Sirens on the freeway

[jprasek]

It's been awhile since EMT school, but it has come up whether running lights and siren on a freeway is effective? I remember being taught that after about 60-65 mph you actually start to out run your siren and it's not as effective far away. Can anyone lead me to the answer or does anyone know the answer. Thanks in advance.

Jim Prasek

[fireman4949]

It's been awhile since EMT school, but it has come up whether running lights and siren on a freeway is effective? I remember being taught that after about 60-65 mph you actually start to out run your siren and it's not as effective far away. Can anyone lead me to the answer or does anyone know the answer. Thanks in advance.

Jim Prasek

Well, let's see...The speed of sound in air is roughly 770 mph.

I don't think you're in much danger of out running the siren!

[BoxAlarm187]

Included in my EVOC instructor's manual is the statement that a 100W electronic siren has only 12 feet of forward penetration at 60MPH. I haven't done the true research to see where this came from, but I feel confident that our Commonwealth wouldn't have me deliver this information if someone hadn't looked into it.

At the VFD, we have a rig that runs 64MPH max, but have a 65MPH highway to respond on. Therefore, when we get on the highway, we simply turn the lights and siren off till we either approach the scene or the exit we're going to use. What good are they if you're going 10MPH slower than everyone else out there?

At work, our engines run faster and easily keep up with highway traffic, so the electronic siren is usually left on simply to comply with the law, not because it does any good at those speeds. The Q-siren on the other hand, is another story.

HTH...

[jprasek]

Well, let's see...The speed of sound in air is roughly 770 mph.

I don't think you're in much danger of out running the siren!

It's not a question of the actual speed of sound, it's the fact that the siren can not project the sound far enough ahead at higher rates of speed. Like I said, I'm not saying this is true or not, I'm just trying to find the answer.

[firepiper1]

I took this from an article about the effectiveness of sirens.

"
You need at least 75dBA of siren level to be audible through rolled up windows and over the car radio. In reality it might have to be 20dB higher if it's a boom car, or a Mercedes, or some other vehicle with very good sound isolation. The maximum warning distance you can get with a siren that begins with a 100dBA at 10' is about 160'.

At 30 MPH (44 ft/sec) closing speed that gives you about 4 seconds of warning for drivers ahead of you. At 60 MPH (88 feet per second) closing speed that only gives you 2 seconds of warning time. That's just the warning time for someone to begin to hear a siren, they still have to react and try to locate the emergency vehicle and then do something about it.

If the car has a loud audio system so that you needed 90dBA of siren level to be audible inside the car, then you only have about 35' of distance where the siren is loud enough to be heard inside the car. You're relying entirely on the flashing lights to warn the kid in the boom car, they likely won't even hear the air horn. At 30 MPH closing speed, that's less than 1 second for the driver to hear the siren and react. At 60 MPH closing speed that's less than 1/2 a second of warning time."

I used to drive a MICU on long distance transports and thought that using lights and sirens on a highway with the flow of traffic was a waste.

[jprasek]

Once again I want to thank everyone that took the time to reply. This is a great forum and I know that if I ever need a question answered or any type of information you are the guy's to go to.

Stay safe out there and once again, thanks.
Jim

[Catch22]

Here's one you'll like. My career department, by departmental guidelines, limits drivers to the speed limit or 60 mph. Now here's the kicker: we protect about 12 miles of 70 mph interstate! A lot of guys will shut down until we get a little closer to the scene in the name of safety; we figure it can't be safe to be running hot when you're travelling slower than the pace of traffic. It doesn't matter a lot on how fast we're allowed to go on paper, anyway; our trucks are governed to about 66 mph.

Anyway, to answer your question, yes it is taught that you can "outrun" your sirens. Like has been said, it's a penetration issue. The siren has to "push through" the air you're cutting through at 60+ mph. Ever hear the difference in a tornado/outdoor siren when the wind's blowing compared to when it's not? If the wind is blowing toward the siren, it'll dampen the sound compared to a calm day when it can travel relatively freely. It's like the wind "carries" the sound away from the source.

[wcfpd2601]

I can't speak for all of my districts stations. But we have told our guys to turn off the siren on the interstate. Our vehicles are governed at 64-68 mph (65 mph speed limit). While it technically is a violation of the IL motor vehicle code to run lights and no siren...at least maybe we can keep people from pulling in front of us, but when people are passing us it's not much use to use the siren.

NOW, a little siren info about speed etc....P.S. long live the "Q"!

How does speed affect a siren?

Vehicles with electronic sirens tend to out run their sound waves at approximately 55 mph. This phenomena is so common in the industry, they call it sirencide. Vehicles equipped with mechanical sirens do not outrun their sound waves. The air or shock wave is accelerated to more than 120 mph allowing the vehicle’s speed to never be a concern.

[DeputyMarshal]

Vehicles equipped with mechanical sirens do not outrun their sound waves. The air or shock wave is accelerated to more than 120 mph allowing the vehicle’s speed to never be a concern.

Pure poppycock.

Sound waves propagate the same way regardless of what method generated them -- vibrating speaker diaphram or rotating siren turbine. Any siren that produces a sound pressure of 100dB @ 10 feet while sitting still will still produce the same nominal 100 dB sound pressure while going @ 60 mph regardless generation method.

