Razing Seattle's viaduct doesn't guarantee nightmare commutes, model says

May 10, 2011, University of Washington
The Alaskan Way Viaduct is slated for demolition for seismic reasons. Credit: Washington State Department of Transportation

Debate about how to replace Seattle's deteriorating waterfront highway has centered on uncertainties in the project's price tag. Drilling a deep-bore tunnel and building an underground highway is estimated to cost around $4 billion, but some worry the final price could be higher, as it was for Boston's infamous Big Dig.

University of Washington have, for the first time, explored a different subject of uncertainty, namely surrounding how much commuters might benefit from the project. They found that relying on surface streets would likely have less impact on travel times than previously reported, and that different options' effects on commute times are not well known.

The research, conducted in 2009, was originally intended as an academic exercise looking at how to assess uncertainties in travel-time projections from urban transportation and land-use models. But the paper is being published amid renewed debate about the future of Seattle's waterfront thoroughfare.

"In early 2009 it was decided there would be a tunnel, and we said, 'Well, the issue is settled but it's still of academic interest,'" said co-author Adrian Raftery, a UW statistics professor. "Now it has all bubbled up again."

The study was cited last month in a report by the Seattle Department of Transportation reviewing the tunnel's impact. It is now available online, and will be published in an upcoming issue of the journal : Part A.

Researchers looked at eight routes that currently include the Alaskan Way Viaduct. Credit: University of Washington

The UW authors considered 22 commuter routes, eight of which currently include the viaduct. They compared a business-as-usual scenario, where a new elevated highway or a tunnel carries all existing traffic, against a in which the viaduct is removed and no measures are taken to increase public transportation or otherwise mitigate the effects.

The study found that simply erasing the structure in 2010 would increase travel times a decade later for the eight routes that currently include the viaduct by 1.5 minutes to 9.2 minutes, with an average increase of 6 minutes. The uncertainty was fairly large, with zero change within the 95 percent confidence range for all the viaduct routes, and more than 20 minutes increase as a reasonable projection in a few cases. In the short term some routes along Interstate 5 were slightly slower, but by 2020 the travel times returned to today's levels.

"This indicates that over time removing the structure would increase commute times for people who use the viaduct by about six minutes, although there's quite a bit of uncertainty about exactly how much," Raftery said. "In the rest of the region, on I-5, there's no indication that it would increase commute times at all."

The study also considered the effects on 14 routes, shown here, that do not include the viaduct. Credit: University of Washington

The Washington State Department of Transportation had used a computer model in 2008 to explore travel times under various project scenarios. It found that the peak morning commute across downtown would be 10 minutes longer if the state relied on surface transportation. Shortly thereafter state and city leaders decided to build a tunnel.

The UW team in late 2009 ran the same travel model but added an urban land-use component that allows people and businesses to adapt over time – for instance by moving, switching jobs or relocating businesses. It also included a statistical method that puts error bars around the travel-time projections.

"There is a big interest among transportation planners in putting an uncertainty range around modeling results," said co-author Hana Sevcikova, a UW research scientist who ran the model.

"Often in policy discussions there's interest in either one end or the other of an interval: How bad could things be if we don't make an investment, or if we do make an investment, are we sure that it's necessary?" Raftery said. "The ends of the interval can give you a sense of that."

The UW study used a method called Bayesian statistics to combine computer models with actual data. Researchers used 2000 and 2005 land-use data and 2005 commute travel times to fine-tune the model. Bayesian statistics improves the model's accuracy and provides an uncertainty range around the model's projections.

The study used UrbanSim, an urban simulation model developed by co-author and former UW faculty member Paul Waddell, now a professor at the University of California, Berkeley. The model starts running in the year 2000, the viaduct is taken down in 2010 and the study focuses on peak morning commutes in the year 2020.

Despite renewed discussion, the authors are not taking a position on the debate.

"This is a scientific assessment. People could well say that six minutes is a lot, and it's worth whatever it takes [to avoid it]," Raftery said. "To some extent it comes down to a value judgment, factoring in the economic and environmental impacts."

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3 / 5 (1) May 10, 2011
How does removing the viaduct by-pass affect access to ground-level properties along the route ??
not rated yet May 10, 2011
Bypass's do not usually have access to "ground-level properties" along the route. Unless you are refering to the increase in traffic at ground level due to the cars that have few alternate routes to take.
not rated yet May 10, 2011
I lived in Seattle and trust me almost no one used the Viaduct to get anywhere the fastest. Everyone goes to I5 already, unless there is a football game, baseball game, soccer game, or concert... although one of those is pretty much every weekend.
5 / 5 (1) May 10, 2011
They would spend the money wiser by expanding I-5 downtown from 2 lanes to 6. Unforetunately, the elites running Seattle have forbid freeway construction into Seattle supposedly to encourage public transportation, however, their real estate empire probably would suffer any reduction in the 2 hr commute time from Pierce county. Meanwhile the Dems are using regulatory obstacles to prevent King County's largest employer Boeing, from moving production to South Carolina. I wonder if China is interested in commerical aircraft production? I wonder how do you say "will work for rice" in Mandarin? Might need to brush up my resume.
not rated yet May 11, 2011

Let's not forget the absolute windfall of prime water- front or -view real estate that will suddenly be available along the footprint of the viaduct, and all conveniently cleared and upgraded by the public dollar- including the tax dollars of all those commuters that will just have to to do the best they can trying to travel from north to south in a heavily-urbanised, multiply geomorphologically-restricted and in no way optimized for surface(much less highway)-traffic Seattle driving corridor.
Regarding Boeing- hey, most of the components that their aircraft are constructed from are already made in China, so between that ugly fact and the oh-so-favored tax status that Boning -I mean Boeing enjoys in the Puget Sound region make that scenario pretty much a wash, once you factor out the jobs in the region- but that's really just a question of time anyway- Boeing, Boeing, Gone!
Essentially, the two issues are facets of the same old "privatize profit, socialize cost" conundrum.
1 / 5 (3) May 11, 2011
Seattle run by progressives, King county run by progressives, Washington state run by progressives. This tunnel idea is as stupid as get go. Want a stupid solution to a problem, just ask a progressive.
not rated yet May 11, 2011
While I haven't read the report, it seems that it focuses on drive times. IMO of equal importance is improving Seattle's waterfront as a source of civic pride, improved access and visitation. The viaduct as it exists effectively cuts off the downtown area from the waterfront with this hideous construction. Seattle is flanked by Portland and Vancouver that have awesome waterfront access and attracts pedestrians, sightseers and tourists. Seattle could be so much better than it is.

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