[npunt]

You're right, not the only thing, but I do think it is by far the primary.

Efficiency-wise a large diesel vis-a-vis traditional car yes the bus wins by a lot, but with smaller multi-person autonomous electric busses I'm not sure if there are any meaningful efficiency gains.

Traffic-wise it may actually be better to have smaller autonomous busses than hulking road giants, because they're not stopping as frequently and they're more nimble. I swear every fifth time I'm around a bus it's stuck idling waiting for a bicyclist or someone to get out of the way, because it's too big to maneuver. Separated bus lanes help, but that isn't a property of the bus as much as urban planning.