What it is and when to use it
Stopping distance is the total distance a vehicle travels from the moment the driver perceives a hazard to the moment it comes to rest. It has two parts: the reaction distance, covered at full speed while the driver recognises the danger and moves a foot to the brake, and the braking distance, covered while the tires decelerate the vehicle. Understanding both parts explains why speed matters so much for safety.
Use this calculator to compare stopping distances at different speeds, to see how wet or icy roads lengthen them, and to check safe following distances or teaching examples. It uses a simple physics model with a constant friction coefficient. It does not model anti-lock braking, brake fade, tire wear, road gradient or vehicle load, so treat results as a baseline estimate rather than a guarantee of real performance.
The formula and its variables
The calculator adds two terms: d = v × t + v² / (2 μ g).
- v: speed in metres per second. The calculator converts your km/h entry by dividing by 3.6.
- t: reaction time in seconds. Around 1.5 s is a common planning figure for an alert driver, and it is longer if the driver is tired or distracted.
- μ: the friction coefficient between tires and road. Roughly 0.7 is used for dry asphalt, 0.4 for wet and 0.1 for ice, though real values vary.
- g: gravitational acceleration, 9.81 m/s².
The reaction term grows in proportion to speed, but the braking term grows with the square of speed. Doubling speed doubles the reaction distance but quadruples the braking distance.
Worked example: 100 km/h on dry asphalt
Speed: 100 km/h ÷ 3.6 = 27.78 m/s. Reaction time 1.5 s and μ = 0.7.
Reaction distance = 27.78 × 1.5 = 41.7 m. Braking distance = 27.78² / (2 × 0.7 × 9.81) = 771.6 / 13.734 = 56.2 m.
The exact total is 97.85 m, shown by the calculator as 97.8 m (about 321 ft); the parts display rounded to one decimal, so 41.7 + 56.2 appears to add to 97.9. On wet asphalt with μ = 0.4 the braking part rises to 98.3 m and the total to about 140 m.
Common mistakes and how to interpret the result
- Leaving out reaction distance. Braking distance alone can understate the real stopping distance by a large fraction at low and moderate speeds.
- Assuming one friction value fits every road. Rain, snow, gravel, worn tires and even temperature change μ, and a lower μ raises braking distance in direct proportion.
- Reading the result as a safe following gap. It is the distance to stop from the moment of perception, so vehicles ahead that stop faster than you can change the margin you need.
- Ignoring slope. Downhill lengthens the stop and uphill shortens it, and this model has no gradient term.