WHATEVER ELSE the candidates for board chairman in Fairfax County may promise the voters, they already have guaranteed that the contest will be anything but dull. After a summer of separate stumping, the two main combatants -- incumbent John F. Herrity and challenger Audrey Moore are sharing stages if not views -- and their exchanges have been heating up at microwave speed. (Two other candidates, James S. Morris and R. Terry Robarge, keep trying to get words in edgewise, but so far have failed to capture significant attention.) Though the pace is quickening, the key topics remain the same: growth and transportation.

For the most part, the growth is over, or already under way, and so the candidates should be concentrating on transportation improvements. The question for voters should be which candidate is best equipped -- in terms of experience, political savvy, realism and ability -- to work with diverse businesses, jobholders, governments and citizens' groups to get roads built.

Mrs. Moore, responding to charges that she's been weak in the past in supporting transportation improvements, has proposed her "transportation initiative," which she says would expedite road projects and literally pave the way for future growth in the county. Mr. Herrity charges that her initiative is nothing new and in fact is a poor imitation of what he and a majority of the county supervisors have been supporting all along.

Who is right? In fact, there is nothing exactly novel in Mrs. Moore's plan. First, she proposes holding a voter referendum on a bond issue in early spring rather than in November 1988, as proposed by the county board. Mrs. Moore also proposes the development of a "comprehensive transportation plan," with a plan to finance it; the county's current plan is insufficient, she contends. Mr. Herrity counters that the county doesn't need another "series of commissions and studies" but needs to start building the roads.

The Moore plan also proposes bringing outlying counties into the Northern Virginia Transportation Commission. Mr. Herrity notes that the outer counties already have agreed to discuss such a regional reorganization at a meeting after the election. Mrs. Moore includes a proposal to establish a transportation commission to advise the county planning commission and to restructure various county agencies to help speed projects.

Planning is essential, that's for sure; but whether voters sitting in traffic will be content to hear the two candidates arguing about organizational adjustments and additional comprehensive studies seems doubtful. Their past records on transportation questions -- as well as their abilities to move bureaucracies -- are more to the point.