Stopping Distance Calculator

Stopping distance is the sum of thinking distance and braking distance. The AASHTO formula is: s = (0.278 × t × v) + v² / (254 × (f + G)), where v is speed in km/h, t is reaction time in seconds, f is the friction coefficient of the road surface, and G is the road gradient as a decimal. The first term gives the distance covered during your reaction time, and the second gives the distance to brake to a halt. See also our calculate Trajectory Total Horizontal Range.

Perception-reaction time is the interval between a driver first seeing a hazard and actually pressing the brake pedal. The standard value used by AASHTO is 1.5 seconds, though alert drivers may react in 0.7 seconds and tired or distracted drivers can take 2.5 seconds or more. Even a small increase in reaction time significantly increases stopping distance at higher speeds.

On dry asphalt, the friction coefficient is typically around 0.80. At 80 km/h with a 1.5 s reaction time on a flat road, the total stopping distance is approximately 75 metres. Dry concrete offers a similar coefficient of about 0.75, resulting in a slightly longer braking distance than dry asphalt.

Wet roads significantly reduce tyre friction. Both wet asphalt and wet concrete have a friction coefficient of around 0.45 — roughly half that of dry surfaces. This nearly doubles the braking distance compared to dry conditions, so drivers should increase their following distance substantially in wet weather.

At 50 km/h on dry asphalt with a standard 1.5 s reaction time and flat road, the thinking distance is about 20.8 m and the braking distance is about 9.8 m, giving a total stopping distance of roughly 30.6 metres (about 100 feet). On a wet road the total rises to around 46 metres.

A downhill gradient increases stopping distance because gravity works against the brakes, while an uphill gradient decreases it as gravity helps slow the vehicle. The gradient is expressed as a decimal (e.g. 5% = 0.05) and is added to or subtracted from the friction coefficient in the formula. Even a modest 5% downhill slope can noticeably extend your stopping distance.

Braking distance is the distance the car travels from the moment the brakes are applied to the moment it stops. Stopping distance is the total distance from when the driver perceives a hazard — including the thinking distance covered during reaction time — to when the vehicle fully stops. Stopping distance is always greater than braking distance alone.

Because braking distance is proportional to the square of speed, doubling your speed quadruples your braking distance. For example, a car doing 100 km/h takes roughly four times the braking distance of one doing 50 km/h. This is why speed limits are so critical in areas with pedestrians or tight junctions.