Where I live, traffic is highly variable. If I leave 5 minutes later one morning, there might be no traffic at all, but if I left another 3 minutes after that, I would be stuck in heavy traffic. If there was some way we could track the rate of traffic over time (say, plot out the mph between 7am-8am) every morning, I bet after a month we could figure out the ideal times to leave for work.
Google maps already uses crowdsourcing for its traffic data, so maybe that's the way to go. I would need a large sample size though. I researched a few cities and Houston, TX has sensors that use the toll RFIDs to determine traffic conditions. There are tons of other applications for this data, too. Think of shipment or delivery companies (UPS...), among others.