OR CDL General Knowledge key terms

The maximum total weight a single vehicle is designed to carry safely, including the vehicle itself, fuel, passengers, and cargo, as specified by the manufacturer. It determines whether a CDL or higher class is required.

The maximum safe total weight of a towing vehicle plus its towed trailer or trailers, including all cargo and occupants, as rated by the manufacturer. It is used to assign combination vehicles to the correct license class.

A CDL category for combination vehicles where the towed unit exceeds 10,000 pounds, such as a tractor-trailer. It also allows operation of Class B and C vehicles with proper endorsements.

A CDL category for single vehicles with a GVWR of 26,001 pounds or more, or any such vehicle towing a unit of 10,000 pounds or less. Includes large straight trucks and many buses.

A CDL category for single vehicles with a GVWR of 26,000 pounds or less that carry 16 or more passengers or haul hazardous materials requiring placards. Small buses and placarded hazmat trucks fit here.

An added authorization on a CDL that permits operation of a specific type of vehicle or cargo, such as passengers, tankers, double trailers, or hazardous materials. Each requires passing a knowledge test and sometimes a skills test.

A CDL endorsement (H) required to drive a vehicle carrying placarded amounts of hazardous materials. It requires a knowledge test, a TSA threat assessment, and a clean record check.

A CDL endorsement (N) required to drive a vehicle with a permanently mounted tank of 1,000 gallons or more, or multiple portable tanks totaling 1,000 gallons or more, used for liquid or gas cargo.

A CDL endorsement (P) required to drive a vehicle designed to carry 16 or more people, including the driver. It requires a knowledge test and a skills test in a passenger vehicle.

A restriction placed on a CDL when the driver tests in a vehicle without air brakes, prohibiting operation of air-brake-equipped vehicles. Removing it requires passing the air brake knowledge and skills tests.

A systematic check of a commercial vehicle's critical parts before driving, covering brakes, tires, lights, fluids, coupling devices, and emergency equipment. Passing this inspection is part of the CDL skills test and required by federal rules.

A braking system that uses compressed air to apply the brakes and spring pressure to release them, common on heavy trucks and buses. It includes a compressor, reservoirs, valves, and slack adjusters.

The linkage that connects the brake chamber pushrod to the brake shoe, transferring air pressure force to the brake. It must be adjusted so the pushrod travel stays within limits for reliable braking.

A failsafe brake chamber that uses a strong spring to apply the brakes when air pressure is lost, parking the vehicle automatically. It is released by air pressure during normal operation.

A loss of braking power caused by overheating, usually from excessive or continuous braking on long downgrades. It can be avoided by using engine braking and lower gears instead of riding the brakes.

A device that uses engine compression to slow the vehicle, reducing reliance on the service brakes on long downgrades. Also called a Jake brake, it may be restricted in some areas by noise ordinances.

Changing to a lower gear to increase engine braking and control speed, especially before descending a hill. It should be done before the downgrade begins, not after the vehicle has already gained speed.

The horseshoe-shaped coupling plate on a tractor that locks onto the trailer kingpin and supports the front of the trailer. It must be checked for locking jaws and proper lubrication during inspection.

The vertical pin mounted under the front of a trailer that locks into the tractor's fifth wheel to connect the two. A secure lock is confirmed by a tug test before driving.

A check performed after coupling in which the driver applies the trailer brakes and gently pulls forward to confirm the fifth wheel is locked onto the kingpin. It is done before a full inspection.

A combination vehicle in which a powered tractor pulls a semi-trailer attached at the fifth wheel. It is the most common Class A configuration and requires careful coupling and turning technique.

The tendency of the rear wheels of a long vehicle or trailer to follow a shorter path than the front wheels when turning. Drivers must swing wide and check mirrors to avoid hitting curbs or vehicles.

The outward path the rear of a vehicle or trailer takes when the front turns sharply, which can strike nearby objects or pedestrians. It is managed by turning slowly and watching mirrors.

A dangerous skid in which the tractor and trailer fold at an angle toward each other, often from hard braking or loss of traction. It is prevented by smooth braking and avoiding sudden steering.

A loss of tire traction that causes the vehicle to slide, often from braking too hard, steering too sharply, or driving too fast for conditions. Recovery requires releasing the brake and steering in the direction of the slide.

The space kept between your vehicle and the one ahead, measured in seconds. For heavy commercial vehicles, at least one second per ten feet of vehicle length plus one extra second is recommended.

The total distance traveled from the moment a hazard is seen until the vehicle comes to a complete stop. It combines perception distance, reaction distance, and braking distance, and grows with speed and weight.

An area around a commercial vehicle that the driver cannot see in the mirrors. Large vehicles have large blind spots, so drivers must check mirrors often and signal before changing lanes.

The areas around a large truck or bus where the driver has limited or no visibility, including directly behind, alongside, and in front. Other drivers should avoid lingering in these spaces.

Setting the side and rearview mirrors so the driver can see the sides and rear of the vehicle and the traffic lane beside it. Mirrors should be checked before driving and adjusted after coupling.

A diamond-shaped warning sign placed on a vehicle to identify the class of hazardous material it carries. Placards alert emergency responders and determine when a hazmat endorsement is required.

The shifting movement of liquid cargo inside a partially filled tank, which can push the vehicle and affect steering and braking. Drivers must slow down and brake gently to control it.

A crash in which the vehicle tips onto its side or roof, often caused by taking a curve too fast or shifting cargo. Large vehicles have a high center of gravity, so speed must be reduced in turns.

The thickness of the rubber on a tire's tread surface, measured in 32nds of an inch. Federal rules require at least 4/32 inch on front tires and 2/32 inch on other tires for commercial vehicles.

The amount of air inside a tire, which must match the manufacturer's recommendation for safe handling and fuel economy. Low pressure causes heat buildup and increases the risk of blowouts.

Federal rules limiting how long a commercial driver may drive and work before taking required rest breaks. They set maximum driving hours and minimum off-duty periods to reduce fatigue-related crashes.

A record of a driver's on-duty and off-duty time, including driving hours, rest breaks, and vehicle inspections. It is used to show compliance with hours-of-service rules.

Physical or mental tiredness that slows reaction time and impairs judgment, a major cause of commercial vehicle crashes. It is managed with adequate rest, scheduled breaks, and following hours-of-service limits.

An intersection where a road crosses train tracks, marked by signs, signals, or gates. Commercial drivers must slow down, look and listen, and never stop on the tracks.

The skill of noticing and anticipating dangerous situations on the road early enough to respond safely. It is tested in the CDL knowledge exam through scenario-based questions about driving decisions.