In our last article we discussed the steps involved in bringing a speeding vehicle to a stop. This complex process includes the time and distance necessary to perceive and react to a hazard, followed by the time and distance necessary for the vehicle to stop. A common question that I hear is “How many feet does it take to stop a vehicle at XX miles per hour?” The answer is a complicated one.
Speed is commonly referred to in miles per hour. But it’s easier to understand speed if we think of it in terms of feet per second. To convert speed in miles per hour to feet per second, we simply multiply by 1.466. For example:
35 MPH × 1.466 = 51 FPS
55 MPH × 1.466 = 80 FPS
75 MPH × 1.466 = 110 FPS
So if you’re driving down the highway at 55 mph, you are actually traveling across the roadway at 80 fps. Let’s take this idea and apply it to perception and reaction time.
An average, sober driver in daylight conditions takes approximately 1.5 seconds to perceive and react to a hazard. If you are traveling at 55 mph, aka 80 fps, this means that your vehicle will travel approximately 120 feet before your foot even hits the brake pedal.
Now your vehicle has to react to the brake pedal being pressed. If you are driving a fire truck equipped with air brakes, the air brake system may take as much as one-half to one second just for the brakes to engage. This lag time is caused by the time it takes for the air to flow through the air lines, into the brake chambers and engage the braking system. At 55 mph, aka 80 fps, this means that you will travel an additional 80 feet before the brakes fully engage and the vehicle begins to effectively slow down.
A fire truck without ABS brakes that is traveling 55 mph on a dry, asphalt roadway takes approximately 393 feet to come to a complete stop. On a rainy day with a wet road, the total stopping distance can increase to as much as 510 feet! The next time you are looking for something to do for drill, go outside and measure off 510 feet. Still want to drive 55 mph on a wet road?
This distance is the same regardless of how long you have been driving a vehicle, or how good you think you are. Once a vehicle enters a skid, physics takes over and everyone inside the vehicle is simply along for the ride. Twenty-five years of experience driving a vehicle is no help as you are skidding across the roadway into another vehicle or a telephone pole. A responsible and experienced driver will recognize this fact and maintain a safe speed at all times.
Daly has been a contributing author to Fire Engineering Magazine,
Pennsylvania Fireman and Firerescue1.com. He has a master’s degree
in safety from Johns Hopkins University. He can be contacted at