Well, this is the weirdest of our whole series on traffic trends: the number of cars and trucks crossing the SR-520 bridge across Lake Washington has held steady for…wait for it…more than 20 years. It was hard for me to believe at first, but it’s the only conclusion you can reach from the state’s Annual Traffic Report series.
In the chart to the right, the red points were marked as “estimates” by WSDOT. The blue dots were the state’s direct counts of actual traffic volumes, averaged over the year. I have no idea how to explain the spike in the late 1980s. [Update: looking at the original pdfs, that spike appears to be an error; WSDOT’s numbers for 1988 through 1992 show several serious anomalies. Also the spike occurred when the state was switching methods of counting traffic volumes on 520, moving from axle counts to vehicle counts.] But the astonishing fact is that average daily traffic volumes on the SR-520 bridge were about the same in 2010 as in 1985. And traffic volumes have held roughly constant—with a few modest ups and downs—since 1990.
Given what’s happened over the last few decades, it seems like a pretty safe assumption that if the bridge remained in its current configuration, traffic volumes would continue to hold steady, rather than climbing inexorably upwards.
And yet, the state’s transportation models—the ones used to justify a project to widen the bridge—show that traffic volumes on the SR-520 bridge are going to go up, starting soon. Take a look at Exhibits 5.1-1, 5.1-2, and 5.1-3, in Chapter 5 of the state’s SR-520 SDEIS. It shows that the state is predicting that traffic throughput on the bridge will grow between now and 2030. Yet those predictions of ever-increasing traffic seem to be directly at odds with the fact that SR-520 traffic volumes in 2010 were five percent below what they were in 1998—the most recent peak.
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The traffic trends don’t change much when you throw I-90 into the mix. Traffic on I-90 across Lake Washington has remained roughly flat since 1998. So as the chart to the right shows, total traffic volumes across the two spans combined have remained steady for about 13 years. If anything, traffic is a bit lighter now than it was a decade ago!
This is a set of facts that the standard traffic models used in the Puget Sound have a terrible time explaining. Those models tend to assume that more people and more money in a metropolitan area mean more traffic, everywhere. But over the past decade, population and the economy have both expanded—and yet traffic volumes on one of the region’s biggest highway corridors have remained flat.
The stark contrast between actual traffic trends across Lake Washington and the state’s forecasts of increasing traffic volumes leads me to a simple conclusion: the state’s transportation models aren’t based in reality. Essentially, those models treat the growth in traffic volumes as a core, unassailable assumption—even though it’s often at odds with the observable facts. Compounding the problem, the public debate over highway mega-projects tends to treat the transportation models’ predictions about future traffic growth as facts — as inescapable conclusions of a rigorous scientific process—rather than as shaky assumptions that often conflict with actual traffic trends.
And that core mistake—confusing assumptions for facts—can make billion-dollar highway projects seem like a necessity rather than a costly luxury.
[Kudos to Sightline staffer Pam MacRae, who compiled the data from the state’s annual traffic reports. And note that the post has been edited slightly after it was first posted; and just to be clear, I am interpreting the the SDEIS projections for “Vehicle Demand” to mean “Average Daily Traffic” — that is, that “demand” should be interpreted to mean actual traffic volumes. The SDEIS is written a bit unclearly; I think this is the accurate interpretation, but f I’m wrong, please correct me in comments!]
Was that spike in the mid to late 90s the result of the issues with the I-90 bridge? Just when did the partially-built replacement I-90 bridge sink?
Good question, Rosemary! I’d forgotten all about that. This Youtube video says that the bridge sank after Thanskgiving of 1990. But the big spike in the traffic chart was in 1988 and 1989.That said, there’s a smaller spike in SR-520 traffic in 1991 that’s probably likely related to the sinking of I-90 the year before. But that’s one of the “estimated” data points, so we don’t actually know what traffic trends were like that year—not from this data series, anyway.
I’d love to see a graph of transit and vanpool ridership to compare with this one. I’m sure you’d see a steady increase of actual passenger traffic.
Clark, did you really cite a YouTube video? It gives the right date, but at least go to an official source like WSDOT (as much as they appear to have problems analyzing traffic demand).
Jessica – Yeah, yeah, everyone’s a critic. First google hit, reliable source, and I’m so busy I gotta move on to something else.
As a former driver of 520 at rush hour, I suspect that traffic volumes remain steady because the bridge is at capacity. It’s always a traffic disaster, so people take other routes. If you add lanes, of course, more people will drive on it (induced demand).
I used to commute across 520 from mid-80s to mid-90s when I worked at Microsoft. At the start there was actually a “reverse commute”. Most of the traffic was from Bellevue to Seattle in the mornings. Those of us going the opposite direction were smug and cruising. Not for long. By late 80s the traffic was equally bad in both direction.I think the decline from the peak has a lot to do with Microsoft and other employers adding transit options. In mid-80s I started taking the bus to work. It took hours. Connections sucked and no express. Lots and lots of waiting. A year later a few friends cobbled together a carpool and even then Microsoft had no carpool spots…though they were very happy to create one for us when we asked right at the front door. Pwn. It by late 80s Microsoft was offering all kinds of great discounts and deals on transit and really pushing the carpooling. Then came express bus service. Now you can get picked up by special Microsoft shuttles around Seattle and driven right to work with wifi and everything.Basically my experience is that Skye is right in his comment. The bridge filled up and now is a nightmare to drive. It is at capacity for personal tolerance. Adding more lanes will just fill up those lanes quickly and increase traffic but not time it takes anyone to get across.
“Those models tend to assume that more people and more money in a metropolitan area mean more traffic, everywhere. “This is the opposite of what is happening in Vancouver BC. In the last decade:– there has been a 23% increase in “trips” into Vancouver and yet “vehicle trips” are down 10%. — the number of kilometers driven by Vancouver registered vehicles is down 29%. — Gasoline sales in Vancouver are down a similar amount. All this while population and the economy have grown rapidly. Transit is up 20%, walking up 44% and biking up 180%. Expanding roads in urban areas is so last century. Waste of money and lost opportunity to improve quality of life for citizens.
Skye-I think that’s basically spot on: SR-520 is full to capacity during much of the day. But as far as I can tell, the Puget Sound traffic models assume that if there’s demand for cross-lake travel, it will keep spilling out into uncongested times—late . That implies more and more commuters traveling at 4 am or noon or 10 pm. But as congested as 520 is, we just haven’t seen that kind of spillover boosting traffic volumes over the past few decades. Barry – Great point about Vancouver! I saw a similar stat the other day in the Vancouver Sun: “in the past 15 years, the city’s overall population, number of jobs and walking, cycling, or transit trips into Vancouver have all risen by 25 per cent. Yet the number of cars coming into the city has dropped by 20 per cent.”I’m trying to track down the source, since I can’t find it on the city of Vancouver’s website. I hadn’t heard 20% before!