And there's a commercial off-the-shelf solution:

https://products.currentbyge.com/transportation-lighting/led...

(which only uses 30W for the heater, so it's still far more efficient than the old-style ~150W incandescent.

If the Minnesota DOT wasn't aware that this could be a problem, they are incompetent.

It's even a problem with little used incandescent lights... on a street where the light is nearly always green, the red light can still ice/snow over and not have time to melt during brief red intervals in cold weather.