Reducing Traffic or Inducing It?

When we build more roads or widen existing roads, drivers and traffic will come. This phenomenon is known as induced demand.

From a guest post by Wes Marshall for the Transportationist

We’ve been discussing induced demand at Strong Towns for quite some time. Including a recent piece that gained a lot of traction where Chuck flipped the notion of congestion on its head (you can also listen to a recent podcast elaborating on this topic).

A persistent bunch, including former Treasury Secretary Larry Summers (aka the Grand Deacon of the Infrastructure Cult) still think that we can build ourselves out of congestion with more highway spending. They conveniently ignore induced demand.

However, induced demand is getting increasingly hard to ignore as the research piles up. While sifting through our Member Blogroll recently, I came upon a post from Strong Towns member Dave Alden’s blog that does a good job highlighting some of the research. Specifically, he points to Stuart Dee, a veteran of the Los Angeles freeways who reviews the current research on induced demand. Dee’s investigation finds that traffic increases as new roads and capacity are added. Writing in Wired, Dee summarizes the conundrum:

“… we humans love moving around. And if you expand people’s ability to travel, they will do it more, living farther away from where they work and therefore being forced to drive into town.  Making driving easier also means that people take more trips in the car than they otherwise would.”

With a growing body of research showing that building more roads and more lanes just encourages more traffic, officials are starting to pay attention to induced demand. The California DOT (aka Caltrans) recently acknowledged induced demand by linking to a policy brief titled “Increasing Highway Capacity Unlikely to Relieve Traffic Congestion.” The brief provides an overview of research findings and was compiled by UC-Davis scholar Susan Handy. Here are the highlights as reported by City Lab:

- There’s high-quality evidence for induced demand. All the studies reviewed by Handy used time-series data, “sophisticated econometric techniques,” and controlled for outside variables such as population growth and transit service.
- More roads means more traffic in both the short- and long-term. Adding 10 percent more road capacity leads to 3-6 percent more vehicle miles in the near term and 6-10 percent more over many years.
- Much of the traffic is brand new. Some of the cars on a new highway lane have simply relocated from a slower alternative route. But many are entirely new. They reflect leisure trips that often go unmade in bad traffic, or drivers who once used transit or carpooled, or shifting development patterns, and so on. Read more

The increased awareness of induced demand is a very encouraging sign. This growing acknowledgement is a key component in changing the conversation. Once we admit that building more highway capacity isn’t a real solution to congestion, we can talk about things that can actually work such as congestion pricing or building strong neighborhoods and cities that do not require such a heavy reliance on congested freeways in the first place.

Cover photo credit: Wikipedia