"Why a bigger viaduct?

Bothell Times:

It's true, a new viaduct would be a lot bigger than the old one.

If an elevated structure replaces the Alaskan Way Viaduct, it would be built around and over the old viaduct, so traffic could keep flowing during 10 years of construction.

The strategy to surround the old viaduct is a major reason the new highway would be 1-½ times as wide as the existing structure in mid-downtown and more than twice as wide near the ferry terminal.

Opponents of a new elevated structure denounce a new viaduct, in campaign materials, as "Bigger, Noisier, Uglier."

Seattle voters are facing an advisory ballot asking if they prefer a new six-lane viaduct or a four-lane tunnel. As proposed by Seattle Mayor Greg Nickels, the tunnel's shoulders would become exit lanes during peak traffic time.

However, Gov. Christine Gregoire last month said an elevated structure with shoulders would be safer. She called it the only viable option to replace the 54-year-old, earthquake-damaged viaduct.

Ballots for the all-mail vote must be postmarked by March 13 to be counted.

In Tacoma, motorists crossing the old Narrows Bridge have enjoyed daily glimpses of rising towers and curving cables during construction of the second suspension bridge, just to the south.

In Seattle, drivers would see new beams and columns going up all around them. There would be closures on nights and weekends, when girders would be lifted into place for the new decks.

The state expects a mere three months of total shutdown, keeping catastrophic congestion from becoming a way of life.

"One of the things this option can do, that others can't, is keep traffic moving on Highway 99 during construction," said David Dye, urban-corridors administrator for the state Department of Transportation (DOT). "That was a very, very important factor."

Construction of a tunnel would close the highway completely for nearly three years.

Elevated-structure supporters such as Warren Aakervik, president of Ballard Oil, call it the only acceptable option, because it would keep fuel, parts and other goods on the move, preserving maritime business.

Keeping traffic flowing during big projects is old hat for the DOT, which has been rearranging Interstate 5 in Tacoma while new lanes are added. But the viaduct is "one of the most elaborate construction-sequencing designs I've ever seen," Dye said.

A wider viaduct would be especially noticeable at South Washington Street, near the ferry terminal.

Project officials plan to "flatten out" the existing curve there, to meet modern safety standards, said Amy Grotefendt, a design-team spokeswoman. That would require stretching the new road deck and its columns farther west, toward Elliott Bay. But on the inland side, the new columns wouldn't shift west, because the entire new structure would have to surround what's there now.

As a result, the width between columns at Washington Street would be stretched to 113 feet — more than double today's 51 feet. But the road deck would occupy only 76 feet and would rest on a crossbeam. Dye said the highway would look as if it were supported by outriggers.

Those who support building a tunnel, or using surface streets instead of a new highway, have mailed out campaign materials that depict an intimidating slab next to Pioneer Square.

The ads incorrectly say that a new viaduct would be "200 percent larger" at that spot; state data show a 122 percent increase, column to column.

On the positive side, Dye said, there would be only half as many columns as today. Each would be wider, for earthquake protection.

At mid-downtown, between Columbia and Madison streets, the width of the structure would increase from 51 feet to 75 feet.

Some other features are driving the bigger design:

• Safety shoulders, which don't exist on the current viaduct, would be built.

• A new interchange at the sports stadiums would require the stacked roadway to start diverging into a side-by-side layout at South King Street — several blocks north of where the spread happens now. So the width there would increase 151 percent — making it about 2 ½ times the current width.

• Overhead clearance would be needed to mount new traffic-message signs for the lower, southbound deck.

• Pedestrian bridges would be built to the ferry dock and mid-downtown.

• Two northbound street lanes and the waterfront streetcar might be moved inland, under the new viaduct. That would open up space for a shoreline promenade but would alter the distance between columns.

The state's strategy creates a dilemma for those who suggest putting the structure on a diet, by reducing lanes or shoulders.

Can you do that and still keep the highway open?

Dye said architectural features such as shapes, column locations and materials can be refined and maybe the deck size can shrink slightly, but the new columns would still need to be outside the old.