Stopping Distance Calculator

Created by Luciano Mino
Last updated: Aug 01, 2022

Our stopping distance calculator will tell you how long a moving car will travel before it comes to a stop.

Finding the stopping distance is a key factor in car safety research 🚗, and in this short article, we will explain everything you need to know about it, including:

  • Braking distance and stopping distance difference;
  • Braking distance formula;
  • The stopping distance formula; and
  • How to calculate stopping distance.

Braking distance and stopping distance

We should consider two similar definitions when calculating the distance before a car stops: the braking distance and the stopping distance.

The braking distance is the distance a moving car travels from when the driver presses the brakes to when the vehicle stops moving.

Similarly, we define the stopping distance as the distance between the moment the driver notices an obstacle (such as an animal crossing the road) and reacts by pressing the brakes and the instant the car stops moving.

Here, the key difference is the driver's perception-reaction time, which we account for in the stopping distance formula.

💡 You can also obtain braking times using the advanced-mode of this stopping distance calculator by inputting the deceleration of the brakes.

Stopping distance formula

If you want to know how to calculate the stopping distance, AASHTO (the American Association of State Highway and Transportation Officials) gives us the stopping distance formula which is used in road design to protect the drivers:

s=0.278×t×v+v2254×(f+G){\footnotesize s = 0.278\times t\times v + \frac{v^{2}}{254\times (f + G)}}

where:

  • ss is the stopping distance in meters.
  • tt is the perception time in seconds. You can read more about perception-reaction times values in the following section.
  • vv is the car's speed in km/h.
  • GG is the grade of the slope, which is positive for uphill and negative for downhill (leave as 0 for flat roads).
  • ff is the coefficient of friction, which is assumed to be 0.7 on dry roads and between 0.3 and 0.4 on wet roads.

🙋 Too complicated? Just fill in the parameters in our stopping distance calculator, and it will automatically do the work for you!

If we don't account for perception-reaction times or road inclination, we get the braking distance formula, which uses the same parameters:

b=v2254×fb = \frac{v^{2}}{254\times f}

Perception-reaction times

Our stopping distance calculator allows you to input any perception-reaction times. However, it would be best if you had some values as guidelines:

  • 1 second - A keen and alert driver.
  • 1.5 seconds - An average driver.
  • 2 seconds - A driver who is tired or an elderly person.
  • 2.5 seconds - Worst case scenario. Even elderly or intoxicated drivers will probably manage to react within 2.5 seconds.
Luciano Mino
Speed
mph
Perception - reaction time
sec
Grade
%
The road is...
dry
Stopping distance
ft
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