I think these may be the most important two sentences from the recent NelsonNygaard report on traffic diversion from Seattle’s Alaskan Way Viaduct:

The State did model the 2015 Program (including the Elliott/Western Connector) with Toll Scenario C, but the results are not included in the SDEIS. In the model forecast including the connector, 38,000 daily trips use the tunnel…[Emphasis added]

AWV cross sectionTranslation:  the state itself says that in the early years, a tolled deep bore tunnel would carry only about a third as much traffic as the existing Viaduct.  The remaining traffic—roughly 72,000 cars and trucks—would be diverted onto I-5 and city streets.

Of course, these figures are based on the state’s transportation models—and I have very few kind things to say about traffic models in general.  Many have proven rigid and unreliable, and none projected the traffic trends we’ve seen in the last few years.

Still, I think there are some very good reasons to pay attention to what the state’s model is telling us here.

Here’s why…

  • The existing Battery Street Tunnel—perhaps the closest proxy for the deep bore in the existing road network—carried about 56,000 vehicles each weekday at last count.  (Data were last collected in June of 2010.)  But about 5,700 of those vehicles got on or off highway 99 just south of the tunnel.  Those ramps simply won’t exist under the deep bore plan—which means that only about 50,000 vehicles that currently go through the Battery Street Tunnel would even conceivably go through the deep bore.

    Then add tolls to the picture.  In nominal dollars, rush hour tolls under the most plausible scenario (Scenario C, described on page 17 of the state’s tolling report) will total $4 in morning rush hour and $5 in the afternoon.  At those prices, I think it’s fair to say that many commuters will choose to avoid the toll by driving on one of the many free roads that parallel the tunnel.

    Those two facts—current traffic levels on and near the Battery Street tunnel, and the fairly high tolls for going through the bored tunnel—make the state’s modeled estimate of 38,000 tunnel trips in the early years seem entirely reasonable.  I wouldn’t take them as gospel.  But they clearly suggest that the tunnel project, as planned, would carry relatively few cars and add lots of traffic to downtown.

    Of course, there are always complications.  For example, perhaps the state’s models underestimate how many cars that currently take the Western and Elliot Viaduct ramps will make their way over city streets to and from the north portal to the bored tunnel.  (That’s what the models predict with an untolled tunnel.)  But in this case, the complication simply underscores the point: no matter how you slice it, a tolled tunnel will dramatically increase traffic on I-5 and on city streets, particularly around the entrances and exits to the tunnel.

    Yet the deep bore is so darned expensive that it leaves almost no money for the necessary (but costly) improvements to I-5, streets, transit, and transportation demand management strategies to help deal with the influx of cars and trucks that the state itself is predicting.