The Fairfax and Loudoun County boards of supervisors, in separate actions yesterday, agreed to link the long-planned Springfield Bypass to a four-lane road extending north of Rte. 7 and into Loudoun County, an action heralding "a new era of regional cooperation," Fairfax officials said.

The agreement, which paves the way for a major transportation route along the Loudoun-Fairfax border near Algonkian Regional Park, was approved 9 to 0 by the Fairfax board yesterday morning, and 5 to 1 by its Loudoun counterpart in the afternoon.

Loudoun officials said they voted on the issue, which was not on the board's agenda, as a gesture of renewed cooperation between the two counties, which have long battled over traffic from Loudoun subdivisions winding through neighborhoods in western Fairfax.

Under the agreement, a southerly leg of Loudoun's Algonkian Parkway would continue into Fairfax as a four-lane road, aligning with the proposed Springfield Bypass where Holly Knoll Drive meets Rte. 7. To accomplish this, the bypass, most of which has yet to be built, would likely be relocated a short distance where it meets Rte. 7.

In the past, Fairfax officials had insisted that the Fairfax portion of the Algonkian Parkway be two lanes.

In return for getting the four lanes they wanted, Loudoun officials will allow Fairfax to close -- should it choose to do so after a public hearing -- Thomas Avenue and Brockman Lane, the two main arteries leading from Fairfax into Loudoun north of Rte. 7.

"This is a solution that benefits both counties and is long overdue," said Fairfax Supervisor Lilla Richards (D-Dranesville), who represents the area.

Under the agreement, travelers heading north on the bypass would be able to cross Rte. 7 and continue on a four-lane road that will snake through several new communities in Loudoun and meet up with Rte. 7 again seven miles to the west.

By the year 2010, the parkway is expected to handle 23,900 vehicles daily, area developers say.

Loudoun Supervisor Steve W. Stockman (R-Broad Run), whose district's residents use Thomas Avenue to get to and from Rte. 7, attempted yesterday to delete from the agreement the provision allowing the closure of Thomas Avenue. Stockman also attempted to delay action until the board's next scheduled meeting. Both motions were met with silence.

"It's our lifeline, and you don't just cut it off for cooperation" with Fairfax County, Stockman said. "If this is the kind of cooperation we get from them, then I'd rather not cooperate."

In voting against the measure, Stockman said about 1,000 cars that travel daily along Thomas Avenue will have to use congested Rte. 7.

Staff writer John Ward Anderson contributed to this report.