A $15 million U.S. Department of Transportation grant awarded to the state will help subsidize a project to add lanes to Interstate 25 in Adams County.

The $44.3 million project involves express lanes for those willing to pay tolls, multi-passenger vehicles and express buses on a six-mile stretch of the freeway between U.S. Highway 36 and 120th Avenue through Northglenn and Thornton.

The grant is one of 47 — totaling $500 million — distributed across 34 states as part of the federal Transportation Generating Economic Recovery (TIGER) program.

Toll lanes exist on I-25 between U.S. 36 and downtown Denver, but CDOT spokeswoman Stacey Stegman said those alone are not enough to accommodate the 175,0 vehicles traveling daily through the area. During peak hours, she said the roadway operates at less than half the posted speed limit.

“It’s totally unpredictable, so the reason why we wanted to build this roadway was to provide some reliability for commuters,” Stegman said. “It’s really difficult to make plans, because it’s hard to tell what the traffic levels will look like, but this project should provide some relief and assurance for driver’s on the road.”

Stegman said construction, which will be funded by state gas tax revenues and $5.4 million in local funds, is expected to begin in fall 2012 and end in fall 2013. She said peak morning and afternoon commuters should experience few disruptions, since most construction work will take place in the evening.

Gene Putman, Thornton transportation planning manager, said the section of I-25 between East 120th Avenue to East 84th Avenue is one of the most congested highway segments in the state.

“I get a lot of calls from people who are disturbed, at the very least, at the amount of congestion and why isn’t there some effort to help put more lanes on I-25,” Putman said. “To me, this a major step forward from a transportation-engineering perspective.”

When the project was first submitted to the federal government two years ago, Putnam said it was rejected after local municipalities only contributed $500,000. This time around, nearly 24 local governments, including Federal Heights, Broomfield, Weld County and Longmont, either agreed to contribute money or wrote project support letters.