We've seen some silly traffic congestion problems in SimCity even when an alternate path is presented for the drivers. Now, it looks the issue's being rectified as here a video from Maxis showing off their new traffic code.
As you can see, drivers will now take an alternate path should a road become heavily congested. It does seem like it takes a little longer than it should to free up the traffic, but this is being tested internally so there's room for improvement. It's definitely a lot better than it was before.
Next fix I would love to see is the people AI. The traffic and the fix of the AI should help the game immensely.