Maine CDL General Knowledge key terms

The total distance a vehicle travels from the moment the driver perceives a hazard until the vehicle comes to a complete stop. It combines perception distance, reaction distance, and braking distance.

The distance a vehicle travels while the driver is recognizing a hazard and deciding to act. At highway speed it can exceed the length of a football field before any braking begins.

The distance a vehicle travels from the moment the brakes are applied until it stops. It grows with the square of speed, so doubling speed roughly quadruples braking distance.

The gap a driver keeps behind the vehicle ahead, measured in seconds of travel time. Large vehicles need at least one second per ten feet of vehicle length at speeds over 40 mph.

The safety margin of empty space maintained around a vehicle on all sides so the driver has room to steer or brake if another road user makes a sudden move.

A pre-planned path or open space a driver can steer into when the lane ahead is blocked or a hazard appears suddenly, chosen before it is needed.

The skill of scanning ahead and spotting developing risks, such as merging traffic, pedestrians, or road debris, early enough to respond smoothly rather than abruptly.

A systematic walk-around check of a commercial vehicle before driving, covering tires, brakes, lights, coupling devices, fluid leaks, and load security to catch defects early.

The three-part vehicle check required for a commercial driver's license skills test: a pre-trip walk-around, an en-route inspection of brakes and signals, and a post-trip inspection.

A sequence of checks on an air brake system, including static leakage, applied leakage, low-air warning activation, and a tug test of the parking brake, performed before driving.

The rate at which a compressor restores air pressure in the brake system. A typical requirement is raising pressure from 85 to 100 psi within about 45 seconds at governed engine speed.

A buzzer and light that activate when brake system air pressure drops below a set level, usually around 60 psi, warning the driver to stop before braking power is lost.

A parking brake held engaged by a strong mechanical spring and released by air pressure. If air pressure falls too low, the spring applies the brake automatically.

The lever arm that connects the brake chamber push rod to the brake shoe, multiplying force. Its travel distance shows whether the brakes are properly adjusted.

A sealed canister where compressed air pushes a diaphragm and push rod to apply the service brakes. It is part of the foundation that converts air pressure into braking force.

A valve that keeps air pressure available for the tractor's brakes if a trailer air line breaks or separates, helping the driver retain control during a trailer emergency.

Two flexible hoses, the service line and the emergency line, that carry air between tractor and trailer. They must be properly connected and supported so they do not drag or chafe.

The couplers joining the tractor's emergency air line to the trailer. They are color-coded and must be locked in place so the trailer brakes release and function correctly.

The horseshoe-shaped coupling plate on the tractor that grips the trailer's kingpin. Its jaws must be closed and locked, checked with a tug test before driving.

The vertical steel pin under the trailer nose that locks into the tractor's fifth wheel. It carries the pulling load and must be fully seated in the coupling jaws.

A coupling check where the driver sets the trailer parking brake, pulls gently against it, and confirms the tractor does not separate from the trailer before driving away.

The tendency of the rear wheels of a long vehicle to follow a tighter path than the front wheels in a turn, which can cause the trailer to strike curbs or pedestrians.

The outward sweep of the rear of a long vehicle during a turn, caused by the vehicle pivoting around its rear axles. It can strike nearby objects or people.

The size of the arc a vehicle follows in a turn. Large vehicles need wide turns, and drivers must begin the turn from the proper lane position to avoid hitting curbs.

A shifting technique for non-synchronized manual transmissions: press the clutch, shift to neutral, release the clutch, then press it again to engage the next gear.

Shifting a non-synchronized manual transmission without using the clutch, matching engine and road speed so gears engage smoothly. It requires skill and is not recommended for beginners.

Changing gears one at a time in sequence rather than skipping gears, which keeps engine speed matched to road speed and avoids lugging or over-revving the engine.

Using a lowered gear and the engine's compression resistance to slow the vehicle, reducing reliance on the service brakes on long downgrades and helping prevent brake fade.

Loss of braking power caused by overheated brake drums and linings after repeated or prolonged braking, often on long downgrades, requiring lower gears and cooling time.

An emergency escape lane, usually filled with gravel or sand and often uphill, built to stop a vehicle that has lost braking ability on a steep downgrade.

An area around a commercial vehicle that the driver cannot see in the mirrors. Large vehicles have much bigger blind spots than cars, especially on the right side.

The areas alongside and behind a large vehicle where the driver has limited or no visibility, so other motorists should avoid lingering there to reduce collision risk.

Setting the flat and convex mirrors so the driver can see the sides of the vehicle, the rear tires, and the traffic lane beside it, checked before every trip.

Operating a vehicle while attention is diverted by a phone, navigation device, food, or conversation. Even a brief glance away at highway speed covers a long distance.

Federal rules limiting driving time and requiring rest breaks for commercial drivers, designed to reduce fatigue-related crashes. Records are kept in a log or electronic device.

Physical and mental tiredness that slows reaction time and judgment. It is a major crash cause, which is why rest breaks and driving-hour limits are enforced.

Tying down cargo with straps, chains, or binders so it cannot shift, fall, or spill. The number of tiedowns and their strength must meet federal cargo rules.

Placing and balancing freight so no axle is overloaded and the vehicle stays stable. Poor distribution can lighten the steering axle or overload the drive axles.

The maximum weight an axle or axle group may carry under federal bridge formula and state rules. Exceeding it risks fines, unstable handling, and tire or brake failure.

The P endorsement required to drive a commercial vehicle carrying passengers, which involves additional training and testing on loading, unloading, and passenger safety.

A passenger who rides standing in a bus that permits it. Standees may only be carried where allowed, and the aisle must stay clear for evacuation.

A marked door, window, or roof hatch on a passenger vehicle used for rapid evacuation. Drivers must know how each exit operates and keep them unobstructed.