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Drive To Survive: Stopping Distances (Continued)
By Chris Daly
Editor’s Note: "Drive to Survive" is 10-part series that will be published in SIZE UP magazine and The NYSAFC Bugle. Click to read the previous stories in this series: "Drive to Survive: An Introduction," "Energy," "Anti-Lock Brake Systems," and "Stopping Distances."
In the last article from this series, I 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 the hazard, followed by the time and distance necessary for the vehicle to stop. A common question that I often 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" (mph). It is easier to understand speed if we think of it in terms of "feet per second" (fps). To convert speed in mph to fps, we simply multiply by 1.466. For example:
So if you are 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 (a.k.a. 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 (a.k.a. 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.
Now the vehicle has to come to a stop. Let’s assume that the vehicle is not equipped with anti-lock brakes (ABS) and you are driving on a dry, asphalt roadway. Under these conditions, if your vehicle is traveling at 55 mph, it will take approximately 193 feet for your vehicle to skid to a stop.
A fire truck 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! 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. 25 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.
About the Author:
Chris Daly is a 23-year veteran of the fire service and a full time police officer who specializes in the reconstruction of serious vehicle crashes and emergency vehicle crashes. He developed the "Drive to Survive" training program and lectures nationally on the prevention of emergency vehicle crashes. Daly has been a contributing author to Fire Engineering magazine and Firerescue1.com. He has a master’s degree in safety from Johns Hopkins University. Click here to email Chris Daly for more information.
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