The Arlington County Board adopted a policy yesterday in favor of narrowing traffic-clogged lower Arlington Ridge Road from four lanes to two as requested by nearby residents, but first ordered a four-month test of how the change would work.

"On balance it's a step that will make it safer," said board vice chairman John G. Milliken of the board's compromise agreement on the test policy. "I do believe it will slow traffic down and will protect pedestrians and property owners."

The board's unanimous action came after dozens of Arlington Ridge residents, who have lobbied for the change for almost 10 years, complained about speeding cars creating safety risks when commuters use the road as a shortcut between Rte. I-395 and Crystal City and Alexandria.

"The main reason we are pushing for the changes is for safety," said Mike Hall, president of the Arlington Ridge Civic Association. "We need to protect our pedestrians and property."

The residents argued that the lower portion of the road, from S. 23rd Street to S. Glebe Road, should be narrowed to match the upper part of the road, from S. 23rd Street to the Shirley Highway access ramp.

Henry S. Hulme, head of the county's public works department, said the project, once estimated to cost about $350,000, would probably cost in excess of $600,000 now. Hulme said that during the test period, the county could put up wooden barriers along both sides of the lower road to test resident and driver reaction to a permanent narrowing.

Board members said they wanted to have a test period on the two-lane policy because they were not convinced it would make the road any safer or the residents in the neighborhood any happier.

The barriers, which will probably go up in about a month, are expected to create traffic snarls as commuters are forced to drive more slowly on their way to and from work.

The board included $30,000 in the fiscal 1984 budget for design studies on narrowing the road, and members said yesterday they would consider a possible November bond referendum to complete the project at their Aug. 13 meeting.

The board is scheduled to continue yesterday's meeting on Wednesday at 8 p.m., and also plans a special hearing Monday at 8 p.m. on Virginia Square Metro stop development plans. At yesterday's session, the board also:

* Voted unanimously in favor of allowing the county's industrial development authority to issue $2 million in industrial development bonds for the Greater Washington Educational Telecommunications Association to buy and renovate an office building at 3700 S. Four Mile Run.

WETA wants to consolidate its administrative, television and radio operations, which are scattered around the metropolitan area, at one site in Arlington. The office building WETA wants to buy is a few hundred feet from a building where the public broadcasting station currently rents space for its television studio. The station plans to move about 60 of its employes who now work in Washington into the renovated facility next December.

* Approved, 4 to 1, revised plans for a proposed $100 million renovation of the Parkington Shopping Center at Wilson Boulevard and N. Glebe Road. Board member Dorothy T. Grotos, who has opposed county plans to aid the renovation by selling $25 million in revenue bonds for an expanded eight-story garage there, dissented. She contended that the county should not be "subsidizing" private developers.

The major changes in the plan include adding a cinema, a food area and a third planned department store, expanding the retail mall from one to three stories and relocating a third office building.

* Approved the first phase of a planned $250 million redevelopment of the Shirlington Shopping Center off I-395 in South Arlington. The first phase includes a new five-story office building at the corner of S. Quincy and S. 28th streets, 370,000 square feet of renovated retail space and 1,075 parking spaces.