Seeking to keep pace with a growing number of states and localities, the Fairfax County Board of Supervisors called on the Virginia General Assembly yesterday to try again during its upcoming session to adopt mandatory seat belt legislation.

With only board Chairman John F. Herrity objecting, the board voted 7 to 1 for a measure sponsored by Supervisor Audrey Moore (D-Annandale) that would require drivers and front-seat passengers to use seat belts.

Two legislators who tried unsuccessfully last year to steer such a bill through the assembly said in separate interviews yesterday that they will renew their effort during the 1986 session, which opens next month. And they were optimistic that the measure would pass this time.

"I think the mood is a lot more favorable than it was 12 months ago," said Del. J. Samuel Glasscock (D-Suffolk). "Some ideas just take a little longer than others to take hold."

State Sen. Charles L. Waddell (D-Loudoun) said he and Glasscock will sponsor the mandatory seat belt legislation, which passed 51 to 49 in the House last year but died in the Senate Transportation Committee on a 9-to-6 vote.

Sentiment has been building for laws requiring the use of seat belts, which safety experts contend save lives and reduce the severity of injuries.

Fourteen states and the District of Columbia have adopted mandatory seat belt legislation during the past year, joining New York and New Jersey, said Mark C. Christie, spokesman for the Virginia Auto Safety Alliance, which promotes the use of seat belts. The District law was signed by Mayor Marion Barry in October and is pending the 30-day congressional review process.

In Virginia, Gov. Charles S. Robb recently signed an executive order requiring state employes to use seat belts. The requirement already had been in effect for state police personnel.

"The statistics are overwhelming," Moore said in asking the board to endorse the legislation. State motor vehicle officials, she said, have estimated that at least 200 victims of Virginia traffic accidents last year would have survived last year in Virginia and an additional 6,500 would have escaped serious injury if they had been using seat belts.

Moore said support of the measure by the state's largest county could alleviate the political pressures on legislators wary of passing legislation opposed by some as too intrusive.

Herrity, the lone dissenting voice on the board, said Moore's proposal was an example of another unnecessary law being "foisted on the public."

"I do not believe this law is enforceable and it's an infringement on people's personal rights," he said.

But several supervisors who usually side with Herrity opposed him on the seat belt issue. Supervisors Nancy K. Falck (R-McLean), Elaine N. McConnell (R-Springfield) and Joseph Alexander (D-Lee) all backed Moore on the measure.

Also supporting the proposal were Supervisors James M. Scott (D-Providence), Thomas M. Davis (R-Mason) and board Vice Chairman Martha V. Pennino (D-Centreville). T. Farrell Egge (R-Mount Vernon) was absent for the vote.