The Fairfax County Board of Supervisors on Tuesday agreed unanimously to release $6 million from the Route 28 special taxing district’s funds to prepare final designs for widening four high-priority areas of the highway.

The request came from the Virginia Department of Transportation and also required similar action from the Loudoun County Board of Supervisors to keep the project on track, county staff said.

The areas involved include: Route 28 southbound between Sterling Boulevard and the Dulles Toll Road; the Route 28 southbound bridge over the Dulles Toll Road; Route 28 southbound between the Dulles Toll Road and Route 50; and Route 28 northbound between McLearen Road and the Dulles Toll Road.

Previously, the Board endorsed using about $1.1 million to begin partial design plans for widening those four areas, county staff said. Based on that work, the overall cost has been estimated at $48.1 million. Loudoun County’s Board of Supervisors approved the request on June 7.