The Arlington County Board voted unanimously yesterday to cut the 1986 real estate tax rate by 1 cent, a decrease made possible by revised revenue estimates in large part tied to the county's rapidly expanding commercial base.

That same expansion figured in the board's unanimous decision to reaffirm a controversial plan to limit access to Arlington Boulevard (Rte. 50) from crisscrossing residential streets. But the board ordered that no permanent change be made to Rte. 50 until measures to improve traffic management and safety are proven to be inadequate.

That vote came in response to growing concerns among residents of neighborhoods along part of the road. They complain that commuters are using their streets as shortcuts to the Rosslyn-Ballston development corridor, which is evolving into a major employment center.

At the same time, the board killed a controversial plan to build a vehicular overpass on Rte. 50 between Highland and Garfield streets.

The board's vote on the real estate tax rate came after its only Republican member, Michael E. Brunner, tried unsuccessfully to persuade the four Democrats to cut the rate by 2 cents.

The 1-cent cut advocated by County Manager Larry J. Brown will decrease the county's real estate tax rate from 95 to 94 cents per $100 assessed value, maintaining the county's status as having the lowest rate in the metropolitan area.

But homeowners will get higher tax bills despite the cut, because assessments have increased an average of 3.9 percent over last year.

The average assessment on a single-family house, $124,176 last year, increased to $129,018 this year, resulting in a $1,213 tax bill -- up $33, or 2.8 percent, from last year. That is the smallest increase in three years, a fact that was attributed to the county's unprecedented commercial growth.

"We are in a booming community, and we're seeing the positive benefits of that. But one of these days, the boom will slow," said board member John G. Milliken, who contended that a 1-cent cut would be more prudent.

It was the fourth year in a row that the board has cut the rate. This year's decision was tied to revised revenue estimates showing that the county will have $9.4 million in extra funds beyond the original budget estimate for fiscal 1987, which will begin July 1.

When a proposed $1.5 million contingency fund is included, the board would have $10.9 million more than it had expected when the proposed fiscal 1987 budget was drafted. Board members indicated that the funds will be used to cut other taxes or make improvements to county and school programs when they adopt the budget on April 26.

Brunner, in seeking the 2-cent cut, said some of the revenue estimates were low, and that the board could easily reduce the real estate rate more and have enough funds to cut other taxes and make significant program improvements.

But the Democrats sided with Brown's recommendation, citing the uncertainties over expected cuts in federal aid and the need to cut business license taxes to meet state-mandated ceilings. They also said they may want to cut the personal property tax of $4.65 per $100 assessed value on cars, boats and office equipment. That tax rate also will be set on April 26.

Brown said that lowering the license taxes would have a greater effect on businesses than would cutting the real estate rate because assessments on the average commercial property rose 18.3 percent from last year. Assessments on Arlington's hotels and motels rose an average of 30 percent.

This year, for the first time, commercial properties will account for more than half, or 51.4 percent, of the county's $10.3 billion tax base. Arlington counts rental apartment buildings as commercial property.

The board's vote to re-endorse the concept of limiting access to Rte. 50 between Glebe Road and Washington Boulevard came as members continued the lengthy process of updating the county's master transportation plan.

As part of that procedure, the board must vote to delete or reaffirm proposed projects that have been in the plan for decades, some of which may never be built because of a lack of funds or changing needs. But projects are often included in the plan as a safeguard, a planning tool that gives developers, residents and the state notice of what the county's intentions are.

Board members indicated that such safeguards were behind the vote to re-endorse a controlled-access Rte. 50, a major commuter route for residents of the outer suburbs and the official dividing line between North and South Arlington.

Although one adjacent neighborhood lobbied for controlled access, others raised concerns that such a move would hamper their ability to get to churches and shops on Rte. 50. They also argued that the commuter congestion came primarily during rush hours and could be controlled by conventional traffic management techniques, such as forbidding turns at certain hours.

The board members, seeking a compromise, coupled their endorsement of limited access on Rte. 50 with the decision not to make any changes until after other traffic control plans prove to be insufficient. The board told county officials to meet with residents to develop a traffic management program.

The board said any changes would have to take into consideration the importance of cross-county connections to residents. "If [traffic's] not a 24-hour-a-day problem, why have a 24-hour-a-day solution? There are other [projects] that have a higher priority for us and are less costly," said Board Chairman Mary Margaret Whipple.