On Tue, 26 Aug 1997 15:11:04 -0500 Martin McCormick
<martin@DC.CIS.OKSTATE.EDU> writes:
>        The same devices are used here to control traffic signals and
>they
>tear the daylights out of AM radio reception as you drive over them.
>What
>one hears is a carrier with lots of 60 HZ modulation on it that drifts
>across
>the station you are listening to as the car drives over the coil.

I don't think the modulation is intentional.  Maybe it is though. Offhand
I think of two major approaches to building such a detector.  The first
one would make the loop part of an oscillator and detect changies in
frequency of oscillation.  The second would make the loop part of a
filter and detect changes in impedance to a signal from a
constant-frequency oscillator.  Most likely the loop characteristic is
tuned in when the loop is installed and any change assumed to be from a
vehicle near it.  Some sort of auto-calibration could be used but it
might get fooled by heavy traffic.

>Some
>motor cycles and bicycles don't have enough metal to register and
>cyclists
>have been seen jumping off their bikes and hitting the crosswalk
>buttons
>when there isn't enough traffic around to cycle the lights.:-)

Which naturally leads to a potentially marketable project idea: some sort
of active or passive device (other than a large block of Fe) that could
be mounted on a bicycle or motorcycle to help it trigger a traffic
detector.  Nearly all traffic detectors currently in use appear to be the
type with a large loop under the road, if the project  adapts to all the
various frequencies and circuits in use it would work most everywhere.

Obviously one would need to build up the actual detector or a reasonable
copy of it to develop this project, since testing in the street isn't
very practical or safe.

>
>        I think the emergency vehicles here have IR-based strobes that
>can control the lights when a police car or fire truck needs to get
>somewhere
>fast.

They use a light detector intended primarily for visible light and
precisely time the rate of the strobe to reject random flashes of light.
School and city buses are also often equipped with such strobes.  The
strobelight detector will extend the green light if already green, or
shorten the green light for the other way if the vehicle with strobe is
approaching a red light.