There is a limit to what buses can do as city size increases – linear versus square

Better bus services and well-enforced bus lanes would be good, but. The number of residents. job opportunities, shops, services etc depends on the area of the city, that is, the square of its size, whereas the number of transport corridors entering the city depends linearly on the size of the city. Therefore, the larger the city, the greater capacity transport system is required. For a large city, buses cannot cope; you need a high capacity, rail-based mode (train or light rail).

Everyone knows this really. London, Manchester or Birmingham could not function without rail – see what happens when there is a strike. I would say that Bristol is about on the edge of what buses can cope with.
David W