When I first saw the traffic overlay on Google Maps, I was struck by the simple brilliance of the idea. As long as you are running the Google Maps app on your phone, you are contributing information regarding your position and speed to the map server. The server can collate this information and annotate it back to the map to show where the traffic hotspots are. Presumably, the server can also learn how traffic patterns develop over time. This would give you more accurate routing that gets you to your destination as quickly as possible.
I once had an idea for a business that would lease space on billboards, overpasses, and other places along the road. A system of speed measuring radars coupled with cellphones would track and report traffic conditions to a central server, where subscribers would have access to the information. I thought at the time that it was a pretty decent idea, but I was obviously outdone by the distributed data collection employed by Google.
I've written before about the possibilities of crowdsourcing to solve complicated problems, and this is another great example. If even a small percentage of drivers contributed their driving information, it would be far more data than even the most comprehensive system of fixed radar units could provide. Unfortunately, in practice, it seems to fall short. More than once while stopped on the freeway, I've been able to pull out the phone, only to see a pretty green line decorating my route.
Plus, if you can crowdsource speed information, why not other information, such as speed traps, accidents, and other hazards?
This question came to me as I was driving recently from San Jose to San Francisco. I ran across a large number of speed traps along a section of Interstate 280 that usually doesn't have any. At one time, that would have provided food for thought for the rest of the trip, but now I usually think, "I bet there's an app for that." I pulled into the nearest gas station, and while I waited for my tank to fill up, I took a look and found the smartphone app Waze.