Stopping distance calculator

How long does it take to stop at 60 mph?

About 566 feet by the AASHTO design formula: 220.5 ft while you react (2.5 seconds) plus 345.5 ft of braking. Highway design rounds it up to 570 ft.

What is the stopping distance formula?

d = 1.47 × V × t + 1.075 × V² ÷ a, with V in mph, t the reaction time in seconds and a the deceleration in ft/s². In metric: d = 0.278 × V × t + 0.039 × V² ÷ a, with V in km/h and a in m/s².

What reaction time and braking rate does it use?

The values road engineers design for in the AASHTO Green Book: 2.5 seconds to see a hazard and hit the brakes, and a deceleration of 11.2 ft/s² (3.4 m/s²). Both can be changed.

Why does doubling speed more than double the distance?

Braking distance grows with the square of speed: 86 ft of braking at 30 mph becomes 346 ft at 60 mph — four times as much. Only the reaction part doubles.

Is this how far my car stops in a panic stop?

No — it is a design value for building roads, not a measurement of your car. A hard stop on dry pavement can be shorter; wet roads, worn tires or a slow reaction make it longer.

How far does a car travel during the reaction time?

At 60 mph the car covers 88 feet every second, so 2.5 seconds of reaction means 220.5 feet before the brakes are even applied. At 30 mph the same 2.5 seconds is 110.3 feet. That distance grows in direct proportion to speed, unlike the braking distance.

Why does the distance double on a slippery road?

Because the braking distance is inversely proportional to the deceleration. If the road allows only half the braking rate, 5.6 ft/s² instead of 11.2, the braking part doubles. At 60 mph the total goes from 566 to 912 feet. Wet, icy or snowy surfaces, or worn tires, lower the achievable deceleration; set it in the form to see the effect.

How is 100 km/h shown in metric?

With the metric formula and the same 2.5 second reaction and 3.4 m/s² deceleration, 100 km/h gives 69.5 m of reaction distance plus 114.7 m of braking, 184 m in all.

Where the formula comes from

The distance is the sum of two parts. While you perceive the hazard and move your foot, the car keeps its speed, so the reaction distance is speed times reaction time. Once the brakes bite, a constant deceleration a brings the car to rest over a distance v² ÷ (2a). In mph the speed is converted to feet per second by multiplying by 5280 ÷ 3600 = 1.4667, which gives the constants 1.47 for the reaction term and 1.075 (half of 1.4667²) for the braking term. At 60 mph that is v = 88 ft/s, and v² ÷ (2 × 11.2) = 345.7 ft, the same as the calculator's braking figure to within rounding of the constants.

Worked example: 30, 60 and 70 mph

With the default 2.5 second reaction and 11.2 ft/s² deceleration, 30 mph needs 110.3 ft of reaction plus 86.4 ft of braking, 197 ft in all. At 60 mph the figures are 220.5 + 345.5 = 566 ft, and at 70 mph 257.3 + 470.3 = 728 ft. Doubling the speed from 30 to 60 mph multiplies the total by 2.9, not 2. A driver who reacts in 1.5 seconds instead of 2.5 at 60 mph cuts the total to 478 ft, saving 88 ft.

What the design values mean

The 2.5 second reaction time and the 11.2 ft/s² (3.4 m/s²) deceleration are the values highway designers use for stopping sight distance, so that a driver can stop for an object on the road. They are intended to cover most drivers and wet roads, not a perfect stop by an alert driver on dry pavement, so a real emergency stop may be shorter, and a distracted driver's may be longer. The figure on this page is a planning number for road design and for understanding how speed matters, not a prediction for your vehicle.

Common mistakes

Mixing units is the first one: with the imperial formula the speed is in mph and the deceleration in ft/s²; with the metric formula the speed is in km/h and the deceleration in m/s². Switching units in the form converts them for you. Second, forgetting that the road grade changes the braking distance: downhill stops take longer, and this formula assumes a level road. Third, treating the table of speeds as a legal safe-following distance. And a heavy truck or a car with poor brakes cannot be assumed to decelerate at the default rate.

Sources: American Association of State Highway and Transportation Officials (AASHTO), A Policy on Geometric Design of Highways and Streets (the Green Book), stopping sight distance (2.5 s reaction time, 11.2 ft/s² deceleration), as reproduced in state design manuals (Michigan DOT, Texas DOT, Indiana DOT). Related: acceleration calculator, kinematics calculator and speed calculator.