Haul Road Stopping Sight Distance Calculator
Estimate the stopping sight distance a heavy or haul vehicle needs — the reaction distance plus the braking distance — for a given speed, reaction time, deceleration and road grade. Because loaded trucks brake weakly and downgrades lengthen the stop, this distance is much greater than for a light vehicle.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the vehicle speed in km/h.
- Optionally adjust the driver reaction time (default 2.5 s) and the available deceleration (default 2.5 m/s² for a loaded haul truck on an unsealed road).
- Enter the road grade as a signed percentage — positive for uphill (helps stopping), negative for downhill (lengthens it) — then read the total stopping sight distance.
How it works
Speed is converted from km/h to m/s by dividing by 3.6. Reaction distance = speed × reaction time. Braking distance = speed² / (2 × (a + g·grade)), where a is the deceleration, g = 9.81 m/s² and grade is the grade expressed as a fraction (grade% ÷ 100). A positive grade adds to the effective deceleration; a negative grade subtracts from it. If the downgrade is steep enough that a + g·grade is zero or negative, the vehicle cannot stop under these assumptions and the calculator returns an error. The stopping sight distance is the sum of the reaction and braking distances.
Worked example
Worked example. A haul truck at 40 km/h (11.111 m/s) with a 2.5 s reaction time and 2.5 m/s² deceleration on level ground: reaction distance = 11.111 × 2.5 = 27.78 m, braking distance = 11.111² / (2 × 2.5) = 24.69 m, so stopping sight distance = 52.47 m. On a 10% downgrade the effective deceleration falls to 2.5 − 0.981 = 1.519 m/s², the braking distance grows to 40.64 m and the total rises to 68.42 m.
Common mistakes
- Entering a downhill grade as a positive number — downgrades must be negative, and they lengthen the stopping distance.
- Using a light-vehicle deceleration (often 3.5–4 m/s²) for a loaded haul truck, which brakes far more weakly, especially on loose or wet surfaces.
- Treating the result as an approved design value — the real requirement comes from the mine's haul-road design standard and traffic management plan.
Frequently asked questions
Why does a downhill grade make such a big difference?
Gravity works against the brakes on a downgrade, reducing the effective deceleration. Because braking distance grows with the square of speed and inversely with deceleration, even a modest downgrade at speed can add tens of metres to the stopping distance.
What reaction time and deceleration should I use?
The defaults (2.5 s reaction, 2.5 m/s² deceleration) are conservative values for a loaded haul truck on an unsealed road, but the correct figures for your site come from the vehicle's braking performance data and the mine's own standards. Wet, loose or steep surfaces reduce the achievable deceleration further.
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