You can, sort of, pretend that every bus takes 50 cars off the road, but that’s not going to work unless you can find 50 commuters who will all decide to take your new bus line… and the way they decide is that they check if the bus is actually going to save them time and money over just driving. This kind of modeling is incredibly computationally intensive, but it works even when you don’t have a closed-form formula for how bus lines impact traffic, or, in general, how individual agents’ behavior affects overall outcomes.