“The cars just disappeared!”

The Alaskan Way Viaduct, Seattle’s unsightly and increasingly unsafe double-decker waterfront freeway, closed for good on January 11th, 2019. Its replacement tunnel, following years of cost overruns and construction debacles, is set to open on Monday, February 4th. (We’ve discussed the tunnel project on Strong Towns before with former Seattle mayor Mike McGinn, who fought unsuccessfully for a surface boulevard instead.) In preparation for that three-week gap, when there would be only one north-south highway route through downtown Seattle instead of the usual two, local public agencies and media outlets did their best to prepare Seattleites for impending #Viadoom.

After all, 90,000 cars a day used the viaduct. Where would they all go?

The answer seems to be that they mostly just disappeared.

“Our travel times have actually been better than when we had the Alaskan Way Viaduct and everyone was commuting in their normal way,” KIRO Radio Traffic Reporter Chris Sullivan said on MyNorthwest, with a note of incredulity. “I’m still trying to figure out where those 90,000 cars went that usually use the viaduct.”

The Seattle Times ran a pretty good account of where they may have gone, though some of the details won’t mean a lot to you if you’re not familiar with the region. The long and the short of it: travelers appear to have adjusted their habits. Some may be working flex hours or telecommuting (a trend that was on the rise anyway). Others may have switched to walking or bicycling. Water taxi ridership apparently tripled.

The real question is, why is anyone still surprised when this happens?

A Familiar Story

In 2007, the I-35W bridge across the Mississippi in Minneapolis collapsed. It was closed for over a year as a replacement bridge was built. News outlets predicted traffic headaches. Guess what didn’t occur? Our friend David Levinson, a civil engineer and professor at the University of Minnesota, found that commute time increased by less than a minute. Levinson has since done extensive research on how commuters changed their behavior in response to the bridge closure.

In 2012, Los Angeles closed a large segment of its chronically congested 405 freeway. News outlets projected Carmageddon. Didn’t happen; in fact, traffic was better than normal.

In 2017, a segment of freeway in Atlanta collapsed due to a fire. News outlets predicted months of traffic woes. Again, following the initial chaos the day of and day after the collapse (as drivers figured out new routines), things arguably got better, not worse. Our friend (and Strong Towns member) Joe Cortright of City Observatory wrote this about it at the time:

So what’s going on here? Arguably, our mental model of traffic is just wrong. We tend to think of traffic volumes, and trip-making generally as inexorable forces of nature. … What this misses is that there’s a deep behavioral basis to travel. Human beings will shift their behavior in response to changing circumstances. … The fact that Carmageddon almost never comes is powerful evidence of induced demand: people travel on roadways because the capacity is available for their trips, and when the capacity goes away, so does much of the trip making. If Atlanta can survive for a month or two without a major chunk of its freeway, that’s a powerful indication that more modest steps to alter road capacity don’t really mean the end of the world.

Fool me once; shame on you. Fool me twice, three times, four times, uhhhhh….

The Real Lesson: The Opportunities We’re Missing

So traffic, it turns out, behaves less like a liquid with a constant volume, and more like a gas which expands or contracts to fill its container. This is what economists refer to as induced demand, and it goes both ways—reduce the system’s capacity, and you can just as easily un-induce that demand for driving trips.

But the point isn’t just to have a told-you-so laugh at the prophets of traffic doom. The real message here is we need to keep these examples in mind when it comes time to decide where we’re going to spend our infrastructure dollars.