I have solar lights in my back yard and I can assure you the sun does not shine all day every day as the lights are sometimes unlit by early in the evening.

Add the dirt & grime of a road to your solar collectors and they likely won't ever get a full charge.

Expense & maintenance are the other things that would prevent traffic engineers from liking this idea.

I've seen reflectors used in warmer states, but here in Kansas, we snow-plow the roads in winter. Anything that would stick-up would be cut off. Anything that would be below the surface would be subject to filling up with water, then being broken when it froze and the ice expanded.