More lanes means a temporary decrease in congestion, which incentivizes people to choose automobiles as their primary form of transportation, which fills up the roads again. Repeat this cycle endlessly as roads are widened.
Basically widening roads doesn't decrease traffic in the long term. This is why LA and Houston and other car dominated places have wider roads than anywhere else and terrible traffic.
Google "Induced Demand" and you will find a ton of articles about this such as this one. http://www.citylab.com/commute/2015/11/californias-dot-admit...
The core issue is that automobile oriented infrastructure infrastructure isn't very efficient at moving people in the first place. If you're going to spend a lot of money to increase transportation capacity, extra car lanes is a poor choice.
