About 12 years ago I developed a concept (on paper/CAD) regarding a solar powered highway sign inc. imbedded LED lettering and heat (via copper heating coil) to provide a highly visual sign for night/inclement weather conditions where snow/ice would not build up on the sign to impair drivers and cause poor decisions when approaching an exit ramp. Insurance claims for accidents due to visually impaired driving are incredibly high in areas subject to winter conditions. This product would be sold to municipalities (state/federal gov't agencies) for use along highways where these conditions are present - and should help reduce insurance claims and injury/death (cost justification).
Here are a few active programs with solar/led signs...
As you can see, I'm a bit late when it comes to incorporating LED into standard highway sign lettering - as this was done as soon as LED mfg. cost came down - but there's still nothing available about incorporating a heating element into the sign underneath the lettering to reduce snow/ice build up during storms (won't matter if the lettering has LED's if it's snow packed).
The power source would be solar cells encapsulated in a mountable (top of sign or on side of overpass/pole (as required) module. Solar cells/power pack would be contained within clear unbreakable plastic in order for maximum sunlight exposure and protection.
I foresee this as a possible kit that can be sold to include the 1) solar module, 2) LED lighting, 3) and heating element strips - as I believe most DOT's build their own signage currently.