Is that even a requirement? Can't you just sync every 5 minutes or so? Or sync more often when the battery is full, and sync less often when the battery is starting to deplete - given enough users that should take care of the power issue. Because it doesn't take a lot of energy to download the congestion information, so like with Bittorrent you only need a couple of seeders.
Bear in mind that you need only one data point to prove there is no congestion: a single car driving at 70 miles/hr rules out congestion at that road.
I expect it will work if all taxis are seeders, and all commuters are leeching. Considering taxis benefit a lot from such a service it's not unrealistic.
The auto-locking problem is another technical nuisance. A firmware update would fix that.
Also, I think the problem should be solved entirely without GPS. You don't want a machine to tell you that traffic is OK, what you really want is to receive a text message whenever there are traffic problems on your usual route on weekdays. "Traffic jam at intersection X" is all you need. When you get that message you just have to take a minute to figure out alternative route. The passive approach is much more appealing. After all, if traffic has been good all week you're not going to check your iPhone the next morning. Just not worth the trouble.