Construction has started on a truck climbing lane for vehicles exiting Interstate 80 onto Jamieson Canyon Road/Highway 12 headed for Napa County.

This $7 million project should reduce traffic backup onto the interstate during the morning commute and other peak traffic periods, said Keith Wayne, a Caltrans spokesman.

Construction will take a full year, but efforts will be taken to minimize the impact on motorists, Wayne said. There will be traffic controls and some detours between 10 p.m. and 5 a.m., he said.

The new truck climbing lane will run 1.25 miles from I-80 to the top of the grade west of Red Top Road in Solano County, Wayne said.

This is a steep grade of 6.7 percent. Heavy vehicles lose momentum as they come off the interstate, backing up other traffic, he said.

When the new lane is finished, slow-moving traffic will be able to get out of the way of vehicles moving at the speed limit, he said.

In preparation for construction, workers have been surveying the roadway and cutting down trees.

A much bigger project, the widening of Jamieson Canyon from I-80 to Highway 29 in Napa, is scheduled to begin in several years. That project is budgeted at $139 million.