Braking Procedures

This month, we continue with the second part of a series of columns on braking procedures for emergency vehicle operators. Part 1 looked at emergency braking procedures (September 1997). Now, we will examine in chronological order those events that must...


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This month, we continue with the second part of a series of columns on braking procedures for emergency vehicle operators. Part 1 looked at emergency braking procedures (September 1997). Now, we will examine in chronological order those events that must occur to bring a fire apparatus to a safe stop. To do this we must first define the terms used in braking.

The first point that must be made is that a fire apparatus will travel a great distance from the point where the operator first realizes the need to stop and where the fire apparatus actually comes to a complete stop. The first part of that distance traveled is referred to as the "perception distance" the distance your apparatus travels from the time you spot a problem until you decide what corrective action to take. Another way to describe this is as the distance the apparatus will travel from the time your eyes spot a problem until your brain receives this signal, processes the information and then makes a decision on what action or actions to take.

The next part of that distance traveled is referred to as "reaction distance," or the distance your apparatus will travel while you move your foot from the accelerator to the brake pedal. This distance can be substantially reduced if the operator puts his or her foot over the brake at the first sign of a potential problem. This is also called "covering the brake."

If the operator covers the brake, the vehicle can save 1.1 feet for every mile per hour of speed. For example, if the fire apparatus is traveling at 50 mph and the driver covers the brake as danger is perceived ahead, the fire apparatus will save 55 feet (50 mph x 1.1 feet = 55 feet). The distance saved by covering the brake could be the difference between having and not having an accident.

If your apparatus is equipped with auxiliary braking devices, you will stop even sooner, as many of these devices are activated when you lift your foot off the accelerator. The time it takes for the operator to move his or her foot to the brake from the accelerator is the driver's reaction time. The average driver's reaction time is three-quarters of a second. However, if the driver is drunk, on drugs, fatigued or has aged to the point that hearing, eyesight or reflexes are affected, the reaction time is going to be substantially longer. Accident investigation experts have said that at night the average driver's reaction time could be as much as 1 3/4 seconds.

The next distance traveled is "brake lag distance"; that is the distance the apparatus travels after you apply the brakes and before they actuate. Brake lag distance is applicable only for vehicles equipped with air brakes. If you have a vehicle equipped with hydraulic-type brakes, there is no brake lag because depressing the brake pedal compresses a liquid that has an instantaneous reaction on the brakes. Air brakes are extremely reliable and completely safe. However, when the brake pedal is depressed in a vehicle equipped with air brakes, there is a momentary delay. This delay occurs when the brake pedal is depressed. It then triggers a release of air from the air-holding tanks, which in turn travels through the brake lines and actuates the brakes.

The last distance we travel is the "braking distance" the distance your apparatus travels after the brakes take hold until the apparatus comes to a complete stop.

By adding all the distances together you have the "total stopping distance." The equation looks like this:

PERCEPTION DISTANCE
+ REACTION DISTANCE
+ BRAKE LAG DISTANCE
+ BRAKING DISTANCE
= TOTAL STOPPING DISTANCE

Although this is the standard formula for computing stopping distances, many other factors can affect how long it will take to bring your fire apparatus to a complete and safe stop:

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