|Drive to Survive|
|By Chris Daly|
|January 28, 2014|
We all know the statistics…each year approximately 25% of firefighter fatalities are caused while responding to, or returning from an alarm. When we examine the reasons for these crashes, it’s as if we are watching summer re-runs of old TV shows…they are always the same. Rollovers, ejections, intersection crashes…we just can’t seem to learn. The problem is that until we decide to learn from past mistakes, the problem will just continue and each year more names will be added to the Fallen Firefighters Memorial at Emmitsburg, Maryland.
Why do fire trucks crash? It’s simple, we drive too fast and we don’t wear our seatbelts. I also believe that fire apparatus operators aren’t trained properly to understand the physical forces of Mother Nature that influence how an emergency apparatus will behave on the road. Fire Departments send their drivers to an Emergency Vehicle Operator Course (EVOC) and think that these classes magically create Superdrivers. The problem is that many EVOC courses fail to address the dynamics and physics behind large vehicle behavior. Those classes that do address these issues are often taught by instructors who don’t necessarily understand them. Granted, there are some great instructors out there, but they just don’t know how to explain these concepts. The vast majority take this section of the class and gloss over it. The usual mentality is “Let’s just get through this chapter quick and go to lunch, no one will understand it anyway.” This creates a generation of apparatus operators who don’t know the limits of driving a large vehicle. I equate it to teaching an EMT class without going over basic anatomy. The student knows how to put on a bandage, but doesn’t know why he or she is doing so. This is why we must concentrate on training our drivers to understand how Mother Nature sets limits on how fast we can drive our apparatus.
Firefighters have “can do” attitudes”…it’s a fact of life. The only problem is that sometimes we apply this attitude in “can’t do” situations. Many drivers of emergency vehicles complete a standard EVOC course, drive for a few years and then think that they can handle an emergency vehicle under any circumstances or conditions. Wrong! At some point, physics will take over and a vehicle will lose control. The point at which a vehicle will lose control can be PROVEN through simple formulas, the same formulas that are used by crash reconstructionists, you know, the same guys who close the road for hours measuring crash scenes with tape measures. Let’s take a look at some of the ways Mother Nature can steal control of your vehicle from you. Some of the words are big and fancy, but don’t be scared…if cops can understand them, so can we!
“Coefficient of Friction” – The coefficient of friction of a roadway essentially measures how “slippery” it is. A dry asphalt roadway usually has a friction value of around 0.8 to 0.9. This value is important, especially when crash reconstructionists are trying to determine how fast a vehicle was traveling from skid marks. On wet or icy roads, these values can drop to 0.2 or 0.3! What does all this mean? Drives must be aware of road conditions because they significantly affect how fast our vehicles can travel. The lower the friction value, the longer it takes a vehicle to come to a stop. Slippery or wet roads will reduce operating speeds by a large margin. You can’t drive the same way on a dry, sunny day as you would on a cold, rainy day. I’ll show you why in a minute.
“Critical Speed of a Curve” - Now this is important! How many times have we heard about a fire truck losing control while rounding a curve? I’ll bet you didn’t know that every curve in the road has a speed known as the “critical speed”. If you go faster than the critical speed, the fire truck drives off the road, no questions asked. It doesn’t matter how long you have been driving, or how good you think you are. If you exceed the critical speed of a curve, the vehicle will lose control. In order to figure out the critical speed of a curve you need only two things, the radius (or sharpness) of the curve, and the coefficient of friction of the roadway. By plugging these two values into a formula, we are able to calculate the critical speed of a curve. This also means that as the curve gets sharper, or the road more slippery, the critical speed goes down. In other words if it’s raining, you can’t drive through the curve at the same speed as if it were dry! SLOW DOWN! Let’s consider a curve with a 150-foot radius; this is a pretty common curve for most of our districts. On a dry day, with a coefficient of friction of .9, the critical speed for the curve is 44 MPH. Drive faster than 44 MPH and the truck will drive keep going straight, off the road surface and into whatever is on the side of the road, instead of staying in the travel lane and safely negotiating the curve. Now let’s say it’s raining and the coefficient of friction for this same curve is .4. Now the critical speed is 29 MPH! How many of us slow down to 29 MPH to negotiate a curve? As we discussed before, it doesn’t matter how long you have been driving, or how good you are. If you exceed 29 MPH, your vehicle WILL slide off the road.
