San Diego County is leading the state in compliance with the law requiring use of seat belts, thanks in part to an education effort in the northern part of the county, traffic safety officials said. More than half the drivers and passengers in San Diego County were buckling up in November, 1986, compared with 42% of the drivers and passengers statewide. Moreover, compliance with the law in San Diego County has actually increased since last year. The educational program, funded by the Office of Traffic Safety, sends speakers to community groups and schools and employers develop mandatory policies on seat belts to reduce health insurance costs. Since the state's seat belt law took effect Jan. 1, 1986, the percentage of motorists who wear seat belts has jumped from 28% to 42%, and the percentage has been increasing, in part because of a recent crackdown by the California Highway Patrol.