This is an important point that is commonly overlooked. Traffic is a sign of something. It’s a sign that the benefits of transportation outweighs the costs of

This is an important point that is commonly overlooked. Traffic is a sign of something. It’s a sign that the benefits of transportation outweighs the costs of transportation. Given this, high traffic often implies that there is pent up demand out there - more cars that would be on the road right now if only the roads could support it. When more roads are built, this unlocks some of the pent up demand, as the cost of transportation has effectively declined due to more available roads. However, this reduction in cost / hassle etc and the previously pent up demand may be so large that in encourages much more driving, such that in equilibrium the traffic could be nearly as bad as before.