At a technical level, I think smart roads are more straightforward. However, I think a purely technical analysis ignores why self driving cars have been so popular.
The cost of replacing the entire US road system with "smart" roads would doubtless be astronomical - as well as increase the maintenance and infrastructural requirements. We've probably had the technical capabilities to create self-driving-capable roads for years, but the costs are probably too high for people to take it seriously "at scale."
Machines that do things "as well" as humans in similar roles have been a science fiction staple for over a century. That machines do focused tasks much, much better than humans haven't stopped people from searching for "drop in" replacements for tasks that currently use a human. Suddenly, the scale is on the level of a single vehicle - very manageable if it works. That's why people were so willing to spend billions of dollars to avoid spending trillions on all that road replacement - because they wanted the worlds they saw in science fiction.
You don't have to replace the entire road system. A hybrid model can also exist. Complicated city streets can be "smart", while self driving tech can be good enough to traverse long stretches of highways (which it can basically already do).
The cost of replacing the entire US road system with "smart" roads would doubtless be astronomical - as well as increase the maintenance and infrastructural requirements. We've probably had the technical capabilities to create self-driving-capable roads for years, but the costs are probably too high for people to take it seriously "at scale."
Machines that do things "as well" as humans in similar roles have been a science fiction staple for over a century. That machines do focused tasks much, much better than humans haven't stopped people from searching for "drop in" replacements for tasks that currently use a human. Suddenly, the scale is on the level of a single vehicle - very manageable if it works. That's why people were so willing to spend billions of dollars to avoid spending trillions on all that road replacement - because they wanted the worlds they saw in science fiction.