Before we talk about 21st Century technology...

...we need to spend some time thinking about the 20th Century technology that got us here.

A controlled-access highway provides an unhindered flow of traffic, with no traffic signals, intersections or property access. They are free of any at-grade crossings with other roads, railways, or pedestrian paths, which are instead carried by overpasses and underpasses across the highway. Entrances and exits to the highway are provided at interchanges by slip roads (ramps), which allow for speed changes between the highway and arterial roads and collector roads. On the controlled-access highway, opposing directions of travel are generally separated by a median strip or central reservation containing a traffic barrier or grass. Elimination of the sources of potential conflicts with other directions of travelers dramatically improves safety, fuel consumption, and travel times.

It turns out that many of the innovations mid-20th Century engineers came up with to maximize throughput (controlled access, divided lanes, etc.) also tend to make the jobs of today's engineers much easier when it comes to autonomous vehicles. This relationship between technologies will be a big factor when we get back to this.