Nelson/Nygaard, a traffic analysis firm that has worked for both Mayor Mike McGinn and the Seattle City Council, has issued the results of city-commissioned study on the proposed deep-bore tunnel that finds traffic impacts on downtown city streets will be worse than the state has admitted and that claims of the tunnel's purported ability to support freight traffic are unfounded. Meanwhile, the state has cut off the city from consultation on the controversial $4.2 billion project.

Peter Hahn, director of the Seattle Department of Transportation, had requested the study to determine the impacts of traffic diversion of the subterranean downtown bypass to replace the Alaskan Way Viaduct. But today Hahn says in a letter to the city council that that state transportation officials have cut off city officials—the state's co-leads and co-financiers for the project—from discussions about traffic concerns and the project as a whole. "WSDOT has indicated that it will not consider any further input from SDOT, and it will not share a draft of the Final [Environmental Impact Statement] until it is presented for signature," Hahn writes in the letter to council members (.pdf). Although possible unprecedented, it comes as no surprise as Mayor McGinn has persistently attempted to stymie the project.

A few highlights from the Nelson/Nygaard report (.pdf):

Drivers Will Have an Incentive to Avoid the Tunnel: If the tunnel were built, the state would have to connect Alaskan Way to the Elliot/Western Avenue corridor on the waterfront—essentially creating a parallel, free, surface-street alternative to driving in the $4-per-trip underground tunnel. The state omitted this connection when evaluating traffic diversion impacts in its own study. The state's draft Environmental Impact Statement has estimated a tolled tunnel would carry 42,000 vehicles a day, but that number is knocked down to 38,000 trips daily with the Elliot/Western connector in place. In other words, the tunnel would accommodate roughly one-third of the 110,000 vehicles that currently use the viaduct. That said, much of the diversion will reportedly occur during midday, when surface-street congestion is relatively low.

The Tunnel Wouldn't Help Freight Mobility: Nelson\Nygaard notes that the freight most directly affected by a viaduct replacement will be trucks commuting from the SODO industrial area to the Ballard/Interbay industrial area. For these commuters, a deep-bore tunnel and a surface/transit plan produce roughly the same travel times.

For other freight paths, the surface/transit option makes more sense because it invests in I-5 improvements. For example: A tolled tunnel would divert approximately 15,000 vehicles daily to I-5 and make no improvements to I-5 for the traffic dump. The surface/transit/I-5 option would increase I-5 volumes by 34,000 daily vehicles; however, the surface/transit model invests in new I-5 lane capacity to increase daily throughput by approximately 30,000 cars. (In short: Tunnel, +15,000 cars to I-5. Surface/transit, +4,000)."

And there are other problems: The northern and southern portals will cause traffic to bottleneck in narrow street grids already plagued with congestion, and there's no funding set aside to resolve this issue. In addition, Nelson\Nygaard's report says, the state's preliminary traffic analysis "has shown that a tolled tunnel is the worst of all evaluated scenarios for greenhouse gases."

"We strongly believe that the information in the report should be considered and addressed by decision-makers before the [final study] is complete," Hahn writes in his letter to council members. With his letter fresh on their desks, Seattle City Council members have three options: stand with SDOT and demand that state officials work collaboratively to address these problems; stand up and admit the deep bore tunnel fails to meet nearly every challenge it was designed to alleviate; or continue to do absolutely nothing. I have calls in to Council President Richard Conlin and Tom Rasmussen, chair of the council's transportation committee. I'll update when I hear back.