“Total Stopping Distance” Total Stopping Distance is the total distance that it takes you to see a hazard, process the hazard in your brain, apply the brakes, and come to a complete stop. To understand this concept we first have to understand speed in terms of “feet per second” instead of “miles per hour”. 55 MPH is equal to about 80 feet per second, so in one second your vehicle will traverse 80 feet. Stop and think about that. It takes the average person around 1.5 seconds to see, process and react to a hazard. So from the time you see the car pulling out in front of you to the time your brain processes the “Uh Oh” and causes your foot to push down on the brake pedal, you’ve traveled about 120 feet! Now you have to apply your brakes and come to a stop. Let’s be honest and figure that you aren’t the greatest at “Threshold Braking”, or working your brakes so well that they don’t lock up. Instead, we’ll assume that you lock the brakes up and start skidding to a stop. Skidding to a stop in a fire truck will take around 194 feet. Let’s add that to the reaction distance and we see that it takes around 314 feet to stop a fire truck while traveling 55 MPH on dry roads. Imagine that…it’s an entire football field. On a wet day, this distance can be as much as 500 feet! Still want to approach a “stale green” light at 55 MPH and just assume that no one will pull out in front of you? I certainly hope not. The thing to remember is that this Total Stopping Distance was calculated using PROVEN formulas used by crash reconstructionists. These formulas use variables for speed, distance, coefficient of friction and braking efficiency. Nowhere in the formula do we multiply for years of experience or how good you think you are. In other words, if you’ve been driving for 40 years and teach advanced EVOC, you’ll still need 314 feet to stop your truck…just like the brand new driver in the pumper behind you!
“Braking Efficiency” An important concept to understand when we talk about Total Stopping Distance for a fire truck is the idea of braking efficiency. When a commercial vehicle, or large truck equipped with air brakes tries to stop, it can’t stop as quickly as a car. There are two major reasons for this. The first is the “lag time” it takes for air brakes to work. In a standard automobile with hydraulic brakes, you apply pressure to the brake pedal and the brakes immediately start to slow the car down. In a vehicle equipped with air brakes, you are actually operating an air valve to start the braking process. It takes up to 0.5 seconds for this air to travel through the brake lines, activate the push rods and in turn apply the brakes. One-half of a second might not seem like a big deal, but as we discussed above, if you are traveling at 55 MPH, in 0.5 seconds you will have already traveled 60 feet before your brakes even start to slow you down. Now you’ve managed to apply your brakes and now you are skidding to a stop. Once again, a large truck will be at a disadvantage due to the composition of the tires. Truck tires are designed for weight and wear. In other words, in exchange for increased durability, traction and braking ability are sacrificed. Essentially, truck tires are more “slippery” against the road surface. So what you say? Let’s pretend you are traveling behind a small car at 60 MPH. A deer runs in front of the small car so the driver slams on the brakes and skids to a stop. It will take the car 171 feet to skid to a stop on a roadway with a coefficient of friction of 0.7. It will take your fire truck 342 feet to skid to a stop on the same roadway. What happens when the small car stops at the 171-foot mark and your fire truck is still skidding for another 171 feet? Your truck will slide into the back of the small car with a tremendous amount of energy and seriously injure the people inside. It is for this reason that apparatus drivers must remember to leave plenty of room between themselves and the vehicle in front of them. You must also remember that you will need twice as much room to stop your fire truck than to stop your own car.
“Seatbelts” I believe that this one is a no-brainer. We all go to crashes in the middle of the night and as we are cutting the deceased occupants out of the wreckage we say to each other, “If they’d just had their seatbelts on”. But what do we do after we put the tools away? We climb into the rig and drive back to the station without our seatbelts! But we’re firefighters and paramedics. We don’t need seatbelts, we are invincible. WRONG! Mother Nature could care less what your occupation is when your fire truck slams into a tree. Or worse yet; when the truck rolls over and you are flung out the open window to land 75 feet down the road. There really isn’t much explaining to do for this particular topic. It all comes down to personal responsibility. Drivers are responsible for ensuring that everyone is restrained, Officers are responsible for ensuring everyone is restrained, and the individual firefighters are responsible for ensuring that everyone is restrained. To not wear your seatbelt is just plain dumb. Put it on, people!!
There are many other aspects of safe driving that we didn’t even touch upon in this article. However, I tried to address the big issues…BUCKLE UP AND SLOW DOWN! By simply addressing these two aspects of emergency response, we could help reduce the number of firefighter fatalities each y ear by 25%. You should also be aware of a new website we’ve created to help teach drivers about the physical forces that affect vehicle performance. Please visit www.drivetosurvive.org and read up on the different aspects of vehicle handling that you should be aware of. We would also welcome any new articles or links that you can find to help increase the safety and awareness of fellow fire apparatus operators throughout the fire service. Remember, buckle up, slow down and drive to survive!!