In the skies, you have complete three dimensional freedom, there are no hard objects apart from other planes and there’s someone (or something) controlling all of the planes and watching they don’t even come close to each other.
Roads are much more complicated. I’m unsure if it will be possible on winding B roads without very advanced AI.
Although, networking might save it. If you had all the autonomous cars wired to a central system, then they could share knowledge of specific roads and risk situations. For example, the system could start off on motorways where the problem is fairly straightforward, then revert to driver control on A and B roads. However, the system would still be active and would learn what drivers do and how they react to hazards and so on. Over the years it would be fine tuned to find the safest possible way to drive every single road in the country. Think about it – it would know where every dodgy junction or concelealed exit in the country is. It would ALWAYS know the road as if it’d driven it a thousand times.
Then, if you had enough uptake, the system would know where all the other automatic cars were, and if there were only automatic cars on a stretch of motorway it could drive appropriately, taking advantage of the fact that ALL automatic cars would be gathering data about the road network wherever they were. So it’d instantly know if there was a problem or risk at any point in the road network. It would also know where all the non-automatic cars were too because it would register a car visually and know it didn’t correspond to one in its system.