Comment by askvictor

> A bus only lane is also a vehicle lane that is sitting empty a good part of the time.

You can make the same argument (in terms of space) about a train track. The real advantage of trains (light or heavy) is permanence. It's easy for the next government to remove the bus lanes because "OMG too much traffic, one more lane will fix it." It's much more difficult to rip out a rail line and convert it to a road.