Tolls to drive through a future Alaskan Way Tunnel would be far higher at peak times than at night or midday, according to the state Department of Transportation.

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Tolls to drive through a future Alaskan Way Tunnel would be far higher at peak times than at night or midday, according to the state Department of Transportation.

A commute from Green Lake to Sodo might cost $3.50 in the morning, and $4 heading home in the afternoon, after completion of the tunnel in 2015. At other times, tolls could be as low as $2 midday or $1 overnight.

The off-peak discount is meant to prevent diversion onto surface streets, including First and Fourth avenues south, or at South Lake Union, said program administrator Ron Paananen. If tolls stayed at a flat rate no matter the time of day, as many as half the drivers would detour, he said.

This scenario — one of five — would raise roughly the $400 million the Department of Transportation (DOT) needs to go along with $2.4 billion in available gas taxes and federal grants. Other scenarios reflect a range of toll rates, or include tolls at nearby interchanges.

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The state has pledged to cover $2.8 billion in design and construction costs, with $300 million more promised by the Port of Seattle, for highway work.

The city of Seattle is to pay hundreds of millions more toward waterfront parks, utility relocation, reconstruction of Mercer Street and other work. Last week, Mayor Mike McGinn proposed a $241 million property-tax vote in May to replace an old sea wall.

The DOT submitted a report with updated cost and tolling estimates to state lawmakers Tuesday and gave news briefings in Seattle.

Engineering is about 15 percent completed now, compared with 5 percent a year ago when a bored-tunnel plan was announced, Paananen said. Construction cost estimates for the bored-tunnel portion have risen somewhat — to $1.96 billion, instead of $1.9 billion a year ago.

One reason is a higher allowance for soil work beneath downtown, where a major mistake could weaken the foundations of office towers. “It was just an area where we were probably light on the estimate last year,” he said.

On a smaller scale, Sound Transit’s Beacon Hill Tunnel for light rail caused voids in the soil beneath homes and streets, because of soil slides or excessive soil removal.

The DOT has conducted test drills to sample the soils downtown, and surveyed 300 buildings, Paananen said. To strengthen the soil, concrete grout can be injected, or underground braces can be added beneath vulnerable buildings.

The second reason for an increase is that the tunnel now will be 640 feet longer in Sodo. The DOT is shifting the Sodo portal a block west to Alaskan Way, instead of First Avenue South — too close to historic buildings that sit on shallow foundations in weak soil.

About $53 million is being saved by simplifying the Sodo interchange, officials said. Instead of a curving truck underpass, a cheaper overpass would be built linking the waterfront to Edgar Martinez Drive South.

In South Lake Union, the highway will emerge a block west of the present Aurora Avenue North, then slant over to meet Aurora north of Mercer Street. This new alignment keeps the Battery Street Tunnel open during construction.

There will be no toll booths, because tolls will be collected electronically.

An important question, to be decided by lawmakers, is whether tolls would be charged only to tunnel users, or also be applied to drivers who use new interchanges at Sodo and South Lake Union but don’t enter the tunnel.

That issue hasn’t come up yet, said Senate Transportation Chairwoman Mary Margaret Haugen, D-Camano Island. She said it’s costly to add tolling equipment to the interchanges. Haugen said the Legislature can wait until next year to authorize tunnel tolls.

Though a tunnel is unpopular with many citizens, both the Legislature and Seattle City Council have chosen that approach to replace the aging Alaskan Way Viaduct, at risk of failure in a severe earthquake.

McGinn, who opposed the tunnel during most of his campaign, was traveling to Washington, D.C., on Tuesday and hadn’t seen the new report, a spokesman said.

The toll strategy, to reduce off-peak traffic diversion, is welcome news to Dave Gering, executive director of the Manufacturing Industrial Council, who favors keeping traffic in the tunnel, not detouring to I-5.

Mike Lindblom: 206-515-5631

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