Reminds me of the story of what happened when traffic lights got "upgraded" to LED to save on energy. First winter all traffic lights got completely covered in snow and traffic ground to a halt. The "waste" heat as was not such a complete waste after all, it was continuously melting snow off the lights.
So, just add heating element when you need it? It's probably more efficient to use LED lights than to have incandescent light bulb for the few days of the year when you need it.
If you need to keep the heat with the wastewater, don't harvest it unless there's excess heat to be harvested.
As an amateur electronics engineer... I think we can do better. I have not checked what is the actual solution, here is what I figured out on my own:
First of all, the yellow light bulb might not be enough to reach with the heat to the red and green lights. And since it is only turned on for a brief moment, might not be providing enough heat. And will still be wasting heat when it is not needed.
Even heating it up when it is cold is a waste of time. I can only guess at the number of nines, but 99.9% of the time you can get away without heating even when it is very cold, because the conditions are not right for the snow to accumulate (read -- it is not snowing, the temperature is not close to zero degrees celcius and the wind is not blowing in the right direction).
The only time the heat is really needed is when the lights are obscured with snow.
So here is a better design: have a photo cell inside the light to detect if the light is reflected back to the photo cell. The cell is isolated from the rest of the inside but has a clear access to a small portion of the glass in front.
When snow starts accumulating on the glass, the traffic light will start getting scattered in all directions including getting reflected back inside the traffic light and into the photo cell.
You can easily discern between light from the outside from light from the traffic light by seeing if it follows the pattern. Ie. constant light from outside regardless of the light turning on or off means it could be something like a sunset shining into the light and we need not worry about it.
What we are looking for is a clear difference in light shone into the cell when the traffic light is turned off vs when it is turned on.
If I can do that there is probably other, smarter people who can figure out even better solutions.
So, just add heating element when you need it? It's probably more efficient to use LED lights than to have incandescent light bulb for the few days of the year when you need it.
If you need to keep the heat with the wastewater, don't harvest it unless there's excess heat to be harvested.