As part of his ongoing mission to undermine plans for a waterfront tunnel, Mayor Mike McGinn often points to huge cost overruns on King County tunnel projects. The Brightwater sewage tunnel has been an ongoing financial disaster, and the cost of the downtown transit tunnel, which opened in 1990, ballooned by 56 percent before it was completed.

But Ron Posthuma, assistant director of King County’s Department of Transportation, wants McGinn to stop using the county as an anti-tunnel whipping boy. That downtown tunnel, which when it opened accommodated only buses but which now houses light rail too, actually came in under budget, Posthuma claims.

McGinn is basing his claims on an Oct. 2009 report from the Sightline Institute that looks at past Seattle-area boring projects.

“Cost overruns are commonplace,” the report concluded, noting that the 2009 light-rail tunnel under Beacon Hill went 30 percent over budget. (Report author Eric de Place brushes off the I-90 tunnel, which was finished 54 percent under budget, saying that the geology underneath the Mt. Baker neighborhood was better understood, as the state had bored through it once before for the freeway.)

But Posthuma says the Sightline report mischaracterizes the downtown tunnel, a project he oversaw at KCDOT. The costs blew up thanks to problems building the stations along the route. The tunnel itself came in under budget. “It was a double-digit percentage [under],” he told Seattle Weekly this week, following a viaduct-replacement briefing before the King County transportation committee. Posthuma couldn’t remember the exact figure.

“Tunneling happens every day, all over the world,” he adds.