HTV Driving Theory key terms

The legal priority one road user has over another at a junction, crossing or narrow section. It is given by signs, markings or rules, not claimed by size or speed.

A solid white line across a lane marking where vehicles must halt for a stop sign, traffic signal or pedestrian crossing. The front of the vehicle should not cross it while stopping.

Rule that traffic already circulating on the roundabout has priority over vehicles entering, unless signs or signals say otherwise. Entering drivers must yield and choose the correct lane early.

A sign that imposes a legal requirement or prohibition, such as a speed limit, no-entry or weight restriction. Ignoring it is an offence, unlike advisory or warning signs.

A sign that alerts drivers to a hazard ahead, such as a bend, steep descent, school or level crossing. It tells you to adjust speed and attention, not necessarily to stop.

A stretch of road where signs or continuous centre-line markings forbid passing other moving vehicles, usually because visibility or space is limited. The restriction applies until the next sign or marking change.

A systematic check of tyres, lights, brakes, fluids, load security and bodywork carried out before driving. It aims to find defects that could cause a breakdown or crash while the vehicle is still stationary.

A written record, often on a company form, describing a fault found on a vehicle and the action taken. It protects the driver and operator and ensures repairs are tracked before further use.

The condition of a vehicle being safe and legal to use on public roads, covering brakes, tyres, lights, steering, emissions and structure. A vehicle that fails any required item must not be driven until repaired.

The depth of the grooves moulded into a tyre, measured to check grip and water dispersal. Minimum legal limits apply and tyres below them, or with cuts and bulges, must be replaced.

A stationary check where air pressure is allowed to settle with the engine off to see how fast it falls. Excessive loss indicates a leak that must be repaired before driving.

The point where the vehicle's weight is balanced in all directions. A high centre of gravity makes a truck more likely to roll over during cornering or sudden steering.

The tendency of the rear wheels of a long vehicle to follow a tighter path than the front wheels when turning. Drivers must allow extra room at corners and when passing kerbs.

The lateral acceleration at which a vehicle tips over rather than slides. It falls when the centre of gravity is high, the load shifts or tyre pressure is wrong.

The maximum total weight of a vehicle including its load, fuel and driver, set by the manufacturer and legal limits. Exceeding it overloads axles, brakes and tyres.

The weight carried by a single axle or axle group, measured at the road surface. Legal limits apply per axle, and uneven loading can overload one axle while total weight looks legal.

A device that caps the maximum road speed of a heavy vehicle, usually by restricting engine power or fuelling. It must be maintained and must not be tampered with or bypassed.

The gap in seconds between your vehicle and the one ahead, chosen so you can stop if it brakes suddenly. Heavy vehicles need a longer gap because of their mass and stopping distance.

The skill of noticing developing risks early, such as a pedestrian at the kerb or a vehicle edging out, and adjusting speed or position before a dangerous situation forms.

The total distance travelled from seeing a hazard to coming to rest, made up of thinking distance plus braking distance. It rises sharply with speed and with a loaded or poorly braked vehicle.

An area around the vehicle that cannot be seen in the mirrors or by direct vision from the driving seat. Large vehicles have several, so drivers must check and use mirrors before manoeuvring.

A driving style that anticipates other road users' mistakes and leaves room and time to react. It includes safe speed, good observation, early signalling and avoiding unnecessary risk.

The methods, such as straps, chains and latches, used to stop cargo moving or falling off during transport. The restraint must match the load's weight, shape and the forces of braking and cornering.

How a load's mass is spread between axles and across the vehicle's width. Correct distribution keeps steering and braking effective and prevents axle overloading or instability.

The weight a vehicle can legally carry, calculated as gross vehicle mass minus the unladen weight. Payload includes cargo, passengers and any equipment added after registration.

The maximum force a strap, chain or fitting is rated to withstand in service, marked by the manufacturer. Restraint systems must be selected so the total rating suits the load.

A braking system that uses compressed air to apply the brakes, common on heavy trucks and buses. It includes a compressor, reservoirs, valves, lines and brake chambers.

A device that removes moisture and oil from compressed air before it reaches the brake reservoirs. Damp air can freeze or corrode valves and cause brake failure.

The loss of braking power when brakes overheat, often after long descents or repeated heavy use. The pedal may feel normal while stopping performance drops, so drivers must use gears and retarders.

A visual and audible alert that reservoir pressure has fallen below a safe level. It warns the driver to stop safely, as spring brakes may apply automatically and service braking is reduced.

A parking and emergency brake that uses a strong spring to apply the brakes when air pressure is released. It engages automatically if air pressure is lost, so the vehicle cannot roll away.

A device, such as an engine brake or hydraulic retarder, that slows the vehicle without using the service brakes. It helps control speed on long descents and reduces brake fade.

A lever that converts the pushrod stroke of a brake chamber into rotation of the brake camshaft. Incorrect adjustment causes poor braking or dragging brakes and must be checked by a competent person.

Legal limits on driving and on-duty time within a day and a longer cycle, with required rest breaks. They exist to reduce fatigue-related crashes and must be recorded accurately.

A daily log showing driving, on-duty, off-duty and rest periods, kept in the required format. It must be current, truthful and available for inspection, often supported by tachograph or electronic data.

The minimum uninterrupted rest a driver must take within each 24-hour period before starting the next duty. It is separate from short breaks and must be taken away from driving duties.

A reflective triangle placed behind a broken-down vehicle to warn approaching traffic. It should be set up at a safe distance, and only if standing on the road is safe.

A written account of a crash, near miss or damage event, recording time, place, parties, injuries, witnesses and vehicle details. It supports insurance, investigation and legal obligations.

Immediate help given to an injured person before professional medical care arrives, such as controlling bleeding, maintaining breathing and keeping them warm and still.

Bringing the vehicle to a controlled halt as quickly and safely as possible when a hazard appears, using firm braking while keeping steering control and warning traffic behind.