> 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.

There are 2 strobe lights atop the ambulance, and they flash 2 times each
in the course of 1 second. The pulse looks something like this:

    __    __       __    __
___|  |__|  |_____|  |__|  |___

   Left Left     Right  Right

I cannot be sure of the duty cycle, but I would bet that the receiver is
not that accurate.

Maybe it just counts the pulses in 5 seconds.