In the paper, The Fundamental Law of Road Congestion: Evidence from US cities, Turner and Duranton decipher this rule:
New roads will create new drivers, resulting in the intensity of traffic staying the same.
Basically adding more roads or more lanes usually does not improve the state of traffic into a city. By making it easy to reach the city, we just increase the capacity to clutter more the city.
Why? Because we can. That's the sad part of it.