VOTERS IN ARLINGTON County become experienced monitors of their local government in no time flat -- thanks to a rotation schedule of elections that puts one or two seats on the Arlington County Board up for election each year. This time the contest is between the county board's vice chairman, Democrat John G. Milliken, who also is endorsed by the Arlingtonians for a Better County, and Peter Espada, an independent who has the endorsement of the Arlington County Republican Committee.

Mr. Milliken's service on the board since 1981 and his strong record of sensible leadership in regional affairs has won praise from Republicans as well as Democrats -- and he is by far the stronger candidate in this contest.

This may explain the failure so far of Mr. Espada to make effective arguments for replacing the incumbent. Much of his challenge has been based on Mr. Milliken's role as chairman of the Metro board -- which happens to be distinguished. Thanks to some delicate negotiations with all the regional officials who served on this transit board, Mr. Milliken and Metro General Manager Carmen Turner were able to obtain an area-wide consensus that has improved Metro's standing with the Reagan administration and thus the hopes of completing the 101- mile subway system as originally contemplated.

Mr. Espada also has attempted to portray Mr. Milliken as being responsible for waste and mismanagement of Metro -- when, in fact, Mr. Milliken has made cost-cutting and more efficient management a theme in his time as board chairman. Mr. Milliken has the support of Mrs. Turner and fellow board members for this effort. On other county issues -- development, money for the schools and taxes -- he has been an independent and thoughtful contributor to board deliberations. Mr. Milliken works well with his colleagues and deserves reelection.

Arlingtonians also are being asked for decisions on four bond issues involving transportation, conservation, park and recreation development and street and highway improvements. This approach to capital funds enables the county to spread the costs over many years, and responsible leaders from all corners of the county are recommending "YES" votes.