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Behind the scenes, plot unfolds for 1-93 opening

If everything goes according to plan, daybreak atop the elevated Central Artery on Dec. 20 should bring the sound of silence -- the end of a six-decade era of driving along the elevated highway through the heart of Boston.

For that to happen, however, project managers for the $14.6 billion Big Dig must stage a sequence of engineering feats that must come together with the precision of a Balanchine ballet.

Project officials still will not commit to a date for the opening of the southbound Interstate 93, but say they are on a schedule for it to start the evening of Dec. 19. Most of the mile-long main tunnel section has been paved, and construction equipment is being removed.

Two big traffic changes will signal the run-up to the opening over the next two weeks. The first is the extension of Cross Street from Atlantic Avenue by Christopher Columbus Park to Hanover Street, allowing drivers to travel to North Washington Street and on to Charlestown.

The second is a new ramp from the Ted Williams Tunnelconnector westbound on I-90 to I-93 south, a significant change that will allow drivers to travel directly from Logan International Airport to points south of the city.

Noncommercial drivers must now use surface streets to get on I-93 south, or the Sumner Tunnel to the Central Artery.

The new southbound tunnel will feature two onramps -- one from Storrow Drive east at Leverett Circle, which will join I-93 south at the base of the Leonard P. Zakim Bunker Hill Bridge near the FleetCenter, and the other from New Chardon Street near the Government Center parking garage. The tunnel will have two exits, one for Government Center and the Callahan Tunnel, and the other a new South Station exit at Purchase Street, which is just now being completed.

City and state public safety officials must then give the new roadway system their seal of approval, including a test of all cameras and air-quality sensors.

Under the opening plan, sometime in the early hours of Saturday, Dec. 20, I-93 south will have to be closed, with traffic diverted through Chinatown.

The last part of the new southbound tunnel must then be lined up with the mouth of the existing Dewey Square tunnel. Drivers will travel through the Dewey Square tunnel as they do now.

Finally, north of the city, the last concrete barriers will be pushed aside so I-93 south drivers can bid farewell to the current bottlenecks on the old double deck, and head down the ample expanse of the Zakim bridge and into the new southbound tunnel.

After the weekend opening, the new roadway system will get tested for real with a Monday morning commute. At the same time, the dismantling of the old elevated Central Artery will begin.

Traffic headaches won't be over, however.

The elevated structures loom over ramps and other parts of the new roadway system, and it's too dangerous for demolition to be done over live traffic, so overnight closures will be scheduled.

Anthony Flint can be reached at flint@globe.com.

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