Actually, if all we factor in is paint then if we would have autonomous vehicles for which 'paint' was critical there would be a higher frequency in the maintenance cycle using automated robot painters. I'm fairly sure if the highways facilitated 100% autonomous operation their maintenance could be automated thus driving down the costs.

But you are right, road maintenance is never cheap.

It would probably be easier to have a swarm like national airtraffic AI, all cars flying, all cargo in blimps than to maintain a complex ground network of "smart roads"