Fairfax County Board Chairman John F. Herrity (R-At Large) and Supervisor Audrey Moore (D-Annandale), meeting last night in their first broadcast debate of the campaign, took turns defending their records and blaming each other for the grueling traffic jams that have turned Fairfax into a commuter's nightmare.

In a one-hour segment on WMAL radio (630 AM) dominated by hard-hitting exchanges on traffic and development, the two major candidates for board chairman, Fairfax's top elected job, repeated the major themes of their campaigns in the sharpest terms they have used in public since they announced their candidacies in April.

Herrity, chairman since 1976, portrayed Moore's plan to slow the pace of growth in Fairfax as an "unrealistic" dream that had been tried in the early 1970s and rejected by Virginia's judges and lawmakers. He took responsibility for joining Moore in voting against road improvements in that era, but stressed that since 1976 he has championed road projects, while Moore continued to oppose them.

Moore, a member of the Board of Supervisors since 1972, repeatedly attacked Herrity for what she called "rush-to-develop policies," "poor management" and "totally unplanned growth" that have "strangled our existing road network."

While Herrity painted his efforts to lure new businesses and office buildings as a key to the county's robust economy and abundance of jobs, Moore challenged him: "This is the man who led the board for the last 12 years. If everything was so great, why didn't he" improve the road system?

Herrity tried to shift the focus away from Moore's linkage of development and traffic. Without growth, he said, "we don't build the schools, we don't build the police department, we don't build anything, and we don't get anywhere."

He also declared he was not to blame for the county's inability to make major road improvements because the state legislature did not give the county the authority to borrow money to pay for them until 1981.

Moore replied: "Why did he support the development . . . of the whole county if he couldn't get the money? He totally mismanaged the growth and development in Fairfax County . . . because he's been damn-the-torpedoes, full-speed-ahead, don't-worry-about-the-roads."

Herrity cited several of Moore's votes against major road projects, including the Dulles Toll Road and the still largely unbuilt Springfield Bypass, as "a huge mistake."

Each time he mentioned her votes, Moore dismissed them as inconsequential, turning again to Herrity's progrowth policies and their effect on traffic.

Refusing to acknowledge that link, Herrity pointed with pride at his record, stressing job growth, the proliferation of parks and libraries, and higher spending for road construction. He said Moore lacks the will and leadership to build new roads, and he belittled her plan to slow development as unrealistic.

In one of the few surprises of the debate, Herrity, who for years has attacked the District of Columbia's handling of problems at Lorton Reformatory in Fairfax, refused to take the bait when moderator Larry Matthews asked him about that issue. Instead, he spoke about accords between the District and Fairfax.

Moore, however, used the question to attack Herrity's often confrontational approach to neighboring jurisdictions, and to promise that her own style would be conciliatory.

Two independent candidates in the race for chairman, R. Terry Robarge and James S. Morris Jr., were not invited to attend the debate. Both said yesterday they were angry at WMAL for the slight.