Traffic AI is horrid in this game, no 2 ways about that. However, 1930 you can upgrade roads with bus lanes, trams and bus’s use it. This pretty much solves any issue you should have with traffic. Though upgrading to that sucks the big one!! Very easy to miss just a little chunk of road. I also place a tram and bus station next to each other at every major stop.
Why would you not have an upgrade the whole network button? This is 2014 not 1989!!