West Oahu-bound drivers have endured years of lane closures, project changes and completion delays while the state has worked to ease congestion through Central Oahu.

Now those thousands of drivers finally have an added lane on the H-1 freeway that should bring some relief.

On Friday state transportation officials announced that after having spent several years widening the freeway’s Pearl City viaduct, they’ve opened a new lane from the Aiea Heights Drive overpass to Waikele. The 2.5-mile Ewa-bound lane will accommodate more than 2,000 cars an hour, according to the state Department of Transportation. It will also help make the commute faster and smoother for drivers by eliminating or at least reducing bottlenecks at several points along that stretch, agency officials say.

Starting Monday, westbound commuters can also use a new afternoon shoulder lane that will be open from 3:30 to 6 p.m. weekdays from the Aiea Heights Drive overpass to the Waimalu/Pearl City offramp (Exit 10), according to the state Department of Transportation.

“We’re really excited that the residents can start using the benefits of that project,” Ed Sniffen, DOT’s deputy director for the Highways Division, said during a news conference Friday at the Kaahumanu Street overpass. “The project has been a long time coming.”

The start of the new lane also means that drivers will not encounter any more “major” lane closures associated with the state’s widening project, although they will likely see overnight closures of up to two lanes through the end of the year to help resurface the new lane and relocate signs, Sniffen said.

Meanwhile, officials with the Honolulu Authority for Rapid Transportation, which oversees construction of the island’s rail transit system, say that drivers will encounter overnight freeway lane closures in the next several weeks approaching the H-1/H-2 merge but nothing major after that. The bulk of rail construction continues to affect nearby Farrington and Kamehameha highways.

DOT officials previously had expected to finish their H-1 improvements by 2013, Sniffen said Friday, but an overhaul to the project after it had already started helped push back its completion by about two years.

Originally, the approximately $90 million project, funded 80 percent by the federal government, was called “PM Contraflow.” It planned to give drivers a 7.2-mile zipper lane between Pearl Harbor and Waikele to help ease Ewa-bound rush-hour traffic.

But months after DOT officials announced PM Contraflow in 2012, they began to privately reconsider whether a zipper lane was the best use of taxpayer dollars to mitigate one of the most notorious commutes on Oahu. State officials spent about $5 million on the zipper lane before suspending its construction in late 2013 and finally scrapping the idea for good by early 2014.

DOT officials have consistently pointed to the PM Contraflow project’s “design-build” contract, awarded to Hawaiian Dredging Construction Co., as the reason for the relatively late switch. Such contracts give extended flexibility to consider more effective approaches after they’re awarded. None of the zipper lane construction directly caused any lane closures, they said last year.

The state expects to save at least $1.7 million in annual operational costs by not having to run an afternoon zipper lane. It further expects to see more benefits to H-1 traffic by having an added lane segment for 24 hours instead of just for the afternoon commute. On Friday, Sniffen called the freeway widening a “better solution, better project, less operational costs,” while acknowledging it also extended the amount of time that drivers had to endure closures and construction.

However, DOT doesn’t have data on how much faster commutes might be with the added stretch of lane and afternoon shoulder, Sniffen said.

The department had expected a 10- to 15-minute savings for drivers based on data going back to the project’s beginnings in 2009, when it was still planned as a contra-flow project, he said.

“As we start monitoring the traffic as we go through, then definitely we can get a better number,” Sniffen added.