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Congestion as Incentive
Erica Barnett, 17 Aug 07

273395204_0b7f2f0021.jpgA few days ago, two lanes of the main freeway arterial through Seattle, Interstate 5, were shut down for construction. They will remain closed at least another two weeks. For weeks preceding the lane closure, local newspapers, blogs, and television have predicted utter traffic chaos and disaster. But despite predictions of "nightmare" traffic, "survival tips" for dealing with the commute, and even an entire blog called "The Clog" dedicated to the closure, Traffic Jam 2007 failed to materialize. (Actual headline on day two of "The Clog": "No Clog Just Yet.") Not only that, but many commuters described the drive as smoother than ever.

What happened? Media and government efforts to sow collective panic can't, on their own, explain the startling reduction in traffic on I-5. According to the state Department of Transportation, of 120,000 cars that normally use northbound I-5 daily, about half simply disappeared. The explanation: Drivers are adaptable. When faced with the prospect of gridlock—and given ample warning and time to prepare—people found alternate routes, rode transit, worked from home, and avoided unnecessary trips.

There's nothing counterintuitive about this. It is, in fact, exactly what happened in San Francisco after the Embarcadero was badly damaged in the Loma Prieta earthquake in 1989; city leaders closed the double-decker freeway down and forced people to find new ways through the city. In the immediate aftermath of that freeway-closing earthquake, city officials predicted massive gridlock for miles in every direction. Instead, people adapted--taking transit, finding alternate routes, changing their schedules--and the gridlock never materialized. Eventually, the city's mayor and city council decided to remove the elevated highway permanently.

I'm not suggesting, of course, that an entire major interstate highway can be removed without traffic consequences. However, it is interesting to note the effect congestion has on people's decision-making processes. Just as congestion pricing, both temporary (as in Stockholm) and permanent (as in London) give drivers an incentive to avoid commuting by car, reducing the number of lane-miles available to drivers give them an incentive to find alternatives to driving alone. What if state highway planners reopened those lanes, but charged a toll? Would some of those single-occupancy drivers stay away? Intuitively, it makes perfect sense that they would.

Incentives, of course, must be positive as well as negative. For congestion-as-incentive to work in the long term, it has to be paired with alternatives that are viable in the long term--flexible work schedules that allow workers to stay home a few days a week, mass transit that is affordable, frequent, and convenient, and an infrastructure that supports bike and pedestrian commuting, among other things.

In Seattle, a debate is raging about whether to tear down a downtown waterfront highway, the Alaskan Way Viaduct, that is, in many ways, similar to the Embarcadero. But that solution, like the I-5 closure, won't work in isolation; removing the viaduct won't be a long-term solution unless the city also makes significant improvements in transit, a waterfront boulevard, and improvements to the surface street grid through downtown.

Seattle (and other cities) has had difficulty recognizing this; despite record transit ridership during the first few days of the freeway lane closure, its main transit system, King County Metro, did not provide additional buses to accommodate the increased demand. If it had, the 60,000 cars that disappeared might have been joined by another 20,000 or 30,000—rendering those two closed lanes ultimately unnecessary.


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It is interesting and encouraging that commuters and others respond when given advance notice of conjestion. Unfortunately, like the famous frog in the saucepan, most won't get out of the water while they (and we) can, unless there is some dramatic event or _very_ ample notice of one to come. I think we need to work on what is coming to be called the "complete streets" approach, where streets are explicitly planned to accomodate many different modalities of transit - walking, cycling, Seque-ing, skating, and oh yes, cars.

Posted by: Dale Lowery on 16 Aug 07

Here in New Zealand the highway agency began annual congestion surveys in the three main cities four years ago. For the first three years the results for the smallest city were excluded because this was the only city that didn't have a reputation for being badly congested yet it was recording the highest levels of congestion of all the cities.

The findings have now been verified as correct. Essentially the study has confirmed the frog in hot water theory. The two bigger cities have arterial motorways feeding into their CBDs. These motorways become heavily congested during the am & pm peaks but flow relatively freely in the interpeak. Consequently drivers are very aware of the congestion and this has successfully created political pressure to improve roads and transit services. The third city does not have any motorways so it's interpeak traffic is constantly disrupted by traffic lights. Consequently it does not have the same extreme difference between its peak and interpeak congestion. But its peak congestion is almost as bad as the other two cities and it's interpeak congestion is much worse than them producing an overall workday congestion equal to the other two cities. This lack of major variability in the amount of congestion has created the frog in hot water situation. Although the actual level of congestion and the economic costs are equal to the other cities the demand to do something about it is missing. And it is the only city where congestion is continuing to get worse. The third city actually has a reputation for being cycle friendly and having an excellent bus system yet it actually has the highest level of car ownership of the three cities.
This certainly fits with the Seattle experience. It also provides a warning that improvements in mass transit can lose their effectiveness over time if it creates a sense of complacency amongst too many drivers that the problems have been solved.

Posted by: Kevyn Miller on 17 Aug 07

This is a clear example of the "Wisdom of the Crowds" as described in the book by James Surowiecki.

I would be interested to know if anybody is planning to follow up on this event with a survey to find out the full breadth of strategies employed by commuters. That is, were there any novel strategies that where not the usual ones we think of such as car pooling, telecommuting and public transport. More specifically, as indicated in the article, tactics that are viable for reduction traffic in short term are not necessarily viable in as long terms strategies -- however, some of the approaches taken by the people affected by road works in Seattle may actually be applicable in as long term solutions.

Even understanding why people do not normally employ these approaches, might provide insight into how the system or strategies could be adapted or supported, so as to actually be viable as and attractive to individuals as default strategies.

Posted by: Stewart Gebbie on 17 Aug 07

I don't have to travel that section of I-5 myself much, but every one of my friends say it's been a huge improvement.

It sounds like Highway 99 is taking the brunt of it and many of the West Seattlites who don't need to go North are loving this.

Because of that pattern, a pet theory of mine is gaining some traction. That is that an enormous amount of congestion is caused by what I call the zipper effect. Specifically when one line of cars needs to move to the left and another line of cars needs to move to the right. Effectively endless streams of vehicles need to swap places. A cloverleaf interchange is a classic example. The I-5 closure basically eliminated at least one zipper merge that I know of (West Seattle to I-5 merging with people coming up from the SODO). Cars can move at high rates of speed when they're safely separated (take any freeway for example), but put them in positions where they have to interact with other vehicles and you'll have congestion.

Time to move away from the 1950s thinking on our highways...

Learning more about Sustainable Living at

Posted by: Nicholas on 19 Aug 07

The congestion pricing has just become permanent in Stockholm, it started on the 1st of August.

Go to (Public Road Administration), click on English and then Congestion Tax.

Posted by: Leo on 20 Aug 07



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