Source: sightline.org

The Washington transportation department plans to replace the Alaskan Way Viaduct along Seattle’s waterfront with a deep-bore tunnel underneath downtown Seattle. There is special concern about the project’s fiscal plausibility; the state has estimated that the entire project will cost $4.2 billion. Yet the estimate is preliminary; at this point, Seattle residents have no way of knowing if the actual costs will be lower or higher than the initial estimate.

As a way to inform the debate over Seattle’s deep-bore tunnel, Sightline has prepared a basic comparison of other high-profile tunnels recently constructed in the area: Mt Baker I-90 tunnel expansion, Downtown Seattle Bus tunnel, Sound Transit Beacon Hill tunnel, and King County Brightwater sewage tunnels. The costs of these tunnels tell a cautionary tale.