We need vehicles that are big enough to carry a decent number of people, so that during rush hour the available road-space can be effectively utilised: After all, we want to reduce the overall cost and blight of roads for a given peak level of demand, which means maximising the number of passengers per square metre of tarmac. On the other hand, away from rush hour, we want vehicles that are small enough to flexibly be able to tailor their route to the demands and needs of their passengers - so that individuals can be picked up and dropped off at their front doorstep - and at a time that suits them, rather than according to an inflexible schedule.

These conflicting requirements call for a mix of vehicle sizes - both small and large. If you take the cost of a driver away, then I think the minibus might well hit a sweet spot in terms of size and flexibility - although that is little more than a hunch at the moment.