Think of them as massive seesaws that construction crews will install above the busy H-1 freeway and Kamehameha Highway near Leeward Community College, forcing lane closures for the next year.

The work to traverse some of the busiest stretches of island roads with three concrete “spans” for Honolulu’s $5.26 billion rail project has never been tried in Hawaii over an operating road, said Karley Halsted, project manager for the West Oahu/Farrington Highway portion of the Honolulu Authority for Rapid Transportation’s rail project.

But the so-called “balanced cantilever structure” that acts like a seesaw until it’s permanently in place is standard in the construction industry for bridges and overpasses, Halsted said, and was used to build a portion of the H-3 freeway.

It was a far better choice than having to close most lanes of the H-1 and Kamehameha Highway to install the kind of rail spans going up over Kapolei farmland, she said.

“This is the least impact to traffic,” Halsted said.

Most of the first 10 miles of rail guideway is being built in Kapolei out of precast segments formed at a casting facility and trucked in, said HART spokesman Scott Ishikawa.

To minimize traffic disruptions, Ishikawa said, “the work over the H-1 freeway connecting Waipahu and Pearl City will all be built in place.”

The work will be done only from 8 p.m. to 4 a.m. Sundays through Thursdays to avoid traffic disruptions.

It will be “rather fascinating work that can be distracting to drivers,” Ishikawa said. So in a HART public service announcement airing on radio and television this month, nighttime drivers are urged to “look ahead, not overhead.”

Lanes already have been closed for the last five months for preliminary work, “and there have been minimal disruptions,” Ishikawa said.

The H-1 freeway has five westbound lanes and five eastbound lanes in the area. Kamehameha Highway has two lanes in each direction.

Construction crews are allowed to close up to four lanes at any time, but “we will never close the entire freeway in either direction,” Ishikawa said. “There will always be a lane open.”

In all, construction crews will install six piers to support five spans from Leeward Community College across the H-1 and Kame- hameha Highway to what will be the new Pearl Highlands rail station.

But only the work to install three spans directly over the freeway and highway will force lane closures.

The first lane closures for installation of the cantilever structures will occur on the H-1, followed by Kamehameha Highway. Exact dates for the closures have not been announced.

Once in place, the spans will sit atop columns that will stand anywhere from 281⁄2 feet to 36 feet above the H-1 and Kamehameha Highway.