When we build more roads, we invite more cars.
If you've ever found yourself stuck in traffic in your metro area, you might want to print out the chart below, tape it to your wall, and use it for dart practice. It comes via a guest post at the Transportationist by Wes Marshall, and it explains so very much of your earthly woe.
The red line represents vehicle flow along a given road. Traffic steadily rises until someone decides the road needs to be widened. Then the original trend line (dotted red) gets replaced with an even greater travel forecast (dotted orange), as we'd expect by creating more road capacity. But the actual new level of travel developed by this widening (solid red) is even greater than the forecast predicted.
In other words, widening a road invites more cars onto it. That principle, known as "induced demand," is captured by the grey arrows showing the gap between a travel forecast and an actual travel outcome. Here's Marshall on the "triple convergence" of induced demand:
First, existing road users might change the time of day when they travel; instead of leaving at 5 AM to beat traffic, the newly widened road entices them to leave for work with everyone else. Second, those traveling a different route might switch and drive along the newly widened option. Third, those previously using other modes such as transit, walking, bicycling, or even carpooling may now decide to drive or drive alone instead.
The roots of this principle trace back to the fine work of Anthony Downs, who decades ago discovered a fundamental law of rush-hour expressway congestion. (Recent scholarship has expanded that law to include a "broad class of major urban roads" rather than just highways or expressways.) In 2004, Downs wrote there are four ways to address the problem, but that three of them—peak-hour tolls, greatly expand road capacity, and greatly expand transit capacity—are "politically infeasible or physically or financially impossible in the US."
That leaves the fourth: live with it. He writes:
Congestion is an essential mechanism for coping with excess demand for road space. We need it! Peak-hour congestion is the balancing mechanism that makes it possible for Americans to pursue goals they value, such as working while others do, living in low-density settlements, and having many choices of places to live and work.
That's not to say cities should just throw up their hands. By creating strong transit corridors, building dense housing near these areas, and charging a cost of driving that takes congestion into account, the situation can and will improve. New roads can help, too, but only for a while. Before you know it, traffic will be bad again, and local government will need new tax revenues to maintain the extra highway capacity that's started to crumble. Hey—watch where you're aiming that dart.