>> As I understand it over here (NZ) the radio antennas on most city traffic
>> lights are to allow buses and emergency services priority - the red cycle
>> can be delayed slightly.
>> I've been trying to find out more details re the frequency & codes used
etc.
>> - but purely for academic reasons of course :-)
>
>It *is* a subject of  absorbing academic exercise, I must agree!
>
>> (The antenna length suggests approx. 150MHz but that's all I've been
able to
>> guess so far)
>
>Maybe try employing a scanner? (Radio, not the image sort.) There are
lists of
>frequency allocations around someplace.

A method that I have seen used here in the US is a system that extends the
green to emergency vehicles by keying off a white strobe mounted on the
front of the vehicle.  The strobe frequency is specific (and unknown to
me).  You can spot these intersections that are suchly equipped- the
optical sensor is mounted in a horizontal tube pointed along the traffic
flow, attached to the same support that the light is connected to.  I used
to see these all over the Washington, DC area, but since I have moved to
Austin, TX, I have not noticed any.

Matt Bennett