For many cities road congestion is a continual issue, but there are ways to mitigate this. One of the biggest sources of congestion can be poorly laid out road networks. In particular, opposing "T" intersections can cause tremendous problems. When laying out roads, it is important to avoid off set "T" intersections as much as possible even if this means losing a couple tiles to roads so that you can bring two opposing "T" intersections together.
Think of it this way, every intersection you have means another set of traffic lights to slow things down. If you merge two opposing "T" intersections you eliminate a set of traffic lights. Another problem with opposing "T" intersections is that it creates a small strip of roadway, which must handle the traffic load for both North/South and East/West traffic. This means that you have effectively cut the traffic capacity of that area in half. A good example of this is an older real life city where roads just kind of happened. If you've ever lived in one of these cities you will know that it can take two or three times as long to get through a pair of opposing "T" intersections than through a single cross intersection, in part because the traffic light timing becomes very complicated.
I've attached a screen capture showing a bad opposing "T" intersection with bus stop and subway station and the best way to bring these two intersections together. Notice that the bus stop stays at final resulting "+" intersection. This is because bus stops are more effective when they are at intersections.