As often as the asphalt topping.

Uh-oh. In other words, there's a smart road ahead, you can 100% trust it. Except when it's not there, because it hasn't been maintained for decades, and it's unclear where the missing pieces are. Can't see what could possibly go wrong.

Plus there's also no incentive for anyone to waylay the smart road off the physical road, because there are no evil people (i.e. with motives that are incompatible with the vehicle's survival).

It's a hint, like lane lines. You're going to be using GPS and LIDAR as well. Yes, some of the lamer self-driving systems put too much faith in lane lines.

Indeed. Which means that we're back to square one, "it's where most people are, which is exactly the spot where it's least needed; whereas in places where more conventional navigation fails, there's minimal incentive to also build and maintain this."

TL;DR: good roads will be getting better, bad roads won't.