SEATTLE – The rickety Alaskan Way Viaduct – shakier since the 2001 Nisqually Earthquake – could be closed for almost four years if a tunnel is built to replace the roadway, according to the state Department of Transportation.

The state indicated previously that the 50-year-old viaduct, which carries State Route 99 for 2.2 miles above the downtown waterfront, would remain open during construction. But new cost estimates indicate shutting down the road could shave years off construction time.

The viaduct-replacement scenario that could require the longest closures involves a section north of Pike Street where the tunnel would end. The tunnel would have to be connected to a bridge leading to the existing Battery Street Tunnel, said David Dye, viaduct-project manager with the department.

Dye said that section alone could require a closure of at least three years.

A 2004 state study, prompted by worried waterfront business owners, found that closing the viaduct could save $300 million to $500 million but would add 25,000 cars daily to downtown Interstate 5 by 2010. It also would increase downtown street traffic by 36 percent, or 33,000 cars a day.

Those numbers haven’t been updated but will be in the Environmental Impact Statement, Dye said.

According to the new preliminary estimates, rebuilding the viaduct, rather than the tunnel option, would be much cheaper and would cause less traffic disruption but would take up to two years longer to build, Dye said.

The official preference of the state and city of Seattle is a tunnel to replace the viaduct.