Sightline's Eric de Place looks to history to answer the common question, "Will the waterfront tunnel go over budget?"

Looking at four recent tunnel projects, the answer appears to be: More likely than not. Of the four the Mt Baker I-90 expansion tunnelthe downtown Seattle bus tunnel, Sound Transit's Beacon Hill light-rail tunnel, and the Brightwater sewage tunnels, only one (I-90) came in under budget. The rest ended up substantially over budget, with the bus tunnel coming in more than 56 percent higher than predicted.

De Place notes:

There’s a lot riding on the current cost estimates. If the project goes over budget, Seattle taxpayers foot the bill -- a curious result, considering that Seattle voters rejected a tunnel replacement option by a 39 point margin. A cost overrun as small as $100 million (just 2.4 percent of a $4.2 billion project) works out to about $167 per Seattle resident -- or almost $700 for a family of four. For a city struggling to avoid deep cuts to basic services, even a relatively small cost overrun could be challenging.

Of course, it is impossible to know in advance whether any project will stay on budget. And that’s especially true for a project as complex, daunting, and unknown as this one. It would be among the widest-diameter bored tunnels ever built, through a seismic fault, directly underneath some of the densest and most valuable urban real estate on the West Coast. Besides, when engineers first made the $4.2 billion cost estimate for the entire Alaska Way Viaduct replacement project -- with $1.9 billion price tag for the deep-bore tunnel -- they had finished only 1 percent of the design. So the current cost estimate is little more than a placeholder.

Scary stuff indeed.