Every few weeks we get a call or email from an exasperated motorist—usually someone who recently was stuck in a time-sucking backup—wanting to know why Caltrans doesn’t add more lanes to existing freeways or build new freeways to reduce congestion. The thinking goes that if freeways had more lanes, traffic wouldn’t be so bad.
Seems logical enough, except it doesn’t work. Numerous studies have examined the effectiveness of this approach and consistently shown that adding capacity does not reduce congestion. On the contrary, it actually increases vehicle miles traveled (VMT).
How can that possibly be? Here’s a three-word answer: supply and demand. When congestion is reduced, the “cost” of driving drops, which is to say, it takes less time. And what happens when prices go down? More people buy! In other words, people drive more, effectively erasing any initial reduction in congestion realized by increasing capacity. This is known as induced congestion.
VMT increases because some people decide to switch to driving from other modes, such as transit. They may be less likely to combine trips or commute outside peak hours. They may even move farther away from work and school, requiring longer trips.
If the futility of adding lanes seems counterintuitive, here’s the part of this discussion that will really blow your mind: reducing capacity can provide social and economic benefits without increasing congestion. Cities in Europe have closed streets in central business districts to spur economic revitalization, and US cities are adopting this strategy as well.
While there are no plans to close any freeways or highways in District 7, Caltrans is increasingly focusing on fixing existing roads and encouraging bicycling, walking and mass transit. Occasionally, adding lanes and building new roadways will be necessary, but such projects will only be undertaken very strategically.
Want to learn more about induced congestion? Check out UC Davis Professor Susan Handy’s excellent and informative brief on this topic here.