Sorry to be so Seattle-centric…but this post about Seattle’s Alaskan Way Viaduct got me thinking. If the Viaduct is closed—whether for construction of a tunnel or a new aerial highway, or to make way for green space and a surface street—what happens to rush hour?  Does traffic in downtown Seattle get hopelessly snarled, and stay that way for at least 3 years?  Or do city transportation pllaners have some reasonable options for keeping people moving through the downtown core, even without a Viaduct?

Traffic studies show that the Viaduct carries about 105,000 daily trips. But most of those trips are at off-peak times when the surface streets have plenty of extra capacity. Sure, a trip along the Viaduct-less corridor would take a little longer than it does now; but the steet grid could easily handle the load.

But at rush hour—particularly the afternoon—there’s precious little extra capacity on the city streets. So the thorniest problem that traffic planners will have to face will be accomodating rush hour trips on the street grid and I-5, during the busiest part of the day.

So, how many trips is that, exactly? And what are the options for dealing with the added load?

Earlier this week, the helpful and responsive folks at the Seattle Department of Transportation sent me some data that may help shed some light. As far as I can tell, it boils down to this: without the Viaduct, transportation planners will have to figure out how to accomodate the equivalent of 11,000 rush-hour car trips through the busiest part of downtown. Can they do it?

  • First, the data. (Feel free to skip the next few paragraphs if you’re not a traffic geek.) Each year, the city collects data on traffic flows on the major arterials around Seattle, including the Viaduct and its on- and off-ramps. The data include figures for average weekday traffic, plus the traffic volume during the one-hour morning and afternoon peaks. Based on those figures, it seems like there are about 12,000 total vehicle trips on the Viaduct during busiest one-hour afternoon rush hour peak.

    Now, without a Viaduct, some of those peak-hour trips will take quite a bit longer—so people will shift their trips to other times of day, other modes, or forego them altogether. Based on published estimates of how much an increase in travel time decreases travel demand, it looks like demand for car trips may drop from 12,000 trips to 10,000 during the afternoon peak hour.

    But some of those trips already begin or end in the Pioneer Square-to-Belltown corridor. A car that currently gets on at the Belltown northbound exit may have travelled through Belltown or downtown. So the peak number of cars added to surface streets after the Viaduct is closed will be somewhat less than 10,000.

    To get a finer-grained look at where the traffic problems might be most severe, you can break down the trips on the former viaduct corridor into zones—Belltown, Downtown Core, Pioneer Square/Stadium, etc.–and look at the actual increase in travel demand in each zone. To me, it seems that the real pinch occurs in the Downtown Core—roughly, from Yesler in the south through Stewart in the north, and Alaskan Way on the west through Boren on the east. That area is already pretty packed during rush hour. With no Viaduct, peak-hour travel demand will increase by somewhere between 6,100 and 7,200 trips. (Note, this is a somewhat conservative estimate—I’m assuming that some former Viaduct trips will “disappear”–i.e., move to other times or destinations—because they’ll take too long; but I’m not assuming the same for trips already on the surface streets.)

    So that’s the one-hour peak. Over the course of a rush hour that lasts at least an hour and a half, that means that transportation officials will have to worry about accomodating the demand for some 11,000 addtional trips in the busiest part of downtown, during the busiest part of the day.

    So, 11,000 extra trips: is that a lot or a little? It’s a lot less than 105,000. But in my mind, it’s still a lot of trips. The existing street grid may be able to hold a few more cars than it currently does. Some tweaks to traffic enforcement (“don’t block the box”), elimination of some street parking during rush hour, and so forth may increase throughput a bit more. Still, even with those improvements, the demand for 11,000 extra trips could really jam up the afternoon rush hour. Even if people eventually adjust to the congestion—by changing schedules or jobs, or switching to transit—the early months could be brutal.

    But if you add in transit improvements, accomodating 11,000 downtown trips seems much more achievable. The bus system already carries 31,000 people out of downtown during the afternoon peak. So getting 11,000 people to shift from driving alone to the bus would boost rush-hour transit ridership by a little more than a third—tough, maybe, but not inconceivable.

    And in theory, at least, there’s ample capacity to handle that many trips in the bus tunnel, which is now closed for service. Once the tunnel reopens, it will be able to handle about 9,000 rush hour trips that right now are travelling on Third Ave. And when light rail starts running through the tunnel, its capacity could grow by a third or more. (To my surprise, it seems that the tunnel may have been underutilized; one estimate, a few years old now, is that the tunnel could carry 18,000 trips per hour (scroll down a bit to find the claim), with everyone seated, in buses alone. If that’s really possible, then the tunnel alone would be meet the post-Viaduct demand.)

    And then there’s Third Ave., which is currently closed to cars during rush hour. If Third reverts to being predominantly a car corridor, it’ll handle at most 2,000 vehicle trips during rush hour. But if it’s kept closed to cars, and is used to handle an extended bus schedule, it can handle at least three times as many passengers.

    So, there are three options—(1) surface street and traffic enforcement tweaks, (2) adding light rail to the bus tunnel, and (3) keeping Third Ave. as a bus-only street even after the bus tunnel opens—that could accomodate most, if not all, of the added demand for rush hour trips. If those options are phased in, as the Viaduct is phased out of service, it could be that many folks wouldn’t notice much of a change to their afternoon commutes.

    Yes, it would be tough to get people out of their cars onto transit. But if city officials have their way—and the Viaduct is closed for reconstruction—they won’t have much choice but to try. There really aren’t many other options.

    And, as I’ve said before—if a combination of transit and street improvements can keep downtown traffic moving, or at least bearable, for a mimimum of three years, why not see if they’ll work as a permanent solution? Why spend billions of dollars to fix a problem that the city’s already solved?