California's DOT Admits That More Roads Mean More Traffic Take it from Caltrans: If you build highways, drivers will come.

"...it’s refreshing—and rare—to see the California DOT (aka Caltrans) link to a policy brief outlining key research findings from years of study into induced demand. The brief, titled “Increasing Highway Capacity Unlikely to Relieve Traffic Congestion,” was compiled by UC-Davis scholar Susan Handy. Here are the highlights:

  • 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."