The Arlington County Board voted yesterday to boost taxi fares in the county by 16 percent, making cab rides there the most expensive in any suburban Washington jurisdiction.

The board's unanimous action, which is effective immediately, boosts the cost of a three-mile trip from $2.90 to $3.40. The fare increase, the first since 1975, affects more than 400 cabs licensed in Arlington as well as many Arlington-based cabs that serve National Airport.

Cabs licensed by Washington and other jurisdictions that serve the airport are not affected by the increase.

The county's transporation commission had urged the board to grant a higher increase, which would have increased the cost of a three-mile trip to $3.60. That recommendation was opposed by half of the 20 cab drivers who attended yesterday's board meeting and by officials of several cab companies. They testified that a 20-cent increase would reduce the number of elderly Arlington residents who routinely used cabs for errands and short trips.

"People on expense accounts can afford to pay anything," said John Melnick, lawyer for the Red Top Cab Co. He urged the board to reject the additional increase because it would "hurt the little people" and cut into cab ridership.

"We do not want management to negotiate for us," countered cab driver David Mooney, who unsuccessfully argued for the higher increase, citing spiraling inflation and steadily increasing gas prices. "We've had the subway put in on us and that took at least 30 percent of our business."

In other action the board, in a closed door meeting, rejected a suggestion from a group of South Arlington residents that the county acquire a five-acre tract of marsh land for a park.

The county could have acquired the land through condemnation or by paying the $1.4 million requested by the Ohio-based Anden Corp., which owns the property. The company plans to build a townhouse development on the site.