Buses really need to be grade separated from traffic, but most cities are reluctant to remove a lane out of a road to make it possible. I believe rio de janero has a huge BRT (bus rapid transit) network. Grade separated, and they are pretty much a subway with rubber wheels and it's waaay cheaper to build even an elevated roadway vs. a tunnel with rails that is more constrained with how sharp you can turn and what elevation changes your track needs to avoid. In Los Angeles, BRT busses are twice as long as well, all the more riders in one go.