One thing I found out is that most problems are caused by cars trying to turn. They can only do so at entrances of depots or train-stations (DEVs confirmed this somewhere in the forum).
To help traffic, I usually build some extra road-depots scattered around the city. This seems to help a lot, although it’s merely a work-around rather then a solution.
I often get a jam at the train stations as a lot of buses and tram’s pass there. If a car tries to turn at the train-station’s entrance and the lane it wants to return into is blocked by a stopping bus/tram, it blocks the traffic on the other lane as well (this in turn could prevent to stopping bus to leave, so the car cannot complete its turn….).
To solve this, I noticed it helps if you don’t connect the train-station directly to a road, but place a small section of road between them. This way, the car can make a full turn in the “entrance and wait for the lane to free-up, without blocking the other lane.