Study: “Roads Cause Traffic”

In a culture that is suffering the consequences of skewed transportation priorities—the US government spent 40 times as much on roads as it did on rail over the last four decades—it is little surprise that the most common way state transportation officials handle traffic congestion is to build new roads or lanes. But as anyone who commutes by car in most US metro areas knows, if you build new lanes, people will use them. Yet another academic study has quantified this phenomenon.

Two University of Toronto economists analyzed reams of traffic, infrastructure and travel behavior data from many US metropolitan regions and found that the total distance people in a given metro travel by car increases in proportion to the Interstate highway mileage in the region. In other words, as the authors conclude, “roads cause traffic.”  This is because, largely in the absence of viable alternative methods of getting around, people tend to gravitate towards living and working in areas with more highways.

This phenomenon was first explained by Lewis Mumford in the mid-1950s, and again by Anthony Downs in 1962. Downs showed that peak-hour congestion on urban expressways always rises to meet the road’s maximum capacity. Mumford summarized the reasons behind this thusly:

All the current plans for dealing with congestion are based on the assumption that it is a matter of highway engineering, not of comprehensive city and regional planning, and that the private motorcar has priority over every other means of transportation, no matter how expensive it is in comparison with public transportation, or how devastating its by-products.

The challenge, as Mumford discovered, is that the public is often more averse to the consequences of the policies necessary to truly curb congestion than it is to having to sit in traffic. When the total effects of such policies are described, many may see it as social engineering. But when improvements are proposed piece by piece, they are a lot more palatable, even desirable. One of these is additional train service.

The Toronto economists were careful to point out that adding transit options has no effect on highway congestion: so long as highway capacity is plentiful and free, there will always be new drivers to take the place of those who switch to transit. This doesn’t mean, though, that more rail routes and frequencies shouldn’t be provided, it just means that more transit won’t reduce congestion unless it is coupled with policies that make it costlier or harder to drive.

Revenue-raising policies such as congestion pricing not only put a price on highway use, forcing drivers to treat their travel choices more like they do their shopping choices, but they also raise funds to pay for rail systems and other safer, healthier travel options—which will be necessary to accommodate the 130 additional Americans expected by 2050. You don’t have to change your worldview or give up your car to realize that we can do better than to spend the equivalent of several days every year stuck in traffic (as the average American does), and that we cannot accommodate our growing population with more highways alone.

Greater public support for forward-thinking policies—and growing train and transit ridership—show that the people get it. Too bad so many of our elected officials are behind the curve.