# How to Calculate the Stopping Distance

Written by jillian o'keeffe
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You can calculate the stopping distance of a vehicle by using an equation that takes into account the speed of the car and the friction between the tires and the road surface. The most variable part of the calculation is the coefficient of friction, which will change depending on tire tread, a wet or dry road surface and whether the car's wheels are still turning or are locked. The variability of the coefficient of friction under different situations means that calculations of stopping distance under normal driving conditions cannot be measured extremely accurately.

Skill level:
Moderate

• Calculator
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## Instructions

1. 1

Choose the speed that you wish to calculate stopping distance for. A faster speed will result in a longer stopping distance, as many people will know from personal experience in an emergency stop situation.

2. 2

Convert the speed (velocity) in miles per hour into feet per second for the purposes of the calculation. This is most easily done by inputting the speed value into an online conversion tool such as CalculateMe.com. Alternatively, arithmetic can be used to convert the values if you take into account that one mile per hour is equal to 1.46666667 feet per second.

3. 3

Take the coefficient of friction as 0.8. This value is a ratio of the frictional resistance force to the normal force which presses the surfaces of the tires and the road together. It varies according to the road and tire surface, but 0.8 is a reasonable value for calculating stopping distances on a dry road when the car wheels are either locked or still turning.

4. 4

Enter your values into the equation: Distance = (velocity)squared / 2 multiplied by coefficient of friction multiplied by gravity.

Or d=v^2/2μg

For the equation, the value of the acceleration due to gravity (g) is 32.174 feet per second per second. (32.174ft s^-2). and the coefficient of friction is 0.8.

5. 5

Take the resulting value of d to be your minimum stopping distance under conditions where your tire treads are new and the road is dry and flat.

6. 6

Calculate stopping distance of your car for wet roads. A suitable rough value of the coefficient of friction with tires of the correct tread depth and efficient brakes on wet roads is about 0.4.

7. 7

Recalculate the stopping distance for different speeds. A lower speed will result in a shorter stopping distance, which is why speed limits exist in built-up areas.

#### Tips and warnings

• Keep tires inflated to the correct pressure and ensure tire tread is within specifications in order to minimise stopping distances.
• Minimum stopping distance calculations will be affected by worn tires or inefficient brakes, as well as oily or icy road surfaces.
• The stopping distance calculation does not take into account reaction speed of the driver.

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