Braking Distance

This is a little more complex. There is calculus involved in actually developing some of the necessary data, so approximations have to be used, at least by some of us. Real braking distances depend on the size and weight of the vehicle, the type of road surface, tire and vehicle condition, whether the road is wet or dry, whether the roadway is flat or not, whether the vehicle is fitted with anti-lock brakes or not, and the skill of the driver. "Average" stopping distances are often reported for a typical passenger car or small truck on dry flat pavement.

There must first be an assumption for the deceleration rate that the vehicle and driver are able to achieve. If you watch a car show like Motor Week on PBS, they routinely test braking distance and it is common to see about 130 feet necessary to stop a vehicle traveling at 60 mph. If you back calculate the deceleration rate indicated by this distance, it is almost 30 feet per second per second (29.9 fpsps), on dry flat pavement in a new vehicle with a professional driver and typically with ABS. This is best case, big time.

So, to calculate the braking distance from initial speed, the time to reach a complete stop is the initial speed divided by the deceleration rate. 60 mph x 1.47 equals 88.2 fps, and that speed reduced at the rate of 29.9 fpsps requires 2.95 seconds. A simple estimate of the average speed during braking is half the initial speed. So, if braking begins at 60 mph, the average speed across the braking event is 30 mph. 30 mph x 1.47 is 44.1 feet per second, and 44.1 fps x 2.95 seconds results in the vehicle travelling 130 feet to a full stop. See, it works.