The capacity of mass transit modes is still worthwhile even in a fully automated universe, once you get dense enough. It's a simple calculation of space usage:

The capacity of mass transit modes is still worthwhile even in a fully automated universe, once you get dense enough. It's a simple calculation of space usage: If you fit more people in a vehicle, you use less road. Depending on mode and scale, you may save energy too. Now, you could postulate that we'll go smaller with AVs, or we'll develop ways to reconfigure vehicles on demand to synthesize a larger vehicle. But those things are not as clearly on the horizon as an immediate post-AV situation where traffic patterns remain roughly the same(ease of trips encourages more trips per day, cancelling efficiencies), but per-vehicle usage rates are up(due to better sharing). In that situation, we get better safety and convenience, and future zoning will require less parking...i.e. you get a global spike in density, which motivates better transit.