I'm imagining buses that go down major arteries like Geary. They'd come often and there would be a lot of them and they'd have dedicated lanes. Seems so much more flexible than fixed infrastructure.
Public transit doesn't need to be flexible though. Individuals might change their routines every day but when you average the traffic for the whole city the throughtput of the major arteries is very predictable. The biggest priority in these situations is making the transport be cheap and efficient and make it scale to demand.
And just because a mode of transportation is on wheels doesn't mean it flexible. If you look at a Bus Repid Transit system[1] you would notice that the buses are specialized for driving in separate lanes that go in a straight line and the bus stops are custom built (and are less frequent than a regular bus).
