RTK [0] systems definitely can have centimetre-level accuracy, though they require a fixed base station within 10-15 miles or so to broadcast corrections. It would also require roads to be mapped to a high level of accuracy.

I have seen self-driving test cars in Silicon Valley (frequently, especially in the last year or so) using these types of systems, so they are at least being tested. I've also heard discussion of putting RTK base stations on cell-phone towers to provide wide area coverage, but I'm not sure if much effort has been put into that. I do know vast areas of the agricultural midwest are covered in RTK networks -- its used heavily for auto-steering control in agriculture.

[0] https://en.wikipedia.org/wiki/Real_Time_Kinematic