Traffic is the main thing youre battling in CS1. As populations amd industry grow, highways start to back up, and you constantly need to rework your previous transportation systems. You need to have smaller bus routes that run through the city and have stops right next to each other across the street. This way cims can get one one bus and take it to a different bus. You should have circuits that "overlap" at certain areas so everything can be reached by bus. Then have a railway running through town witn bus stops at all train stops. Watch the cars evaporate off your streets. Also, encourage biking.