Aerials, Apparatus, Daly, Pumpers, Rescues

Total Stopping Distance, Part 2

Issue 3 and Volume 21.

As you may recall from Part 1, you need three pieces of information to calculate the stopping distance of a fire apparatus: the speed of the vehicle, the braking efficiency of the vehicle, and the coefficient of friction (COF) of the roadway.

Now that we have an understanding of the factors that affect the stopping distance of a fire truck, let’s examine some real-life examples. How will the vehicle’s speed or how will the road conditions affect an emergency vehicle’s stopping distance?

Speed

First, let’s examine speed. Many fire apparatus operators don’t realize the significant impact that vehicle speed has on stopping distance. Every time you double your speed, the stopping distance quadruples!

Assuming that you are on a a dry asphalt road and with a COF of 0.70 and a braking efficiency of 65 percent (because of the air brakes and truck tires), consider the following stopping distances:

  • 20 miles per hour (mph) equals 29 feet of skid distance.
  • 30 mph equals 65 feet of skid distance.
  • 40 mph equals 117 feet of skid distance.
  • 50 mph equals 183 feet of skid distance.
  • 60 mph equals 263 feet of skid distance.

You must also remember to account for the 1.6-second perception and reaction time, which gives us the following perception and reaction distances:

  • 20 mph equals 46 feet of perception and reaction distance.
  • 30 mph equals 70 feet of perception and reaction distance.
  • 40 mph equals 93 feet of perception and reaction distance.
  • 50 mph equals 117 feet of perception and reaction distance.
  • 60 mph equals 140 feet of perception and reaction distance.

Total Stopping Distance

After calculating the skid distance and the perception and reaction distance, add the two together to determine the total stopping distance of a fire truck at each particular speed:

  • 20 mph equals 75 feet of total stopping distance.
  • 30 mph equals 135 feet of total stopping distance.
  • 40 mph equals 210 feet of total stopping distance.
  • 50 mph equals 300 feet of total stopping distance.
  • 60 mph equals 403 feet of total stopping distance.

Although it may not seem like a big difference when you are behind the wheel, you can see that driving 40 mph instead of 30 mph will add 75 feet to your total stopping distance. Seventy-five feet could be the difference between stopping safely and slamming into another vehicle.

Road Conditions

Now let’s examine how the road conditions will affect the stopping distance of the fire truck. Obviously the slicker the road, the longer it is going to take the fire truck to come to a stop. However, we tend to jump behind the wheel, turn on the magic lights and sirens, and think that they will protect us from the effects of driving recklessly in inclement weather.

We have previously calculated the skid distance of a fire truck that is traveling on a dry, asphalt road with a COF of 0.70 and a braking efficiency of 65 percent (because of the air brakes and truck tires). Now, we are going to compare the stopping distance of a fire truck on wet roads (COF of 0.40) vs. the stopping distance of a fire truck on dry roads (COF of 0.70).

Charts 1 and 2 tell us that if we are traveling at 60 mph on a wet road, we may skid an additional 190 to 200 feet because of the wet road conditions. This extreme difference in stopping distance may come as a shock when a fire apparatus operator suddenly finds himself trying to bring the fire truck to a sudden stop because of a hazard that suddenly appears ahead. While the driver may be able to stop the rig in time on a dry day, the wet road may contribute to just enough extra stopping distance to cause the vehicle to crash.

CHRIS DALY is a 25-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 (www.drivetosurvive.org), which he has presented to more than 14,000 emergency responders across the country, and lectures nationally on preventing emergency vehicle crashes. Daly has a master’s degree in safety from Johns Hopkins University, is a Fire Apparatus & Emergency Equipment editorial advisory board member, is a contributor to Fire Engineering, and has presented at FDIC International for the past 10 years.