2 ideas and then 1 idea:
1. GPS devices need more information about real-time traffic information. The best source for this is other GPSes that are also stuck in traffic. If they could communicate, they could intelligently route traffic to optimally spread traffic over primary and alternate routes. There are products that do this. iPhone and Android would also be excellent candidates.
2. To share this information, they need to be networked. They can form an ad-hoc wifi network when near one another and nodes with internet connections like internet phones and nodes near municipal wifi (which could be embedded into traffic lights?) can act as up/down links to a central database of traffic information.
3. In lieu of available real-time information, devices could sync with the central database by docking. A Bluetooth-enabled GPS could send its daily traffic recordings to your phone, which syncs itself to your computer when you carry it inside to home or work, which communicates this information to the internet. Pattern recognition is applied, and then traffic patterns are downloaded via the same channels to your GPS. It can use the traffic patterns to avoid areas of likely congestion during your route. (And you could cut out the middleman if your phone is your GPS.)