The final portion of a six-part construction project to open up Route 28 between Route 7 and Interstate 66 gets underway Tuesday, when state and local officials will help break ground on the Route 28-Sterling Boulevard interchange.

The latest phase will involve the construction of a partial cloverleaf interchange and a four-lane overpass for Sterling Boulevard. The project is scheduled to be completed in spring 2007.

The Route 28 project dates to 1985, when state transportation officials unveiled plans to turn the 14 miles of road between I-66 and Route 7 into a limited-access expressway. The road was widened from two lanes to six over a four-year period that ended in 1991.

Then, in 2002, businesses and owners of commercial property in the Route 28 area agreed to pay higher property taxes to help finance six new interchanges, thereby getting rid of six of the 10 traffic lights on that stretch of road. The tax revenue is covering $130 million of the $200 million price tag; the Virginia Department of Transportation is paying the rest.

Four of those interchanges have been completed and are open; work on the interchange at McLearen Road began in February and should be completed next summer.

More than 100,000 vehicles travel daily on the segment of Route 28 just north of the Dulles Toll Road, a number that is projected to double by 2012.

As a result, further improvements are being considered, including the addition of four interchanges and further widening of Route 28, to eight lanes.

Tuesday's groundbreaking ceremony, which will include officials from Shirley Contracting Co. and Clark Construction Group, is to be held at 8:30 a.m.