I think the problem with that is that to be generally useful, autonomous vehicles need to operate in environments with plenty of entities that are not participa

I think the problem with that is that to be generally useful, autonomous vehicles need to operate in environments with plenty of entities that are not participating consistently in the standard: Human driven cars, pedestrians, bicyclists (about half of whom dart around unpredictably, ignoring other vehicles and traffic control signals), motorcycles (like bikes but faster), the weather, etc. So, yes, much cheaper and easier to implement, but also not very useful because they won't be able to drive very many places people want to go.