Decades of traffic data across the United States shows that adding new road capacity doesn't actually improve congestion. The latest example of this is the widening of Los Angeles' I-405 freeway, which was completed last May after five years of construction and a cost of over $1 billion. "The data shows that traffic is moving slightly slower now on 405 than before the widening," says Matthew Turner, a Brown University economist.The article explains in detail the various studies that have reached the same conclusions. (And, no, sorry, public transportation is no panacea.)
The main reason, Turner has found, is simple — adding road capacity spurs people to drive more miles, either by taking more trips by car or taking longer trips than they otherwise would have. He and University of Pennsylvania economist Gilles Duranton call this the "fundamental rule" of road congestion: adding road capacity just increases the total number of miles traveled by all vehicles.
This is because, for the most part, drivers aren't charged for using roads. So it's not surprising that a valuable resource, given away for free, leads people to use more of it. Economists see this phenomenon in a lot of places, and call it induced demand.
So why does traffic increase when new road capacity is added? Turner and Duranton attribute about half of the effect to people's driving decisions. "Think of it as if you made a bunch of hamburgers and then gave them all away," Turner says. "If you make hamburgers free, people will eat more of them."Another analogy: Make it harder to drive to Cabot and Benton and maybe people will live in Little Rock. But don't tell that to Cabot Mayor (Hon.) Lance Hines, who nominally represents Little Rock on the City Board but seems determined to place commuters' needs first.
By way of illustration, consider the following situation: there's a store where you know you can save $10 on something you need to buy, but it's 10 miles away. If you assume there will be terrible traffic and it'll take 30 minutes to get there, you'll just buy the product at a closer store. However, if a new lane gets added to a highway that will speed your journey there, you'll decide it's worth it.