Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!mips!cs.uoregon.edu!ogicse!ucsd!brian
From: brian@ucsd.Edu (Brian Kantor)
Newsgroups: sci.electronics
Subject: Re: IR receiver on traffic lights ?
Message-ID: <31802@ucsd.Edu>
Date: 12 Apr 91 15:51:28 GMT
References: <1991Apr12.042157.6685@engage.enet.dec.com> <1991Apr12.063837.7120@bradley.bradley.edu>
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Organization: The Avant-Garde of the Now, Ltd.
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The "Opticon" sensors in use throughout San Diego County for traffic
light control by emergency vehicles use a photocell in a focusing
assembly that's pointed down the road.  They ignore changes in ambient
light levels and look for flash rates of either 10 or 16 hz, which the
bus or emergency vehicle generates with a very bright xenon strobelight.

The two different rates are used for different purposes - one is for
emergency vehicles and causes the lights to go green in that direction
and red in all others.  The other flash rate is designed for use by
transit buses and simply extends the green cycle so that the bus won't
have to stop suddenly if the light changes.  To my knowledge, the
second isn't actually being used around here.
	- Brian