The new Highway 99 tunnel opens 10 years after Gov. Christine Gregoire chose a deep-bore tunnel to replace the Alaskan Way Viaduct and nearly two decades after an earthquake irreparably damaged the elevated highway.

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The new Highway 99 tunnel is set to open Feb. 4, 10 years after Gov. Christine Gregoire chose a deep-bore tunnel to replace the Alaskan Way Viaduct and nearly two decades after an earthquake irreparably damaged the elevated highway.

Once opened, drivers will pass up to 200 feet underground through a 1.7-mile tunnel between South Lake Union and Sodo.

Here are answers to a few of the most common questions Traffic Lab has received about the $3.3 billion project.

Traffic Lab is a Seattle Times project that digs into the region’s thorny transportation issues, spotlights promising approaches to easing gridlock, and helps readers find the best ways to get around. It is funded with the help of community sponsors Alaska Airlines, CenturyLink, Kemper Development Co., NHL Seattle, PEMCO Mutual Insurance Company and Seattle Children’s hospital. Seattle Times editors and reporters operate independently of our funders and maintain editorial control over Traffic Lab content.

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Do you have a question you don’t see answered? Email us at trafficlab@seattletimes.com.

When does the tunnel open Monday?

The Washington State Department of Transportation (WSDOT) expects to open the tunnel to vehicles early Monday, but the agency hasn’t given a specific time. WSDOT says the process will take several hours but the tunnel will be open for the morning commute.

Is the state celebrating the tunnel opening?

Free walking access and festivals at the viaduct and tunnel are scheduled Saturday, two days before cars take to the highway. The north and south hubs of the tunnel will open to visitors Saturday at 11 a.m., and people can walk the soon-to-be demolished viaduct and new tunnel beginning at 12:30 p.m.

As many as 100,000 people are expected Saturday. The free tickets allow access at a specific time to spread out the crowd. Walkers without a ticket (all tickets have been claimed) can line up at the Seneca Street ramp or the Battery Street tunnel and wait to get on the viaduct. WSDOT suggests visitors without a ticket show up between 1:30 and 5:30 p.m., but the agency won’t guarantee access past 6:30 p.m.

Ribbon-cutting ceremonies at the south end start at 11:30 a.m.

A 5-mile fun run begins at 7:30 a.m. Saturday. Cost is $45 and tickets are available.

On Sunday, the Cascade Bicycle Club is sponsoring a 12.5-mile ride from 8 to 11:45 a.m. That event is sold out, but the club’s Emerald City Ride on May 26 offers another chance to bike the tunnel.

Are there downtown exits inside the tunnel?

No. However, drivers exiting the tunnel at South Lake Union can turn right at Dexter Avenue North, then continue south across Denny Way toward downtown. Or they can take the Sodo interchange on the south end and head into downtown on First Avenue South or Alaskan Way South. Or instead of entering the tunnel, they can depart Highway 99 using new interchanges just before the tunnel portals, where WSDOT signs mark the exits as “Downtown.”

Could exits from the tunnel to downtown be added later?

No. The tunnel is 200 feet below street level mid-downtown. Adding exits would require huge amounts of money for minimal traffic improvement. Before the viaduct closed this month, 25,000 cars a day took the Seneca-Columbia exits. Viaduct bus routes that carried 25,000 or more people a day at the Seneca-Columbia exits will use new waterfront bus lanes along a rebuilt Alaskan Way South, starting in 2020.

How wide is the tunnel?

The tunnel is 32 feet wide on each of two levels. There are two 11-foot-wide southbound lanes on the upper deck and two 11-foot northbound lanes on the lower deck.

An 8-foot shoulder is on the waterfront side of both decks. Shoulders are positioned that way because the emergency-evacuation doors and passageway were built along the waterfront side of the tunnel. On the inland side, the shoulder is 2 feet wide.

Overhead clearance is 15 feet, 9 inches below the traffic-message signs.

What’s the speed limit?

The posted limit is 45 mph, to be reduced by electronic lane signs when traffic is heavy. Seattle police are responsible for enforcement.

