Per Tekkie®:
Just today I witnessed an ambulance blow two red lights on a main
thoroughfare.... no siren in this case, no lights.... just relying on
some new system they have in place that turns the light suddenly green
for the emergency vehicle - only fly in the ointment was the lights did
not turn green until *after* the vehicle had blown through them.

It's called traffic preemption. There is a acknowledgment sent back to the
EV that the signal is giving them the green.

Seems like one of those things that sounds good in a sales presentation,
but may not work so well in practice.

The EMS driver is relying on oncoming traffic to heed a red light that
suddenly appears with no yellow light, no warning whatsoever... just
"Blink !" and it's red.... and, in the case I witnessed, the EMS driver
is even proceeded through the light *before* it turned green for him.
--
Pete Cresswell