Congestion in a city, especially from a AI's buses getting stuck, has become a major problem for your buses.
Trams provide a dedicated right-of-way that is open even when a street is blocked be stuck buses.
One way to control bus operation costs is to set a minimum load percentage at one of two of th stops on a route. This prevents the bus from just speeding around the route with very light loads.
However, is a minimum load percentage is set without a maximum wait time being set, as the AI plays do, the bus can block the station for a long time. This backs up all the other buses using the same station in the same direction. The AI players often have this issue.
So, if the AI player jams a street your buses also get stuck. The solution is to move to a tram line to service the problem area. The trams maintain a clear right-of-way no matter how congested or stuck the surface traffic becomes.