At some point people might stop bothering with car ownership. That would open up a number of interesting possibilities. Imagine removing all parking on some (not all) streets during the day, thus doubling or tripling their bandwidth, and also increasing the number of cars that can be operated at the same time. I'm not saying that's the answer, but that there are lots of things that could be tried here.

I'm hoping that once self driving cars become common, we can remove parking spaces and replace them with protected bike lanes. A while back I bought an electric bike to ride around Boston, but I quickly returned it because I didn't want to end up as a splatter on the side of the road. Self-driving cars can't come soon enough.