Stopping Distances, Part 1


When most people think about “stopping distance,” they usually picture a vehicle skidding to a stop in a haze of blue smoke. In reality, bringing a vehicle to a stop is a more complicated process than you think.

The first step in this process occurs when a driver perceives a hazard in the roadway. In other words, we must sense that there is a problem up ahead, such as a vehicle pulling out in front of us or a small child running across a lawn. Not only must we see this hazard, but our minds must recognize that the approaching situation is a potential problem. A driver who sees a tractor-trailer crossing the roadway may immediately realize that this is a potential hazard and begin the braking process. However, if a driver sees a small child running across a lawn, he may not immediately recognize that this child may suddenly change course and run into the street. In this situation, the driver may continue at his current speed and not begin the braking process until it is too late. It is for this reason that emergency vehicle drivers must constantly scan the road ahead of them and always be alert for potential problems.

Factors such as fog, rain, snow or smoke can obscure a driver’s view of the road ahead and contribute to delayed perception times. There are numerous examples of fire apparatus crashes that occurred while the vehicle was driving through a smoke-filled street. When approaching fire scenes where the roadway is obscured by smoke, drivers must be extra cautious of possible hazards that they cannot see. Drivers also must moderate their driving habits in the event of bad weather. Slow down — what you can’t see may kill you!

The amount of ambient light may also affect a driver’s ability to perceive a hazard in the roadway. The common belief is that it is harder to see while driving down the road at night, or at dusk. In reality, this is not always the case. The issue is not necessarily how much light is available to a driver’s eyes, but rather more complicated factors such as the contrast of what lies in the road ahead. For example, a driver may be able to see a road sign better at night than during the daytime due to its reflective properties when struck by approaching headlights. On the other hand, a pedestrian who blends into the surrounding environment may not be perceived until it is too late to stop. The lesson to be learned is that drivers must slow down at night!

After a driver perceives a hazard ahead, he must decide how to react. The driver must determine what action is most appropriate for the situation. A driver could slow down, brake, speed up or attempt to steer around a hazard in the roadway. No matter what action the driver takes, it will involve sending signals from his brain to his hands or feet. Once his hands and feet receive these signals, muscles will need time to react. The time it takes for the body to send signals to the appropriate muscles and begin the evasive action is what is known as “reaction time.”

While every person is different, the average “perception and reaction time” is around 1.5 seconds for a sober driver in daylight conditions. The amount of time needed can increase during poor weather conditions, at night, or when a driver is impaired by drugs or alcohol. The more time it takes to react, the longer the total stopping distance will be. On an average day, 1.5 seconds will have gone by before you’ve even touched the brake pedal.

Once a driver perceives a hazard in the roadway and then decides how to react to this hazard, additional time is needed for the vehicle to react. Let’s say the driver decides to press down on the brake pedal and attempt to bring the vehicle to a stop. In a vehicle equipped with hydraulic brakes or air brakes, it takes time for the brakes to engage. Whether it’s hydraulic fluid flowing from the master cylinder, or air flowing to the brake chambers, additional time is needed for the braking process to begin. All the while, the vehicle is hurtling toward the hazard. It should be noted that air brakes typically take more time than hydraulic brakes to fully engage. In a commercial vehicle, or fire truck, stopping distances will be longer due to the additional time it takes for air to travel through the air lines, enter the brake chambers and begin the braking process.

Once the brakes have engaged, the vehicle will initiate the braking process. Depending on whether or not your vehicle is equipped with an anti-lock brake system (ABS), the wheels may or may not lock up and begin to skid (Read: “Burning kinetic energy key to safe driving” for more information). The distance needed for a vehicle to come to a stop depends on several factors, including the vehicle’s original speed, the road conditions and the brakes’ efficiency.

As you can see, the concept of “total stopping distance” is more complex than just a set of skidding tires. Total stopping distance includes the time needed to perceive and react to a hazard, the time it takes for a vehicle’s mechanical parts to engage, and finally the distance needed for the vehicle to skid or come to a stop. It is for these reasons that apparatus drivers must strive to avoid distractions and maintain a heightened sense of alertness for potential hazards in the road ahead. By maintaining a safe speed, drivers will allow themselves the time needed to see and react to an approaching hazard. In my next article, I will explore these concepts in more detail.

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