Fairfax County Board Chairman John F. Herrity was charged Saturday with driving 34 miles per hour over the legal speed limit on Rolling Road in West Springfield -- the fourth traffic infraction with which the county's top elected official has been charged in the past 15 months.

Herrity, whose driving style is legend in county government circles and has earned him at least three recent convictions, was ticketed about 9:30 a.m. after a Fairfax County police officer reported clocking him driving 74 mph in a 40 mph zone, officials said.

Herrity, who is in the midst of a tight reelection campaign against Supervisor Audrey Moore (D-Annandale), said yesterday he was stopped while driving to an Independence Day parade in Fairfax City. He said he will probably admit the charge and pay a fine rather than contest the ticket in court.

The fine for speeding is $20, plus $2 for every mile per hour over the speed limit, according to the county magistrates' office.

In addition, the Virginia Department of Motor Vehicles may assess "points" for moving violations and may apply punitive measures, ending in license suspension, for increasing point totals, Fairfax County Commonwealth's Attorney Robert F. Horan Jr. said in response to a question last night.

Herrity said he does not believe that he is in danger of having his driver's license revoked, but he acknowledged that his style behind the wheel is becoming a problem. "I'm trying," he said. "I do think I need to . . . slow down."

The chairman was convicted in Fairfax General District Court of running a stop light in Fairfax City in March, driving 76 mph in a 55 mph zone in December, and failing to maintain control of his vehicle in an accident in April 1986.