I recall seeing that option discussed on The Daily Planet with Jay Ingram back in the early 00s. They interviewed a research engineer on self-driving vehicles,

I recall seeing that option discussed on The Daily Planet with Jay Ingram back in the early 00s. They interviewed a research engineer on self-driving vehicles, probably in the context of the 2004 DARPA Grand Challenge. He'd clearly been working in the space for years and was very knowledgeable. IIRC, they had some very successful prototype road systems back in the 90s (or maybe earlier) that were based on implanting magnetic markers into the road at regular intervals. The big problem was cost. There's miles and miles of roads and the cost to upgrade and maintain enough miles was prohibitive. With that said, perhaps now that the cars themselves are 90% of the way there, the upgrades to the roadways could be more limited. I wonder if adding machine-readable markers to particularly important or problematic areas might be reasonable at this point.