The reason is that buses - both inner-city buses and coaches - often have to operate in narrow streets in city centres during the rush hours, when many other vehicles, motorbikes, bikes, and pedestrians are on the road.
Grunden er, at busserne, såvel bybusserne som turistbusserne, ofte kører i de smalle gader i bymidterne i myldretiden, når der også færdes mange biler, motorcykler, cykler og fodgængere på gaden.