After three years of closures and construction, commuters on the 91 Freeway in Corona will finally see some relief.

The new toll lanes and general lanes are opening Monday, March 20.

The $1.4 billion project on one of the region’s most congested freeways has added two toll lanes and one general-use lane in each direction on an 8-mile stretch from Interstate 15 west to the Orange County line.

The westbound lanes will open that day at 4 a.m., while the eastbound lanes debut at 5 a.m., Riverside County Transportation Commission officials announced Friday, March 10.

Anne Mayer, executive director of the commission, said transportation officials are delivering on their promise to open the new lanes by spring.

“It’s the perfect day — it’s the first day of spring, it’s the equinox,” she said. “We’re getting it open as we projected we would do.”

The milestone represents the culmination of a decade of work on the biggest project the agency has ever handled. Crews also built or widened 32 bridges, improved five interchanges and realigned and repaved local streets. A March 31 dedication ceremony for local officials is planned.

“It’s very, very challenging to build a project of this complexity and magnitude under live traffic conditions,” Mayer said. “This team has done an amazing job.”

But commuters shouldn’t start a party just yet.

There’s still a final big push that will involve multiple freeway lane and ramp closures in both directions as crews prepare the lanes to be used. The most significant work will start Thursday, March 16, and continue through that weekend.

On Sunday, March 19, in particular, drivers can expect delays of at least an hour. From Saturday night, March 18, until the lanes open at 5 a.m., the eastbound highway will be reduced to two to five lanes at various times, Project Manager David Thomas said.

Travelers should avoid the area and take the 57 or 60 freeways or instead use Metrolink, officials suggest.

Crews will be completing the striping on a 4-mile section from the county line to the 71, finishing some paving and bridge rails, Thomas said.

Work will be also be completed by March 20 on a connector from the 15 Freeway to the 91 as well as the last two ramps at Serfas Club Drive and Maple Street in Corona, he said.

Corona Mayor Dick Haley welcomed the announcement.

“I’m just happy that it’s going to be finished,” he said.

Over the course of the construction, drivers have suffered through the “Corona Squeeze” — during which the size of lanes was narrowed from 12 feet to 10.5 feet — and “Coronageddon,” a 55-hour shutdown of the entire freeway last February. The project also required numerous detours and ramp and road closures that sometimes caused traffic to back up onto city roads.

Because the project went through the heart of the Corona, it disrupted many residents’ lives, Haley said.

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