Well, I would imagine their focus isn't so much automation so much as congestion. While automating X amount of cars will reduce congestion a little, a mass transit system (automated as it may be) would reduce congestion significantly.

So, unless google has been testing high capacity transit vehicles, 4-person max vehicles aren't what MV is excited about.

Take a look at the website for the study: https://mountainviewagtfeasibility.com/

At the bottom, there are a couple mock ups of different types of transportation considered. They include: Rubber-Tired Automated People Movers, Monorail, Cable-Propelled Automated People Mover, Maglev, Autonomous Transit Vehicles, and Personal Rapid Transit, etc...

Basically everything is on the table.

Given that Google has autonomous vehicles running on all the streets of Mt View, it seems like that would be the natural choice.