There's reason to think they would be. When people get into a shared public transit vehicle, usually, they're not all going to the same place. The vehicle has to take an indirect route, it has to start and stop many times along the way to let people on and off. A person will often have to transfer between multiple lines along the way and that will often involve a bit of waiting. There are usually many routes that have very low utilisation rates- carriages will be mostly empty most of the time, in part due to the fact that you can't deploy half of a bus for low-use routes, in part due to the rigidity of the route scheduling.
Small self-driving vehicles, instead, can take a person straight where they need to go. The vehicle can then pick up someone else nearby and take them straight where they need to go, and so on, all day. They'll also benefit from economy of scale of production, if more units are produced, the cheaper they individually become, smaller units, then, can be cheaper.
That's the vision I have in my head, anyway.
The main potential issue that I can see is commuting patterns. If a lot of people are going in the same direction at the same time, batching their trips together with larger densely packed shared vehicles might make sense.
But I'm not sure that's a legitimate way for cities to be. With affordable inner-city housing, mixed-use planning, this commuting pattern wouldn't pop up - people would be taking short trips in all different directions even during peak commuting times.
(Although I cannot personally understand why) A lot of people seem to like living in suburbs, and conventionally planned cities have land pricing issues that make living in them prohibitively expensive for most ordinary people who work in them (I will discuss a potential solution to this in a later post). These ideal mixed use cities that I am mentally situated in, might not exist, and might not come to exist before autonomous vehicles start to compete with existing public transit.
So I'd like to know what relationship suburb size has to the commuting pattern problem - how large can the suburbs get before fixed-route busses become more efficient? (and also, are they really better, even with large suburbs? Is there that much route agreement in any city?)
Visions of self-driving cars currently look like variants of the cars we have, improved for not needing to have a driver's seat and forward visibility, often you'll see 8-seaters that're expected to be shared to some extent. I think that might be unrealistic, if it is, we need to see more analysis of single-occupant cars like the toyota i-road. Aside from using fewer materials and using less energy during acceleration (they're lighter), they'd also have the advantage of being able to to drive two to a lane, reducing congestion.
If we can answer these questions well, I think we will be much better informed about the future of urban transit than we are now.
: although, regenerative braking is a thing. I'm not sure there's really an upper limit on how efficient that can be made, so maybe weight isn't so much of an issue.