How to Calculate Braking Distance

by Contributor

Drivers must pay attention to their surroundings. This includes calculating the braking distance from the cars and objects around a driver. Knowing your car's the braking distance to avoid accidents can save lives and money.

Determine the speed the car is traveling--the higher the speed, the greater the braking distance. For this example, a speed of 70 miles per hour will be used.

Establish the stopping distance. This is the distance from your car to the object in front of it. To estimate stopping distance when driving, remember that the average car length is 15 feet. So four car lengths is roughly equal to 60 feet. When driving 70 mph, the stopping distance is 102.7 feet per second (fps = 1.467 x mph).

Institute a deceleration rate. This is the rate that applying the brakes slows the vehicle. This rate is typically 20 feet per second.

Establish the stopping time. When traveling at 70 mph, which equals 102.7 feet per second if the deceleration rate is 20 fps, the stopping time equals 102.7/20 = 5.135 seconds.

Ascertain the thinking distance, which is reaction time as it relates to distance. Reaction time can take an average of 2 seconds to set in and realize that there is a problem. The reaction time can vary given the age of the driver, condition of the car, distraction in the vehicle and condition of the road. The formula to make a precise calculation 102.7 fps x 2 (reaction time in seconds) = 205.4.

Calculate the total braking distance. This formula is 1/2 the initial velocity in feet per second multiplied by the time required to stop, which is 0.5 x 102.7 x 5.135 = 263.68. The calculated thinking distance is 2 x 102.7 = 205.4. Add the two numbers together. 469.08 feet is the total braking distance.


  • check To estimate stopping time, pick a fixed object (a bridge, building or tree) and start counting the seconds when the car in front passes the fixed object. Stop counting when the driven car passes the fixed object.

About the Author

This article was written by the It Still Runs team, copy edited and fact checked through a multi-point auditing system, in efforts to ensure our readers only receive the best information. To submit your questions or ideas, or to simply learn more about It Still Runs, contact us.