On Fire Apparatus Stopping at All Intersections

By Chris Daly

Over the past few months, we’ve seen a number of incidents related to emergency apparatus failing to stop at a negative right-of-way intersection. These intersections are defined as intersections where the emergency vehicle did not have the right-of-way—stop signs, red lights, etc. I know some of you are saying “but we ALWAYS have the right of way,” and to that I say, “You’re wrong.” I’ll need to deal with that in an entirely different article.

At any rate, let’s examine the science behind why you MUST ALWAYS come to a complete stop at a negative right-of-way intersection. This isn’t opinion, this isn’t an argument, it’s cold, hard facts. We’ll start by asking ourselves “what gives us the right to operate as an emergency vehicle?” The answer is obviously “our lights and sirens.” If our lights and sirens aren’t effective, we can’t expect to be given the right-of-way. Note: I said “given the right-of-way” because it must be given to you—you can’t demand it. Again, that’s another article entirely.

So, what if I told you our sirens are only effective to a range of about 50 to 80 feet when approaching a 90-degree intersection? You’d say I’m crazy, but it’s true. This fact has been confirmed in countless studies and I’ve gone out and proven it myself. We would need an entire book chapter to discuss why this is the case (stay tuned), but here are some highlights.

In order for a siren to be effectively heard by a civilian driver, it must enter the passenger compartment of a vehicle around 10 decibels higher than the ambient noise level inside the vehicle. Ambient noise is the amount of noise usually heard inside the vehicle while it’s driving down the road. One of the biggest factors in determining how loud it is inside the vehicle is how fast the vehicle is going. The faster the vehicle, the louder it gets because of the noise from the tires rolling on the road and the engine doing work. Throw in a radio, a conversation or the HVAC system and you can imagine how loud it can get.

So, the siren sound must enter the passenger compartment around 10 decibels higher than the ambient noise in the vehicle. However, the body of the vehicle will reduce the sound transmission from outside the car to inside the car upward of 30 to 40 decibels, depending on the design and insulation properties of the vehicle. Therefore, if the ambient noise of the passenger compartment is 70 decibels at 40 miles per hour (mph(, the driver will need the siren to arrive at their ear at around 80 decibels to be heard effectively. If the body of the car reduces the siren by 35 decibels, the siren must arrive at the driver’s window at 115 decibels. That’s really loud.

The final piece of the formula is how sound is transmitted through the air. As sound travels through the air, it loses six decibels every time the distance is doubled. Keep in mind that this is a BEST case. In reality, it usually loses more sound because of reflection off buildings and other obstacles that may block the sound between the source and receiver. So if the siren starts out at 132 decibels 10 feet away from the siren (around the output of a mechanical siren), and we need the siren to arrive outside the driver’s window at 115 decibels in order to be effectively heard by the civilian driver inside the vehicle, the siren will only be effective up to 80 feet away from the emergency vehicle.

If it takes a civilian driver traveling 45 mph approximately 200 feet to perceive, react, and skid to a stop on a dry road after hearing the siren, are we giving enough “notice of approach” before we blow the red light and pull out in front of them? Not even close. This assumes the driver immediately detects where the siren is coming from, which probably won’t be the case because of a concept known as “reflection.” In reality, it will take the driver some time to figure out where the siren is coming from, which will add to this total stopping distance. This stopping distance also assumes the civilian driver slams on their brakes and skids to a stop, which isn’t something we should be causing the public to have to do. In order to provide enough notice of approach so that the civilian can slow down safely at a more reasonable rate, we would actually have to provide around 300 feet of approach notice. That’s an entire football field.

What about our lights? Well, aside from the middle of nowhere, most of our intersections have obstructions on all four corners: trees, buildings, phone poles, etc. In many cases, civilian drivers cannot see our apparatus approaching until we are pulling right up to the intersection. Furthermore, most drivers will focus on a small area directly in front of the vehicle, which means they have to rely on their peripheral vision to see emergency lights that may be approaching from their left or right. Depending on the layout of the intersection and the approach path of the civilian driver, they will not have enough time to see the emergency vehicle and initiate an evasive maneuver when approaching a steady green light.

This was a very brief explanation of why we MUST COME TO A COMPLETE STOP at negative right-of-way intersections. Sirens are great for when you live two miles away from the firehouse and you shut off the lawn mower only to hear a siren in the distance and realize you left your pager in the dining room. Relying on your siren to alert a motorist traveling 45 mph toward a steady green light in a sound-insulated car is inviting disaster. For those interested in a more detailed explanation, feel free to let me know. I’ll be glad to share the studies. Be safe.

CHRIS DALY is a 24-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 is a Fire Apparatus & Emergency Equipment editorial advisory board member. He developed the “Drive to Survive” training program (www.drivetosurvive.org) which has been presented to over 14,000 emergency responders across the country and lectures nationally on the prevention of emergency vehicle crashes. Chris has been a contributing author to Fire Engineering Magazine and a regular presenter at FDIC.

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