One could argue every passenger on a bus saves some fraction of a car trip. Even if the total number of car trips saved is a much smaller number than the total number of cars, there's a 'breakeven' point above which the pain of the buses is less than the pain of the bus passengers in cars. I bet in most cities, we've hit that breakeven point, regardless of bus behavior.

<wild speculation> I bet that number is about 6 passengers per bus, excluding environmental concerns. </wild speculation>