Essentially, Silver uses math to prove that we are driving less than one would expect when taking into account low gas prices and the crappy economy.
For fifteen consecutive months, the average miles per American has dropped. Some of this can be attributed to high gas prices in summer 2008 (remember those days of $4+ a gallon?!), and the current high unemployment rate (don't need to drive to work if you don't have a job).
When Silver put together his model, taking into account fewer jobs and less expensive gas, he found that Americans were driving even less than the model predicted - potentially meaning that culture is shifting away from cars. Fascinating stuff - definitely check out the article, and Silver's website.