Carmageddon? When Planned Road Closures Aren’t the End of the World


Frontier Group intern Adam Martin wrote this blog post.

The Massachusetts Department of Transportation is planning a $1.1 billion project to reshape I-90 and the Mass Turnpike, the main artery into Boston from the western suburbs. The project would replace a crumbling viaduct on the highway – a necessary fix to an aging piece of infrastructure. But will the construction be an impending disaster for commuters?

Major road construction projects are usually met with warnings of “carmageddon” and nightmarish visions of suffocating bottlenecks. But while our gut instincts tell us that shutting down a lane or two of traffic will lead to traffic hell, the experience of similar closures around the country suggests that “carmageddon” is far from inevitable – especially when good alternatives are available.

Boston isn’t the first city to close or narrow major arteries for construction overhauls. Several years back, Los Angeles closed 10 miles of I-405 on two separate weekends. As one of the most heavily traveled freeways in the world, many dreaded its impending closure while others urged drivers to stay away. But on both weekends, the anticipated congestion failed to materialize. Many drivers, heeding warnings of the impending closures, either took roads far from the 405 or stayed home altogether. According to an article in the University of California’s Access magazine, traffic volume on I-405 North was 61% below the baseline of pre-“carmageddon” traffic volume during the first closure and traffic on I-405 South was 74% below the baseline. Granted, the closures were short-term and limited to the weekend, when much travel is optional. But even so, the response to the closure demonstrates people’s ability to respond to changing circumstances.

Another example was the recent closure of Seattle’s Alaskan Way Viaduct, a major artery that carried 90,000 cars every day. Again, many drivers anticipating the closure rationally changed their behavior to avoid the rush. Some turned to biking and public transit, while many gave telecommuting a try. In the end, the so-called “Viadoom” many people anticipated never came to Seattle.

These cases sound counterintuitive, but real-world experience and social science assert that these are not mysterious anomalies, but are rooted in established theory.

Why? One reason is induced traffic, the idea that as more road capacity becomes available, there will be increased demand to use these new roads. People who might have avoided driving on the road or shifted their commuting time due to congestion might take to the road once again. People who had taken transit might drive. Businesses might move to the outskirts, drawing new drivers. Soon, people flood the new road, causing congestion to reemerge.

The closure of a road can cause the same thing to happen in reverse, as people change their habits. No one likes sitting in traffic and if there’s a fear that “carmageddon” looms large, wouldn’t you want to avoid it rather than sitting on your hands and complaining? Some people will experiment with different routes, others will try biking, and another group might invest in a transit pass.

A second possible reason is Braess’s Paradox. While this paradox is a complicated mathematical idea, it basically argues that adding a shortcut to reduce traffic will entice more drivers to choose that route out of a self-interested desire to save time, which creates congestion and worsens everyone’s commute. Even if the road gets more congested as more drivers take it, drivers will still pursue the “shortcut” because they hate feeling like they’re missing out on a seemingly quicker commute. If Braess’s Paradox suggests that more roads lead to more congestion, then road closures could produce the opposite effect.

When considering the Mass Turnpike project, it’s easy to write off examples like those of Seattle and Los Angeles as being the result of idiosyncrasies unique to those cities: “Seattle traffic is nothing! Boston drivers are the worst! Outsiders couldn’t survive one minute on these streets!” But while every city is different, time and again, the closure of a road has not led to gridlock but rather to people taking steps to avoid the worst out of desperation and necessity. As Massachusetts moves forward with the Turnpike project, the priority should be to give the road’s users as many options as possible to avoid the construction – including maintaining full rail service along the corridor and expanding transit service during construction – whether or not “carmageddon” ever really comes.   

Photo credit: Oregon Department of Transportation, CC-BY-2.0