[TulliusCicero]

> If everyone can share a small fleet of self driving vehicles

They cannot.

> I don’t see how self driving cars would take up significantly more space than mass transit while on the road.

They would take up more space because they're cars. Cars can't get to similar efficiencies of space compared to buses unless you compared packed private cars with sparsely ridden buses. And then of course, there's trains.

Perhaps someday we'll have all self driving cars that can safely coordinate and pack themselves into smaller spaces on the road, but that's decades away at a minimum, since you'd first need to get rid of all manually driven cars.

[xnx]

Trains are a very inefficient use of space. Trains often run nearly empty, and the tracks can only be used by trains. A road (utilized by autonomous vehicles) can be used at any time and any schedule by any vehicle, including the possibility of giving automatic intersection right of way to pedestrians, or safely sharing road space with personal transport (bikes, scooters, etc.)