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From:	Mike Linnig <LINNIG%ti-eg.csnet@RELAY.CS.NET>  27-Mar-1987 15:29:50
Subj:	[727]  Traffic Light Sensors

Does anyone out there know anything about the photo sensors that you
can see on top of traffic lights?   

I have heard that they detect emergency vehicles (who use a special strobe)
and switch the traffic light to green.

Are the strobes coded or do the sensors just detect a certain flash
frequency?  I tried watching the strobe of an emergency vehicle but it
is too fast to tell much with out a video tape.  The strobe's flashes
do seem to be in irregular bursts, indicating a code of some kind.

I also wonder if the codes change (if they are coded) from city to 


From:	wbaker%ic.Berkeley.EDU@BERKELEY.EDU  14-May-1987 08:28:42
Subj:	[1073]  Re: Digest of accumulated msgs about traffic light sensors 

	So basically, nobody REALLY knows whats going on with these
things, just there is alot of folklore about them floating about.
Not too usefull if you ask me...

	And to quote Doug Humphrey:

	"... I doubt seriously that the pattern of a strobe light is
used for IFF (sic) in the case of traffic controllers since strobe
lights are hard to modulate reliably due to the fact that they are
based on high voltage systems that generally use the ionization of gas
to determine when the strobe goes off, and are thus not very accurate
in a timing sort of way."

	I might suggest you recheck your facts, and possibly reconsider.
A visit your local service station should be convincing enough, however
if not then a trip to the local camera store might offer more evidence.
A book on high-speed photography might also be in order; most book stores
carry them.


From:  14-May-1987 14:40:19
Subj:	[1175]  Re:  Digest of accumulated msgs about traffic light sensors

  The traffic lights here in the DC metro area are activated by
stobes but use a multiple repetition sequence (i.e. two flashes
per sec. followed by a three second blank) to ferret out phreaks
and other undesirable signals.  The strobe must also be of a
considerable candlepower (i.e. a photo flash won't even get close).
 During rush hour the real busy intersections are radio synched
in order to keep the flow of traffic steady.  The freq. is
somewhere in the 490 MHz area.  The actual information is only a
simple set of 20 - 25 tones that are transmitted in  pre-set
intervals over 2 - 3 minuets and then repeat.  All the
associated traffic lights have directional antennas aimed at
the base station (which is on Ft. Reno Dr. and Wisconsin Ave.
for anyone interested).
  It strikes me that one could use a scanner to find the tone
associated with ones favorite traffic light and just use
a low power x-mitter to override the traffic light as one approaches.


From:	Fred Blonder <>  14-May-1987 23:18:15
Subj:	[1116]  Re: Digest of accumulated msgs about traffic light sensors

	Date: Fri, 17 Apr 87 17:00:38 CST
	From: (Paul Pomes - The Wonder Llama)

	. . . One possible variation on using a timing light to
	trip the lights would be to filter out the visible portion
	of the spectrum leaving UV and IR.  Depending on the
	sensitivity of the detector and the transmission properties
	of intervening materials, the sensor could be triggered by
	an invisible means.

The "obvious solution" (well, I admit there'd be problems) would
be to have a directional SOUND sensor on the traffic lights which
listens for a siren.  Since non-emergency use of a siren is already
illegal in most places, coupled with he fact that it's difficult
to use a siren without anyone noticing ( :-) ) traffic-light phreaks
won't (shouldn't (mightn't)) be much of a problem. It'd also be
one less thing to hang on emergency vehicles.
					Fred Blonder (301) 454-7690

From:	McNelly.OsbuSouth@Xerox.COM  15-May-1987 16:10:16
Subj:	[491]  Re: Digest of accumulated msgs about traffic light sensors

I heard a rumor that their answer to "traffic light phreaks" is to set
the traffic lights to turn red for all four directions upon detection of
the strobe.  Emergency vehicles can still proceed through the empty
intersection, and there is negative incentive for traffic light phreaks
to mess with the lights.

-- John --

From:	Jack Ostroff <OSTROFF@RED.RUTGERS.EDU>  21-May-1987 11:23:54
Subj:	[1044]  more on traffic light sensors

From my experience of having driven ambulances and a fire truck (both as a
volunteer, not a professional)  changing all four directions to red might 
decrease problems with traffic light phreaks, but green really helps.  Even
emergency vehicles with lights and sirens on are supposed to stop before
proceeding through a red light.  (I know it doesn't always happen that way,
but if an emergency driver doesn't stop at red light, any accident is 
considered his fault.)
 The second problem is with having the lights respond to the siren.  Most 
emergency vehicles use electronic sirens - which can produce several kinds
of sounds (wail, yelp, hi-lo) and drivers frequently keep switching between
them to try to get the attention of oblivious drivers of nearly sound-proof
cars.  Such sensors would have to respond to all modes of all makes of sirens
used in that area.


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