A dangerous 3.5-mile section of Rte. 1, north of Fort Belvoir, will be widened and reconstructed at a cost currently estimated at $12.5 million, the Virginia Department of Highways and Transportation announced last week.

But state highway funds are in short supply, and work on the project is not expected to begin until 1985 or 1986 at the earliest. No funds for the Rte. 1 improvements are presently included in the state's long-range 1980-84 five-year highway construction program.

The section -- to be widened from four to six lanes and given a 16-foot grassy median -- extends north from the entrance to Fort Belvoir to Mount Vernon Road.

Fifteen feet of highway right-of-way will be acquired by the state to add to the existing 80 feet.

State officials say statistics show this section of Rte. 1 has a number of accidents higher than most other parts of the route and well above the state highway average.

The most recent statistics, for 1979, show that in the 3.5-mile section 335 accidents and 175 injuries per 100 million vehicle miles travelled have taken place, compared to the state averages of 235 and 127 respectively, according to state officials.

The new section will include curbs and gutters and a wide bicycle trail and walk.

The expanded road will follow the present highway location except opposite the historic Woodlawn Plantation. There the road will be widened on the eastern side of Rte. 1, now occupied by pastures and a horse stable.

Of the $60.6 million in funds allocated during the 1980-81 fiscal year for projects in the Culpeper district -- which includes most of Northern Virginia -- almost half are earmarked for I-66 construction, with another $7 million for work on I-95 and I-495 projects.