Surely a person driving 18 people will emit less than each person driving themselves? I can see how dollars can be a proxy for macroeconomic stuff, for example by comparing countries, but it doesn't make much sense for comparing individual units of transport.

I don't understand why we have to do napkin maths to guess what's more polluting. There is countless research showing buses emit less than individual cars per passenger per km, self-driving or not.

And there's no reason buses can't be self-driving as well, if anything, them driving slower, on dedicated lanes and on fixed routes would make it much easier.