Opinion: level 5 autonomy would be cool, but it's not where most of the value of self-driving cars is. Most of the value in being able to travel by auto without attention is in trips longer than 15 minutes. Commutes, tourism, vacations. Shipping. Most of which is certainly highway driving. If it's possible to automate highway driving under most conditions (and safely transition either to a stop or human-piloting when…
Onboarding cars onto trains in a driver friendly way should also be considered. This way you don't exclude non self driving cars, and still have good coverage (taking into account that railway systems aren't far behind highway coverage in many countries).
But one needs to plan and book a travel in advance. There are also capacity limits.