Automated or not, mass transit is just geometrically way more efficient, there's no way around it in dense urban environments.
Your argument might be valid for a mid-sized american city, or a suburb, but it doesn't work for a european city, or even a dense american downtown.
Actually diesel busses are only "efficient" when they are full. They're barely as efficient as single occupant cars just on fuel use when you factor in the bus running underutilized much of the day. That's not counting the cost of the driver, and assuming bus capex is efficient.
So even a commute-sharing system like that being tested in Waze would be more efficient than busses.
Buses take as much space as 3 cars. Even if you can manage to optimize everything perfectly to pack 4 people in these cars at all time, you only move 12 people using the same amount of road of a bus transporting 60 to 90 people, much more in articulated vehicles. There is just not enough room in dense cities.
You have to consider the overall efficiency of a bus network, not one particular trip. In urban centers where buses are run on frequency, not timetable, a transit line is attractive because it runs often. People take the bus in rush hours in part because they know they can depend on it all day. These seemingly low-efficiency trips where the bus is half empty are actually very important to the overall efficiency of the network, because they induce demand.
The case for transit is a space argument, not a fuel argument. Mass transit is simply the most space efficient way of moving people around.
Currently we have about the worst utilization of vehicles possible. Drivers drive alone, and park their cars for long intervals. This is why one can claim that trains move enough people to fill 20 car lanes. At the other end of the spectrum, diesel busses are noisy, smokey, need a professional driver, and go from underutilized to packed, depending on time of day.
A mix of automated vehicles will be able to much more closely approach optimal load factors at all times of day. 6 or 8 passengers can ride in a car-sized automated vehicle. That makes streets a lo-capex alternative to rail.