UPDATE: The Seattle Department of Transportation (SDOT) has now backed away from its previously published Viaduct traffic counts!! The agency now says that it has no idea how many vehicles used the Viaduct in 2012. See our update on Viaduct traffic data—and be advised that this post, and particularly the chart below, are based on data that SDOT no longer supports!

Bertha’s woes are hogging the spotlight. But while everyone’s been looking down, something going on up in the air may prove just as important in the long run: traffic volumes on the Alaskan Way Viaduct have collapsed since the state started its construction project.

Take a look at the trends, courtesy of the Seattle Department of Transportation’s traffic maps:Viaduct traffic count - with warning

Astonishing! Viaduct traffic fell by 48,000 trips in 3 years—a reduction that, I’m sure, many transportation planners would have thought unthinkable.

Where did the traffic go?

  • I think there are 4 major reasons that traffic in the corridor has declined:

    1. A shift from cars to buses. Metro says that bus ridership in the Alaskan Way Viaduct corridor has grown by 33,000 trips per day since the agency added new transit service with mitigation money. That’s a huge increase. Many drivers have parked and jumped on the bus.
    2. Diversion to surface streets and I-5. According to SDOT, the north-south surface streets west of 5th Avenue added about 6,500 trips in 2012, compared to the prior year. At the same time, WSDOT’s Ramp and Roadways reports show that I-5 added about 4,000 additional car trips per day between 2010 and 2012.
    3. A long-term decline in car traffic. According to data from SDOT and WSDOT, north-south car traffic through downtown Seattle peaked in 1998 and has been declining gradually ever since. On the Viaduct itself, traffic volumes slid by nearly 7,000 vehicles per day from 2009 through 2011—suggesting that a significant share of the trips that disappeared from the Viaduct in 2012 were part of a longer-term, secular decline in car travel.
    4. Trips “disappeared” due to traffic delays. When WSDOT tore down the southern end of the Viaduct in last 2011, it created a bottleneck that slowed rush hour Viaduct traffic to a crawl. Faced with those sorts of delays, some drivers may have started to look for other destinations, or consolidated trips to avoid wading through the congestion.

    I’m sure that many of the city’s transportation planners are shocked by these numbers. I mean, until SDOT released the Viaduct traffic counts, nobody had even noticed that traffic on the Viaduct had declined by so steeply. There was no fanfare, no outcry from the business community or residents about lost capacity on the Viaduct choking the life out of the city. (Compare that to the hue and cry during the Viaduct debate, when many folks believed that diminishing the corridor’s traffic capacity would bring the city to its knees.)

    But there’s a real ray of light here. At this point, nobody knows if Bertha will ever get moving again, let alone complete her job. But given these figures, maybe it doesn’t matter. Seattle has seamlessly adapted to losing the first 48,000 trips on the Viaduct. No one even noticed. No one even noticed that 40 percent of the Viaduct’s traffic just disappeared! Could accommodating the loss of another 62,000 be that hard if we, I don’t know, tried even a little?

    Seattle can survive, and even thrive, without a viaduct or a tunnel.