SEATTLE - After months of planning and preparation, the northbound Interstate 5 closures through downtown Seattle started Friday night.

Northbound lanes will remain closed for 19 days as construction crews complete one of the largest road projects in state history.

On Saturday morning, crews were on schedule, according to the state Department of Transportation, and traffic was light. At 10 a.m. workers started paving the Spokane Street on-ramp, a project which was expected to be done by 10 p.m. Crews also began removing the first aging expansion joints on I-5.

Lane closures began at 9 p.m. Friday night. By 10 p.m., three lanes of northbound I-5 were closed between Spokane St. and I-90, and crews began closing: the Columbian Way on-ramp to northbound I-5, the West Seattle freeway and Spokane Street on-ramps to northbound I-5, the northbound I-5 ramp to I-90 and Dearborn.

The lane closures are expected to throw the commute in and around Seattle into a tailspin. The state says the only way to avoid gridlock from 4 a.m. to midnight each day is to significantly reduce the number of cars on the road.

It seems people are getting the message.

Traffic volumes in the work zone were light Saturday morning, but building steadily.

During Friday's evening rush, there was a 15 to 20 percent reduction in traffic.

"I am going to ride my bike and take the water taxi," says one West Seattle commuter.

West Seattle is going to feel the first real impact. During stage one of the closure this weekend, there will be no way onto I-5 north from the West Seattle Bridge. The northbound collector-distributor and the off-ramp from northbound I-5 to I-90 will also be closed, along with two left lanes of the main freeway.

Monday morning, the West Seattle Bridge, I-90 ramps and the collector-distributor lanes re-open, but the 4th Avenue S. off-ramp shuts down.

What it's all about

The elevated span of roadway is 40 years old and needs repairs. The steel plates that hold the road together are failing. They're called expansion joints because they expand and contract with the weather. The fix will add years to the life of the roadway.

DOT officials have re-timed more than 90 signals on several main alternative routes: Rainier, Fourth Avenue, Airport Way South, Beacon Avenue and East Marginal Way. Airport Way will also have a northbound HOV lane and have multiple lights go green for more than three minutes during peak travel times. 

WSDOT emergency center ready to roll

WSDOT will be monitoring everything from their emergency operations center in Shoreline, which is loaded with monitors, cameras and traffic staffers.

There will also be troopers and incident responders standing by on the roads. WSDOT has also established a phone bank for questions from commuters.

"We've had people from as far away as Missouri call, concerned about coming to our state," said Interim Washington Transportation Secretary Paula Hammond.

The big X-factor is the weather.

"We have to have dry conditions for the bonding of the epoxy to the bridge. We're very nervous about the weather," said Hammond.

She says the work to replace the expansion joints on I-5 can be done in inclement weather and some wiggle room has already been set in case of rain.

Hammond didn't express a lot of concern about Mariners or Seahawks traffic during the shutdown, and believes the massive media blitz and those warning signs on the freeway will help people avoid the crunch.

I-5 closure to impact Southcenter, Eastside, Everett

Other main detour routes could also be impacted, if you believe the traffic analysts at Kirkland Inrix. While they have some good news for worried drivers, they predict problems for some drivers who detour I-5 by driving up 405 and crossing over 520.

At Inrix, a Kirkland firm that supplies both real-time traffic data and predictions of future flow, scientists say there will be plenty of headaches – potentially at Southcenter, Everett and Kirkland.

"We're expecting that next Monday isn't going to be as bad as a doomsday scenario that have been talked about so far," said Oliver Downs, a traffic scientist with Inrix.

Using historical traffic data and statistics, Inrix researchers created a simulation of traffic when the state shuts down northbound I-5 near the I-90 interchange.

The major side-effect: As drivers avoid I-90, they will clog up State Route 520 trying to get to Seattle.

Researchers say as those people merge onto I-5, it will exacerbate the so-called Mercer Mess, backing up traffic on I-5 southbound.

"And that will propagate all the way up to Everett for drivers coming to the south," Downs said.

The cascading impact continues: Because there's a backup on I-5, scientists say drivers will head for 405 and that will lead to backups in Kirkland near the Totem Lake area.

Another area of concern: Southcenter.

"Where I-5 northbound meets 405, we're expecting significantly increased congestion there as people decide to take 405 northbound instead of I-5," Downs said.

Researchers say that will clog traffic on State Route 167.

And this whole scenario could change if the unpredictable happens.

"A minor event such as an accident or a stall or somebody running out of gas in the construction zone could have the ability to impact the overall system and cause a catastrophic problem much more so than usual," said Bryan Mistele, CEO of Inrix.

Inrix scientists say the saving grace here is human nature. People always want the quickest route. Researchers have seen it during past closures, such as most recently the shutdown of I-90 for the Blue Angels.

And because people will adjust, that's why Inrix is not predicting a doomsday scenario.

More parking for Auburn commuters

The city of Auburn wants to make it easier for residents to take Sound Transit to avoid the I-5 crunch.

The city is freeing up 45 parking spaces in the Auburn Transit Center garage for Sounder commuter rail users.

Those spots are normally used for downtown Auburn merchants and residents on a leased basis.

The spaces will be available for the duration of the I-5 closure, while Sound Transit adds extra trains.

Crews motivated to finish early

During the 19-day around-the-clock closure, crews working for Concrete Barrier Inc. will break apart old concrete surrounding 34 old, worn-out expansion joints, install new expansion joints and repave 1.13 miles of northbound I-5 between Spokane St and I-90.

"We have spent months planning and talking to the public; we are hopeful that everyone knows and has plans to get around Seattle during this difficult time," Secretary of Transportation Paula Hammond said.

Hammond said she has every reason to believe the public will cooperate.

"We are so fortunate to have a driving public that pays attention to closures and is truly plugged in," said Hammond. "In the past when we have asked them to make smart decisions about their commute, they have. For that, we are grateful."

"People must know that if the contractor gets ahead of schedule, the dates and times of lane closures will change. Those changes could happen as early as Wednesday," said WSDOT Chief Engineer Paul Johnson said.

Contractor Concrete Barrier Inc. has plenty of motivation to get in, do the work, and get out early. For every day they finish early, the contractor will receive a $100,000 bonus.

"It's a win-win for everyone," said Hammond. "They get the money and drivers get the freeway open early."

By the numbers

Thirty-four expansion joints will be replaced. The total length of those joints adds up to 2,162 feet.

Almost nine acres will be paved, requiring somewhere between 38,000 and 56,000 cubic feet of concrete.

More than 100 workers will make sure it gets done.

On an average weekday, 126,000 drivers travel the northbound lanes of that section of I-5. 134,000 vehicles go southbound.

Drivers can now get real time traffic and weather information by dialing 511 from most phones. Callers can also get statewide construction, mountain pass condition, and state ferry system information, as well as toll free numbers for passenger rail and airlines.

KING 5's Jim Forman, Bernard Choi and Chris Daniels contributed to this report.