Well, look at cat’s eyes (sometimes called Botts’ dots in the US, I think?)

I assume they’re more expensive to install than just painting a few lines, but they’re very robust and long-lasting, and they’re fantastic for human drivers. It’s not a stretch to imagine something similarly useful for computerized cars, that links to a standard road database.

Hacking is a risk, sure. I envisage you’d lock it down by having a cryptographically signed master map; if the observed beacons diverge from the map, the autopilot system would refuse to proceed. (OK, I guess that allows a DoS attack at least.)