Re: Light Spectrum and some lateral thinking
The Sun is only really a problem when it is directly behind the traffic light. It outputs on all frequencies from long radio waves to X rays so just using something outside of the visible range doesn't solve anything. Fortunately the Sun is only behind the traffic light from certain direction/distance combinations, so as long as the hidden signal was strong enough to be visible from a couple hundred feet away you don't have to worry about the Sun because you'll have some views of the signal without the Sun overwhelming it. When it is very low in the horizon it might not be visible to the car until you get close so maybe the car will need to slow when approaching those intersections near both dawn and dusk but that's not a big deal.
Whatever frequency you use it should be 1) able to travel well through fog/rain/snow at least 50 feet or so and 2) directional enough that the car can tell it is coming from the traffic light (to avoid bad guys trying to fake signals and cause trouble - you could encrypt it with a private key, but with millions of traffic lights it would only be a matter of time before it became known)
Biting the hand that feeds IT
