There are some nice solar studs around, saw them at the safety place Friday, and they are pretty nice, but never going to be a power source, just having enough power capture and storage to light some ultra high efficiency LED's during the night, and there they are fine.
There are others that are completely embedded in the road surface, with only a thick profiled glass top with integrated optical path visible, so that you have a white ( or orange for the median markers) light in one direction, with the other direction having a red light visible to the vehicles. They show low battery ( enough installed so that you can see those that are ageing) by changing to a flashing mode, though i would guess they do that in any case as night progresses, as I never travel this road after 11PM.