4.2 Following Gaps, Separation and Stopping Distance

Separate the safety rule from the useful measuring method

The legal obligation is to follow far enough behind another vehicle to stop safely if necessary. The required distance depends on speed, grip, visibility, brakes, load and the possible actions of traffic ahead. A heavy vehicle should maintain a useful crash-avoidance space, rather than follow at a gap selected only to discourage cars from merging.

The heavy vehicle handbook recommends a four-second following gap. Choose a fixed roadside object, begin counting when the rear of the vehicle ahead passes it, and check that at least four seconds pass before your vehicle reaches it. Increase the margin in wet weather, poor visibility, difficult road conditions or any situation reducing stopping ability. Four seconds is a planning guide, not a guarantee and not a universal statutory minimum stated as seconds in rule 126.

Convert time to distance

Speed in metres per second equals speed in kilometres per hour divided by 3.6. Multiply that speed by the selected time gap to estimate distance. At 100 km/h, four seconds corresponds to about 111 m. At 60 km/h, the same time gap is about 66.7 m. This explains why a fixed distance that seems large in town can be a very short time margin at highway speed.

These figures describe gap distance, not certified stopping performance. Reassess when speed changes. If another vehicle moves into the space, ease back safely to restore the gap instead of accelerating to close it. More room ahead can also reduce the need for severe braking when a vehicle follows closely behind.

The separate long-vehicle separation rule

Road rule 127 generally requires a long vehicle, including its load or projection and measuring at least 7.5 m, to keep at least 60 m behind another long vehicle outside a built-up area. The rule has exceptions for a multi-lane road, a built-up area and overtaking. It also specifies a different distance in a road train area; identify the applicable area rather than applying that distance everywhere.

The separation gives other traffic room to overtake and return between long vehicles. It is not a replacement for a greater gap needed to stop safely. If 60 m is insufficient for the conditions, increase it. If the vehicle ahead is an ordinary car, the special long-vehicle pair rule may not apply, but the safe-following obligation still does.

Understand stopping distance

Reaction distance is travel while the driver perceives the hazard and begins effective braking. Braking distance is travel while the vehicle slows. Total stopping distance includes both. Air-brake response, driver alertness and system condition can affect the real result; a simplified calculation should never be presented as a guaranteed truck performance table.

For an illustrative model, assume speed 80 km/h, reaction time two seconds and constant braking deceleration 4 m/s². The speed is 22.22 m/s. Reaction distance is 22.22 multiplied by two, or 44.44 m. Braking distance is speed squared divided by twice deceleration: 22.22 squared divided by eight, or about 61.73 m. Total is about 106.2 m. These are stated teaching assumptions, not an official legal stopping distance or a measured result for a particular truck.

If speed rises to 100 km/h with the same assumptions, reaction distance becomes 55.56 m and braking distance about 96.45 m, totalling approximately 152.0 m. The braking term increases with the square of speed. If deceleration is lower on a slippery road, braking distance grows further. This demonstrates why slowing early creates space more effectively than merely preparing to brake harder.

Mass increases the energy the brakes must absorb. However, mass alone does not prove a fixed stopping-distance multiplier; brake capability, tyres, road grip and vehicle condition also matter. Avoid comparing a loaded truck and a car using an invented universal distance. The practical decision is to allow for the actual vehicle and conditions.

Preserve space in traffic

Leave room when stopping behind a queue so you retain visibility and a possible escape if the vehicle ahead fails or rolls. Watch traffic beyond it. Approach bends and crests at a speed allowing a stop within visible clear road. If a following driver pressures you, maintain a safe plan rather than use brake checks or an unsafe shoulder.

Key decisions

Measure a useful time gap, apply the separate long-vehicle rule where relevant, and increase the margin whenever stopping or visibility becomes less reliable.

Official sources

Road Rules 2019, rules 126–127; handbook crash-avoidance guidance. Sources checked 10 October 2026.