By next January, Metrobuses could be using a soon-to-be completed section of I-66 in Northern Virginia, six miles of road from the Beltway to the Ballston subway station in Arlington. The move, according to area transportation planners, could save commuters up to 25 minutes on nearly a dozen Northern Virginia-to-Washington bus routes.

The plan to open the Beltway-to-Ballston section of I-66 to Metrobuses, recently given preliminary approval by local governments and the state Department of Highways and Transportation, will give commuters a preview of the time savings expected when all 10 miles of the new Northern Virginia section of I-66 officially opens next June.

Next month, Metro is expected to hold public hearings on the plan, which would require changes in at least 10 bus routes, all originating or ending in Fairfax County. No hearing dates have been set.

The plan, if approved, would go into effect Jan. 4. Although no specific routes have been identified, county transportation planners say the areas most likely to be affected are Vienna and Oakton and routes outside the Beltway in the Lee Highway, Arlington Boulevard and Leesburg Pike corridors.

The six miles of I-66 from the Beltway to Glebe Road near Ballston are expected to be finished by October. Since Metrobus routes are changed only on a quarterly basis, January would be the first date the bus changes could be implemented, according to Metro spokesman Cody Pfanstiehl.

The final say on the changes will come from Fairfax officials, since the county, in effect, contracts for service from Metro, and must be willing to fund any changes. In the last nine months, the county has made several reductions in Metrobus service.

"The nice thing about this is that we've slashing bus service and now we can add something . . ." said Christopher Jenks, an associate transportation planner for the county. "I-66 will probably mean savings in dollars as well as huge savings in time, from 10 to 25 minutes on at least 10 bus routes."

While only Metrobuses are involved in the current plan, buses owned by private companies and charter buses also may be allowed to use I-66 before the official June 1982 opening, according to officials of the Northern Virginia Transportation Commission, which has helped negotiate the Metrobus plan.

When the entire 10-mile section -- stretching from the Beltway to Roosevelt Bridge just beyond Rosslyn -- officially opens, there will be several restrictions for rush hour traffic. The restrictions were imposed by U.S. Secretary of Transportation William Coleman in 1977 when he approved federal funding for the controversial project.

Under those requirements, I-66 will be open only to buses and four-person carpools during morning and evening rush hours. At other times, the road will be open to all traffic except trucks.

The project will cost $273 million when completed, with federal highway funds providing 90 percent of the funding, and Virginia highway funds the remaining 10 percent.