The most costly road construction project in Utah's history will begin in the spring of 2010.

The I-15 corridor in Utah County, from American Fork to Spanish Fork, will get a full reconstruction. It will include an added lane to both sides of the freeway and new bridges, according to Utah Department of Transportation spokesman Nile Easton.

The 20-mile project will cost $2.6 billion and has been funded by the Legislature. The Utah County stretch of I-15 is "most critical because it is the most congested," Easton said.

Utah will begin right-of-way acquisition, which Easton said is labor intensive and "sets the stage" for the project. Consequently, he said, UDOT wants to ensure it is done correctly.

Forty-three miles of I-15 will be widened and rebuilt, from the south Payson interchange to the 12300 South interchange in Salt Lake County, but the Utah County portion is the only section that was funded.

Federal Highway Administration officials gave the project the green light Friday after an exhaustive, four-year environmental study.