Tyler Duvall, the recently departed USDOT acting undersecretary of policy, boils down traffic congestion in a National Review column today— here's the money quote:
The root cause of congestion is quite simple: Prices aren’t set to balance supply and demand. Imagine if consumers paid the government a weekly food tax that permitted them to take all the food they wanted from any grocery store. This policy would create massive food shortages. Yet, by and large, that’s how we pay for highways in America. As long as highway prices have little to do with highway costs, congestion will be inevitable.
Read on for his explanation of how variable (read: demand-based) road pricing offers a powerful solution.
If you're looking for a primer on transportation innovations along these lines, don't miss Adrian and Sam's recently released book, Mobility First: A New Vision for Transportation in a Globally Competitive 21st Century.