State transportation officials are warning of what will likely be a year-long traffic nightmare starting this fall for H1 commuters in town.

In September, construction will start on what’s dubbed the “H1 Rehabilitation Project” — an effort to fix and repave a 3.5-mile stretch of the state’s most heavily used highway between Ward Avenue and Middle Street.

The work will take place overnight, from 8 p.m. to 4 a.m., and allow construction crews to shut down the highway in one direction at a time for up to 120 nights throughout the project, state Department of Transportation officials say. It will also mean nightly partial lane closures during the yearlong effort.

“There is no good time in doing this much-needed work,” DOT Director Glenn Okimoto said this morning at a news conference to get the word out.  State officials have avoided the work in the past because of the large inconvenience it would mean for commuters, state Highways Adminisrator Alvin Takeshita added.

The work in town will also coincide with H-1 traffic further west from the state’s ongoing “PM Contraflow” project — highway deck repairs in the Aiea and Pearl City areas that have commuters facing lane closures there from 7:30 p.m. to 4:30 a.m.  That schedule is expected to last through April, state officials said.

The in-town rehabilitation project will repave an asphalt-concrete layer about 12 inches deep along that H1 stretch — a material that’s stronger than typical asphalt and designed to last 10-15 years, instead of 7 to 10 years, to better handle the 200,000 vehicles that use the corridor each day, said Jadine Urasaki, deputy director for capital projects.

The $42 million project, paid for mostly with federal dollars, will also reconfigure H-1 to have four lanes in both directions from Punahou Street to Middle Street. It will further replace existing highway lights with modern models that better contain the light to the highway and are more environmentally friendly for migrating birds, officials said.