[gsnedders]

You need 187 buses per hour to have the same capacity as the Piccadilly Line has on-peak (using the capacity of the New Routemaster), or, alternatively, three buses per minute. It's hard to imagine any way in which that is practically workable. I think the most frequent bus service in London currently is scheduled for 30 buses per hour, by way of comparison.

Add to this the fact that average road speed in Central London is about a third of average Underground speed, hence you're quite possibly looking at making journeys three times as long, even ignoring the extra congestion that all those buses would cause.