Iraq Heavy Vehicle Driving Theory key terms

The maximum total weight a vehicle is designed and permitted to carry, including the vehicle itself, fuel, driver, passengers and cargo, as set by the manufacturer and shown on the vehicle plate.

The share of a vehicle's total weight carried by a single axle or axle group, measured at the ground. Exceeding the permitted axle load damages roads and is an offence.

The way cargo weight is spread along and across the vehicle so that no axle is overloaded and stability is maintained. Heavy items should sit low and be spread evenly.

The point where the vehicle's total weight is considered to act. A high or shifted centre of gravity makes a truck more likely to roll over in turns or sudden manoeuvres.

The practice of tying down, blocking or containing cargo so it cannot shift, slide, fall or blow off during travel, using straps, chains, tarpaulins or side gates rated for the load.

The maximum force a strap, chain or tensioning device is designed to hold safely. The total lashing capacity must be sufficient for the weight and type of cargo being carried.

Movement of an unsecured or poorly distributed load during braking, acceleration or cornering. It can cause loss of steering control, rollover or cargo falling onto the road.

Carrying more weight than the vehicle or its axles are permitted to bear. It lengthens braking distance, damages tyres and roads, and is punishable by fines and load removal.

A braking system that uses compressed air to apply the brakes. Air pressure is stored in tanks and released by the brake valve to push brake chambers and shoes or pads against the drums or discs.

The cylinder on each wheel where air pressure acts on a diaphragm and pushrod to apply the brake. A ruptured diaphragm or leaking chamber reduces braking force on that wheel.

The linkage that connects the brake chamber pushrod to the brake shoe mechanism. Correct adjustment keeps pushrod travel within limits; too much travel means weak or delayed braking.

A buzzer and light that activate when air tank pressure falls below a safe level, warning the driver that brakes may not apply fully or that the spring brakes may engage.

A parking and emergency brake that uses a strong spring to apply the brake when air pressure is released. It engages automatically if air pressure is lost, stopping the vehicle.

Loss of braking power caused by overheating of brake drums, discs or linings after repeated or prolonged heavy braking, common on long downhill grades when brakes are overused.

Using a lower gear and the engine's resistance to slow the vehicle instead of relying only on the foot brake. It reduces brake heating and fade on long descents.

An engine retarder that opens exhaust valves to create resistance and slow the vehicle without using the wheel brakes. It is often restricted in built-up areas because of noise.

A dangerous situation in which the tractor and trailer fold against each other, usually after heavy braking or loss of traction on a slippery road, causing loss of control.

Side-to-side swinging of a trailer, often caused by uneven loading, high speed or crosswinds. The driver should ease off the accelerator and avoid harsh braking or steering.

Loss of tyre grip on the road surface, causing the vehicle to slide. It can be caused by wet or icy roads, sudden braking or steering, or excessive speed in a curve.

When tyres ride on a layer of water instead of the road, causing loss of steering and braking control. It is more likely at high speed, with worn tyres or in heavy rain.

The total distance travelled from the moment a hazard is seen until the vehicle stops. It is the sum of thinking distance and braking distance and increases with speed and weight.

The gap kept between your vehicle and the one ahead, allowing time to react and stop safely. It should increase with speed, load, poor weather and poor road conditions.

An area around a large vehicle that the driver cannot see in the mirrors. Other road users in this zone are invisible to the driver, so mirrors and checks must be used carefully.

A speed that allows the driver to stop within the distance seen to be clear, taking account of road, weather, visibility, traffic and load. It may be below the posted limit.

A systematic check of tyres, brakes, lights, fluids, mirrors, load and air pressure carried out before starting a journey to confirm the vehicle is safe and roadworthy.

A physical inspection in which the driver walks around the vehicle looking for leaks, loose parts, damaged tyres, insecure load and burnt-out lamps before departure.

A written record of any fault found on a vehicle, submitted to the operator so repairs can be made. Serious defects must be reported and the vehicle taken out of service until fixed.

The condition of a vehicle being safe and legal to use on the road, with working brakes, lights, steering, tyres and bodywork. It is the driver's and operator's shared responsibility.

The height of the rubber pattern on a tyre, which clears water and maintains grip. Worn tyres below the legal minimum increase skidding risk and are unsafe on wet roads.

Rules limiting how long a driver may drive and work before taking a required rest. They exist to prevent fatigue-related crashes and must be recorded accurately.

An instrument that records driving time, rest periods, speed and distance. It provides evidence that a driver has complied with hours-of-service and rest rules.

A written or electronic record of a driver's working, driving and rest hours for each day, kept so that compliance with hours rules can be checked by enforcement officers.

A state of physical or mental tiredness that slows reaction time, reduces attention and impairs judgement. It is a major crash cause and must be managed with proper rest.

Being physically and mentally able to drive safely, free from illness, alcohol, drugs or extreme tiredness. A driver who is unfit must not take control of a vehicle.

A mark on the vehicle body or a painted line on the road indicating the maximum legal height or weight limit. Drivers must know their vehicle height to avoid bridge strikes.

A sign or structure showing the maximum vehicle height permitted on a road, bridge or tunnel. Striking a low bridge can cause serious injury and major damage.

A traffic sign showing the maximum permitted weight for a vehicle or axle on a road or bridge. Ignoring it risks structural failure and is a traffic offence.

The regular use of side and rear mirrors to monitor traffic, blind spots and the load. Mirrors should be adjusted before moving and checked frequently while driving.

The steps taken after a crash: stopping, securing the scene, warning other traffic, giving first aid, calling emergency services and exchanging details without admitting liability.

A reflective triangle placed behind a broken-down or crashed vehicle to warn approaching traffic, especially on highways or at night when visibility is poor.

A written account of an accident or near miss, recording time, place, vehicles, injuries, witnesses and damage. It supports insurance claims and official investigations.