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Bountiful • The state's largest highway construction project of the year began Friday with a much-bigger-than-usual ceremonial shovel — a heavy machinery excavator.

That's appropriate for the big $117 million project that will add express lanes to Interstate 15 through south Davis County and rebuild key interchanges and bridges there. Officials warn the construction could bring gridlock unless 20 percent of daily commuters use alternate routes or take the FrontRunner commuter train.

Taking turns at the controls of machinery to officially start the project Friday were Utah Transportation Commission Chairman Jeffrey Holt, Utah House Majority Leader Brad Dee, R-Ogden, and Sen. Stuart Adams, R-Layton.

"This project will improve our highways and strengthen the economy with improved traffic flow," Carlos Braceras, executive director of the Utah Department of Transportation, told the crowd at the groundbreaking ceremony.

The project will add express lanes to 12 miles of freeway from North Salt Lake to Farmington, and will extend the nation's longest continuous stretch of express lanes to a total of 72 miles from Spanish Fork to Layton.

"We move more people in express lanes than we do general-purpose lanes" by encouraging carpooling, Braceras said. "So this will be able to help us address some of the congestion here as well as to help us clean our air with less pollution."

It will also replace aging bridges at 2600 South, 1500 South, 500 South and 400 North. Those bridges have shed some concrete onto the roadway, Braceras said. "So these are some of the bridges we've really needed to replace." The project will also reconfigure the interchanges at 2600 South and 500 South in Bountiful to improve east-west movement.

UDOT officials have said they need about a 20 percent reduction in normal traffic to avoid standstills during the project, and are urging use of the parallel Legacy Parkway as an alternative, along with FrontRunner.

To encourage train use during construction, the Utah Transit Authority is offering free seven-day passes to Davis County residents. They may register for the passes, while supplies last, at rideuta.com/I15-Davis-Pass.

UDOT said it plans to keep I-15 open with no restrictions during major commute times, and most lane restrictions and major work will take place at night. The project is scheduled for completion in fall of 2015.