Stopping Distances, Part 2


In part one of this article, we discussed the steps involved in bringing a speeding vehicle to a stop. In this part, we’ll focus on the complexity of the process, which 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 question I commonly hear is, “How many feet does it take to stop a vehicle at X miles per hour?” The answer is complicated.

Speed is commonly referred to in “miles per hour” (mph), butit is easier to understand speed if we think of it in terms of “feet per second” (FPS). To convert speed from mph to fps, we simply multiply mph by 1.466. For example, 55 mph x 1.466 = 80 fps. So if you are driving down the highway at 55 mph, you are also traveling at 80 fps.  Let’s apply this idea 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 (80 fps), 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 brake system may take as much as 0.5 to 1 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 (80 fps), you will travel an additional 80 feet before the brakes fully engage and the vehicle begins to effectively slow down.

Let’s assume that the vehicle is not equipped with anti-lock brakes (ABS) and you’re driving on a dry asphalt roadway. Under these conditions, if your vehicle is traveling at 55 mph (80 fps), it will take approximately 193 feet for your vehicle to skid to a stop.   

A fire truck traveling 55 mph (80 fps) 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! 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.

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