Cherries and Blueberries

February 27, 2006 8:30 AM

How do emergency vehicles control traffic lights?

I've heard that emergency vehicles can control traffic lights, and have plenty of anecdotal evidence confirming this--that they can switch the lights from red to green and so forth.

But how does it actually work?

posted by dead_ to travel & transportation (15 comments total)

Some lights have a sensor on top to receive a special signal (infrared?) the emergency vehicles can emit. There is some info floating around the web on how to "hack" such a transmitter - not a good idea.
posted by exogenous at 8:34 AM on February 27, 2006

they have a button that they can push, which sends out a raidio signal that controls traffic lights.

I hear that those same buttons can be purchased online and from the back of the same sorts of magazines that have cable descrambler boxes...
posted by jaded at 8:36 AM on February 27, 2006

Here is a wired article on the topic.
posted by justkevin at 8:38 AM on February 27, 2006

With the strobe strobe lights. They look for bright flashes.
posted by delmoi at 8:48 AM on February 27, 2006

Old lights look for flashing lights, which is why sometimes you can "fake them out" by flashing your highbeams. Modern traffic lights work a little different, as stated above.
posted by blue_beetle at 8:55 AM on February 27, 2006

"Old lights look for flashing lights, which is why sometimes you can "fake them out" by flashing your highbeams. Modern traffic lights work a little different, as stated above.
posted by blue_beetle at 8:55 AM PST on February 27 [!]
"

No, you can't. Snopes article on it. Civil engineer friend of my father (California Dept of Transportation) also explained the same thing to me many years ago: Your high beams aren't going to do squat.

And I haven't seen any emergency vehicles that are so equipped. At least not the AMR ambulances or city fire department vehicles around here. There are simply policies in place to handle emergency vehicle operations. And one of those policies is to slow waaaay down at an intersection and look for traffic.
posted by drstein at 9:47 AM on February 27, 2006

See the Wikipedia entry on Traffic Light Preemption.
posted by MeetMegan at 9:55 AM on February 27, 2006

delmoi is right (except for only one "strobe").
posted by JeffK at 10:27 AM on February 27, 2006

Snopes might be wrong for some systems. There is also a slower 6 or 7 pps system out there that I recall reading about. A human can DEFINITELY flash their highbeams at this rate (don't believe me? Watch some kids play streetfighter and tell me I'm wrong). But that being said, a properly designed system should turn all the lights RED, not GREEN (or so I recall seeing the last time an ambulance went through the lights locally)! If the entire intersection turns red, nobody will (legally) use it, but the ambulance, cop car, etc can still use it at will. Much better that way...
posted by shepd at 10:32 AM on February 27, 2006

In addition to the strobes, they may also be using GPS.

I don't know how far this has gone, but about 10 years ago I was working in traffic engineering and there was a technology coming on-line whereby there was GPS enabled communication between an emergency vehicle and a nearby traffic signal allowing the signal to switch phase to favor the vehicle as it approached.
posted by hwestiii at 10:45 AM on February 27, 2006

(Also, keep in mind that in most places, emergency vehicles don't control the traffic lights — they just have standing permission to run red lights if they've got their siren on and can find a break in traffic.)
posted by nebulawindphone at 12:50 PM on February 27, 2006

Snopes might be wrong..

Heretic!
posted by Rash at 1:00 PM on February 27, 2006

There is also a slower 6 or 7 pps system out there that I recall reading about. A human can DEFINITELY flash their highbeams at this rate (don't believe me? Watch some kids play streetfighter and tell me I'm wrong).

A human may be able to press a button that fast, but an incandescent bulb (as opposed to a flash tube) of the type used in headlights cannot turn on and off that fast. So even if you could flick your headlight switch that fast, it would do nothing.
posted by Rhomboid at 2:43 PM on February 27, 2006

Some cities, eg. Tampere in Finland, use a centralized system where the route the ambulance is taking towards the hospital is automatically cleared of traffic using traffic lights. (Tampere also has a short rapid with multiple dams going through the city and the Emergency Response Center can automatically drain the specific section of the rapid when they receive a call about a person in the water.)
posted by insomnus at 2:48 PM on February 27, 2006

Ahhh... good point Rhomboid. :-)
posted by shepd at 9:29 AM on February 28, 2006