Posting this here instead of hi-jacking the headlight thread
We tried LED tail lights on the trucks here, but the lights kept getting covered in snow when driving through snowstorms, and the drivers would get ticketed by the cops as soon as they got back into clear weather, so we had to switch back to incandescent tail lamps
The lights aren't getting snow that falls ON them, they are getting snow sucked onto them from the vortex created by the squared off rear of the trailer. The trailer can be totally impaced with snow in under an hour during a snowstorm, and there's not always somewhere safe that you can pull over and wipe the snow off (Think of two lane highways for example).
There was a lengthy discussion about the problem with one of the LED tail light manufacturers (I think it was Truck-lite) on a call-in show a few years back. Their rep thought it was snow falling on the light rather than being sucked onto the light and recommended flush mounting the lights rather than having them recessed by an inch. They have to be recessed by an inch so they dont get smashed off at the loading docks - but that wouldn't have fixed the problem anyway, since the problem is snow being blown onto the lights, not building up from the ledge below.
one of the drivers suggested the heater as well, but the light manufacturer said that if the trailer was designed for LED lights, the whole point was to reduce the weight and cost of the trailer by using lighter gauge copper wiring, and adding a heater to the circuit would require that all of the wiring be changed to the heavier gauge stuff.
School buses in this area get to use LED lights because they aren't as tall - they have what looks like a spoiler on top, and it's sole purpose in life is to blow air down the back of the bus to keep snow off the lights. That would be the solution for a truck trailer too, but we can't put a wing on top cause they would get knocked off by older bridges that have no extra clearance. Not sure if they get any other aero benefits from their spoilers, but the main purpose is to blow snow off the LED tail and warning lights.
In regional driving in more southern areas that dont get quite as much snow as northern Wisconsin, LED lights are very nice - they allow making the trailer lighter by using less copper in the wiring, the alternator can be lighter, the engine doesn't have to work as hard for lights, parking lights can be left on longer without running the main engine.