There is unquestionably a difference in sound patterns/directionality generated by electronic versus mechanical sirens but there's no magical voodoo projecting unique "shock waves" in front of mechanical sirens in front of a vehicle at 120 mph.

"Outrunning the siren" is a factor of the closing speed versus available reaction times as very nicely explained a few posts back. Don't be deceived by a mumbo-jumbo explanation on a website devoted to selling mechanical sirens...

[Weruj1]

Law here says to have the lights on gotta have the siren on too. So we usally run lights only or both the siren and lights, and if traffic is all backed up really bad .............no lights or siren as there is nowhere for the traffic to go.

[ffmedcbk1]

Fire engines usally are slower than traffic. Why leave the lights flashing and siren blaring unless you are requesting the right of way?

Turn them off unless coming to slowed traffic or nearing the scene. It is just too confusing.

I've seen private EMS running down the freeway with sirens on all the time, not going faster than normal traffic. Why do it? Personally, I'll just say either they are happy to use the siren or no one has ever told them not to use it on the interstate.

[ValleyMedic44]

What is your policy and/or belief on L&S going through a construction zone on a controlled access highway where traffic is reduced to one lane?

My company has no policy by I tend to turn them all off while proceeding through the "chute". I feel that it is safer AND quicker to proceed with traffic instead of slowing to squeeze past vehicles.

[Eng34FF]

What is your policy and/or belief on L&S going through a construction zone on a controlled access highway where traffic is reduced to one lane?

My company has no policy by I tend to turn them all off while proceeding through the "chute". I feel that it is safer AND quicker to proceed with traffic instead of slowing to squeeze past vehicles.

If the construction zone is like most I've seen, there is no place for people to pull off anyway, so lights and sirens just create a jam. My last department has a bridge that is similar, no shoulders. The SOP was to turn off lights and proceed with traffic until we got to the other side.

[DennisTheMenace]

I took this from an article about the effectiveness of sirens.

"
You need at least 75dBA of siren level to be audible through rolled up windows and over the car radio. In reality it might have to be 20dB higher if it's a boom car, or a Mercedes, or some other vehicle with very good sound isolation. The maximum warning distance you can get with a siren that begins with a 100dBA at 10' is about 160'.

At 30 MPH (44 ft/sec) closing speed that gives you about 4 seconds of warning for drivers ahead of you. At 60 MPH (88 feet per second) closing speed that only gives you 2 seconds of warning time. That's just the warning time for someone to begin to hear a siren, they still have to react and try to locate the emergency vehicle and then do something about it.

If the car has a loud audio system so that you needed 90dBA of siren level to be audible inside the car, then you only have about 35' of distance where the siren is loud enough to be heard inside the car. You're relying entirely on the flashing lights to warn the kid in the boom car, they likely won't even hear the air horn. At 30 MPH closing speed, that's less than 1 second for the driver to hear the siren and react. At 60 MPH closing speed that's less than 1/2 a second of warning time."

I used to drive a MICU on long distance transports and thought that using lights and sirens on a highway with the flow of traffic was a waste.

If traffic was at a stand still the 2 seconds would applly at 60mph, but since they are likely going within 5-10 MPH of your speed (Which translates to a 5-10 mph closing speed, not 60mph closing speed) the reaction time available is slightly longer. We run lights and sirens on the highway, sometimes they are effective, sometimes they are not. At night the lights are highly effective though even if the siren might not be.

[bchap611]

We usually run just lights down the Interstate because quite frankly not much catches Interstate driver's attention anymore. If we get up close enough to someone who still hasn't seen us quiet yet we flip the siren a couple times it usually gets their attention to pull over. Only thing bad about that is some people tend to freak out when they see fire trucks/ambulances so you don't want to cause more harm than what you're on the way to fix so to speak lol.

BFD
Engine Co. 611/Medic 610

[volfirie]

A run on the freeway? Turn the noise off and just run with warning lights. Can't keep up with the traffic in most of our vehicles anyway! But running with the liights shows that we're on a call, not just taking the truck for a run.

[HeavyRescueTech]

sirens at freeway speeds are generally useless.

that being said, if i'm traveling up the shoulder lane of a gridlocked highway, i will keeps the lights on and use the airhorn only to get peoples attention. not to move aside, but to make people aware that i am coming (and hoping they stay where they are).

the wail siren is useless when traveling at highway speeds. the yelp and other faster sirens are better (for people hearing them), but their range is significantly shorter than the wail.

[dfdems]

I have found that if you split the lane a little bit to the left or right of the lines people are more apt to see you coming up in their rear view mirrors long before they hear you coming.

[FIREMAN36054139]

according to our state law in Alabama if your lights are on your noise maker better be on also

[Bernhard]

.....The air or shock wave is accelerated to more than 120 mph allowing the vehicle’s speed to never be a concern.......

I can agree with that! We use the air sirenes (runned by an extra compressor) at the firetrucks, while police has electronical sirenes.

We are not allowed to shut the sirenes off and only run with lights, but we do it if we have a call out at night, cause we don´t wonna wake up whole village.


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