Until we have roads designed to fully participate in assisting self-driving cars, and cars that communicate with each other, we'll have serious issues that will make it unsafe, and thus impractical.
The cost to upgrade roads appears to be a significant hurdle. A google search says:
"There are approximately 4,071,000 miles (6,552,000 km) of roads in the United States, 2,678,000 miles (4,310,000 km) paved and 1,394,000 miles (2,243,000 km) unpaved."
And there's this:
>> einrealist 18 hours ago [-]
>> I am more fearful of trolls, tricking the technology.
That's even more difficult to address.
We have to evaluate the cost/benefit of implementing this once we get the tech to the point of near total awareness of real world conditions. It might make sense to implement it on major highways, but probably not on rural roads and neighborhood streets because it's not something we can skimp on.
To make it cost effective the roadway tech has to be for the most part "dumb". We cannot rely on "smart" tech that requires complex communication systems or dumb tech that's easy to hack, like lines painted on the road or stop signs and traffic lights that AR can "see".
I think we'd be better off working on implementing assistive safety technologies for automobiles and mass transportation infrastructure like high speed railways.