Seeing and communicating46
- Managing space40
- Seeing hazards49
- Distracted and Aggressive Driving71
- Night, Fog, Winter and Hot Weather124
- Railroad Crossings and Mountain Driving65
- Emergencies, ABS and Skids102
- Accident Procedures and Fires49
- Alcohol, Drugs and Illness41
- Hazardous Materials Rules for All Drivers42
- Transporting Cargo Safely79
- Bus Inspection, Loading and Hazardous Materials74
- Driving a Bus: Stops, Crossings and Prohibited Practices56
- Air Brake Parts: Compressor, Tanks and Foundation Brakes52
- Warnings, Spring Brakes, ABS and Dual Systems59
- Inspecting Air Brake Systems61
- Using Air Brakes51
- Driving Combination Vehicles Safely50
- Combination Vehicle Air Brakes and ABS59
- Coupling and Uncoupling63
- Inspecting a Combination Vehicle24
- Pulling Double and Triple Trailers16
- Coupling, Inspecting and Brake Checks for Doubles34
- Tank Vehicles62
- HazMat basics and who does what48
- Hazard classes, labels and lists64
- Shipping papers and package markings49
- Placards and placard tables40
- Loading and unloading HazMat58
- Bulk packaging and tanks30
- HazMat driving and parking rules72
- HazMat emergencies and reporting60
- Danger zones and mirrors34
- Loading and unloading students47
- Emergency exit and evacuation34
- Railroad-highway crossings40
- Student management, ABS and special situations47