SEATTLE — Here in the capital of the Pacific Northwest, where commercial jets were born but now the mayor brags about biking to work, questions over commerce, climate change and community are converging on a couple of fateful miles of asphalt.

Is the Alaskan Way Viaduct overlooking Elliott Bay on the western edge of downtown just an elevated relic, another old road that needs to be replaced? Or is it something more, a symbol whose fate will help shape not only this city but the rest of urban — and New Urbanist — America?

“It’s not just a highway anymore,” Mayor Mike McGinn said. “It’s about the type of city we are and what are our priorities.”

Another question: Is Mr. McGinn a visionary or an obstructionist?

For a decade, since a 2001 earthquake exposed the viaduct’s vulnerability, this ambitious but frequently indecisive city has been debating what to do about it. In 2009, the city, county and state came up with a plan to replace the viaduct with a $3.1 billion, 1.7-mile tunnel that would run beneath downtown skyscrapers and behind a seawall protecting it from Puget Sound. In a city inclined to dither, something was getting done.