Big Dig ready to open last major roadways

The beleaguered Big Dig's last major road opening happens next weekend with the debut of the final configuration of Interstate 93 southbound through the city.

The changes, set to take place March 5, weather permitting, include the reopening of the onramp to I-93 southbound from Congress Street, a new lane configuration in the southbound tunnel, and an extended car-pool lane on the southbound side of the Leonard P. Zakim Bunker Hill Bridge, officials said yesterday.

The opening ''is the last major milestone, the most significant milestone that we have left on this project," said Turnpike Authority chairman Matthew J. Amorello, who said the project remains on schedule for substantial completion this September.

The I-93 southbound tunnel opened more than a year ago in a temporary configuration to allow for the dismantling of the elevated Central Artery.

Next weekend, a fourth lane will be added to both the Zakim Bridge and the southbound tunnel, adding capacity for drivers coming from the north on I-93 or over the Tobin Bridge, which is currently experiencing backups from an 18-month rehabilitation project. The extra lane is also expected to help traffic flow better around Exit 23 (Purchase Street).

In addition to the added lane on the bridge, the car-pool lane on I-93 southbound will be extended nearly half a mile onto the bridge, feeding into the new fourth lane. That change is expected to end the infamous car-pool lane merge on the lower deck of I-93 southbound, which typically backs up traffic approaching the bridge.

By the end of March or the first week in April, Big Dig officials said, they plan to open the two new cantilevered lanes on the northbound side of the Zakim, plus the final direct connection from the westbound Mass. Pike to I-93 southbound.

Mac Daniel can be reached at mdaniel@globe.com.