Comment by toast0

When it started, from what I've heard, the design goal was for part-time self-driving. In that case, let the human driver do the more variable things on surface streets and the computer do the consistent things on highways and prompt the user to pay attention 5 miles before the exit. They found that the model of part time automation wasn't feasible, because humans couldn't consistently take control in the timeframea needed.

So then they pivoted to full time automation with a safe stop for exceptions. That's not useful to start with highway driving. There are some freeway routed mass transit lines, but for the most part people don't want to be picked up and dropped off at the freeway. In many parts of freeways, there's not a good place to stop and wait for assistance, and automated driving will need more assistance than normal driving. So it made a lot f sense to reduce scope to surface street driving.