While the long-awaited Metro rail system is helping increase property values along its path, real estate along the 9.6-mile corridor of the long-fought Interstate Rte. 66 in Arlington is not faring as well, realty brokers and county officials indicate. Some homeowners along the route, which cuts across north Arlington south to Rosslyn, say they are worried that their property values will be permanently hurt once the road is open.

Although county records show that property in the I-66 area is still appreciating, it has not been rising nearly as fast as other land values in Arlington -- where values climbed an average of 18 percent last year -- and is taking longer to sell.

When construction on the final I-66 link to the District resumed in 1978, after being blocked for years by county residents, Arlington tax assessors automatically subtracted 25 percent from land values of properties abutting the road's right-of-way, said James R. Vinson, real estate assessments director.

A sampling of property values for that area conducted recently by Vinson's staff revealed that the average assessment for 96 properties in the corridor was about $64,000 last year and $75,000 this year. That compares with a countywide average of $73,000 in 1979 and $87,000 this year.

Realtors interviewed, as well as tax assessor Vinson, believe values will be restored after the highway's scheduled completion in 1982. Peter Blann, of the Routh Robbins realty firm, contends that people will then discover that the highway is "not the monster it's supposed to be." He estimated that price increases along the route have been limited to 8 to 9 percent since construction started.

Richard Frank a broker for Century 21-Rowan and Estes, says he recommends I-66 properties to bargain hunters.

"All you have to do is look at the Beltway," he said. "You've got the same situation (along I-66) now as you did there when it was first built. Now there are $250,000 homes in McLean backing up to the Beltway."

He added, though, that he advises owners of homes alone I-66 not to sell now if they can afford to wait.

It is, he observes, the worst possible time for them to sell.

About a dozen homeowners in the corridor said in interviews that they were skeptical that the interstate will ever help their propertys' values, even when the factor of increased access is considered.

Realtor Becky DePriest and her husband bought their home at 2733 N. 23rd Rd. for $73,000 three years ago "in hopes the road wouldn't go in," she said. She said the access route will benefit them little since they would "have to go nearly to Washington to get on it anyway."

However, she believes her neighborhood will be luckier than others along the route since residents there fought the road actively and won some concessions from the planners.

"We're the best-protected neighborhood," she said, looking across the street as workmen put finishing touches on a park that runs beside the highway.

Although she is pleased with the landscaping being done along the road, she noted that Virginia's decision to erect high-intensity lights along the entire length of the roadway -- instead of just at intersections -- will be another factor that will make it harder to sell houses there.

Some Realtors indicated that they steer clear of properties next to major arteries.

"I don't even show houses close to major highways," said Ben Hester, a broker for Home Sweet Homes in Falls Church, "especially to families with children . . . . "When you get there the people won't even get out of the car."

Last year, two agents with the Shannon and Luchs real estate company were brought before the Northern Virginia Board of Realtors on charges of unethical conduct after they sent letters to homeowners in the I-66 area suggesting that they consider relocating. The board did not uphold the charges.

Their supervisor, broker Robert Pannier, said a survey he conducted last spring showed that I-66 properties were on the market an average of 120 days, compared with 27 days for houses in other areas of Arlington.

Pannier contends that his findings provided justification for his agents' letters.

Meanwhile, many homeowners who found unsuccessfully against the highway extention are now bitter about the effect on their property.

Tamara Trops, who lost part of her yard at 1601 N. Quincy St., said she believes the only way I-66 property owners will get good prices will be to sell their land for commercial use.