How much is the toll?

Tunnel trips will be free until sometime this summer.

Then rates will vary by time of day, from $1 each direction in overnight hours to $1.50 for the morning commute and $2.25 at the peak afternoon commute. Private transit and registered van pools are free, but carpools will pay the toll.

Overhead scanners charge the toll as vehicles travel along the highway, as on the Highway 520 bridge.

Motorists need a Good to Go pass to avoid a $2 surcharge per trip, for those billed by mail. Order passes at GoodtoGo.org; call 866-936-8246 (7 a.m.-6 p.m. weekdays, 9 a.m.-1 p.m. Saturdays); or go to 4554 Ninth Avenue N.E., Seattle (University District), or 13107 N.E. 20th St., Suites 3 and 4, Bellevue (8 a.m.-5 p.m. weekdays, 9 a.m.-1 p.m. Saturdays).

Is cellphone service, Wi-Fi and radio reception available inside the tunnel?

Phone service and radio reception will be available, although WSDOT will override broadcasts to announce any emergencies inside the tunnel. No Wi-Fi is planned.

Are there alternatives for drivers who don’t want to use the tunnel through downtown?

One option is to drive the newly repaved four-lane Alaskan Way on the waterfront, although viaduct demolition and road construction will restrict some segments to two lanes.

First and Fourth avenues remain available but will be more crowded than usual, at least until waterfront bus lanes and increased Sound Transit light-rail capacity arrive in 2021. Interstate 5 is expected to absorb some traffic that diverts from Highway 99.

Will transit use the tunnel?

King County Metro has no near-term plans to use the tunnel. Buses that traveled the viaduct will change to surface streets, so they can reach busy downtown stops.

Employer-sponsored buses, airport shuttles, tour vehicles and registered van pools probably will use the tunnel to avoid downtown congestion.

A long-term Metro plan suggests tunnel express routes by 2040.

What are the hazards inside the tunnel?

Curves within the tunnel will limit drivers’ ability to see clogged traffic ahead, which could lead to rear-end crashes.

A prolonged blaze from a vehicle fire may release toxic fumes and cause embedded moisture in the concrete to explode like popcorn kernels. The tunnel is equipped with sprinklers, a fireproof mineral surface on ceilings and ventilation fans to suck out smoke. Petroleum tanker trucks are banned.

Emergency exits, marked in green, lead to a protected passageway where people can walk toward the nearest tunnel end.

Can the tunnel survive an earthquake?

Yes, that’s the whole point. The tunnel is engineered to withstand a once-in-2,500 year earthquake, when tectonic plates slip in the Cascadia Subduction Zone. The tunnel’s 2-foot-thick, reinforced-concrete rings would resist shear. By comparison, the viaduct gradually cracked and sank 6 inches since the Nisqually earthquake in 2001. The viaduct might have collapsed had the quake lasted a half-minute longer, as depicted in a 2009 project video.

Could a tsunami flood the tunnel?

Unlikely. Although a Pacific tsunami would destroy coastal towns like Westport, Tokeland and Long Beach, waves would dissipate by the time they reach Puget Sound.

That leaves the Seattle Fault under West Seattle, Elliott Bay and Beacon Hill as the main tsunami threat. A worst-case, magnitude-7.3 quake could send water 10 to 16 feet above sea level, barely high enough to reach the Sodo tunnel entrance, said T.J. McDonald, hazards-information coordinator for the city’s Office of Emergency Management.

Local researchers anticipate a more likely 6.7 quake, enough to kill 1,600 people if unreinforced brick buildings tumble. Such a quake would cause dangerous currents on shorelines but not create a tsunami.

What will the tunnel be called?

For now it’s the Highway 99 tunnel. A few people suggest “Christine Gregoire Tunnel” because she chose the deep-bored tunnel as governor in January 2009. Naming authority rests with the state Transportation Commission.

Got a question?

If you have a question we haven’t answered here, please send us an email at trafficlab@seattletimes.com. And for more information about the Highway 99 project, go to st.news/tunnel.