The Prince William County Board of Supervisors, hoping that traffic frustration finally has overcome county voters' dislike of debt, unanimously agreed yesterday to place a $42 million bond referendum on the ballot this fall.

"Its time has come," said Supervisor John D. Jenkins (D-Neabsco). "People are tired of sitting in traffic, and they're ready for something to be done."

Most of the money from the bonds, $30 million, would be spent on road improvements, which traditionally have been the responsibility of the Virginia state government. An additional $12 million would be earmarked for a $4 million police and fire training center and for other capital improvement needs.

The bond referendum must be approved in Prince William Circuit Court before it is placed on the Nov. 4 ballot, but officials said they regard this as a formality.

Jenkins and other county officials were optimistic about the referendum's prospects despite the recent history of Prince William voters to reject bond referendums. Most recently, in 1984, voters rejected a $20 million road bond by a wide margin.

"The people are more geared into it this year," said Supervisor G. Richard Pfitzner (D-Coles). "Before it was kind of strange: 'The county paying for roads?' "

Recent surveys of county residents by a board-appointed citizens committee showed a majority of the respondents supported bond financing to pay for improvements to Rte. 234 and other severely congested Prince William roads.

Jenkins and Pfitzner promised that county supervisors will campaign aggressively for the bond referendum's passage. While most supervisors backed the 1984 referendum, many observers said the politicians made only modest public efforts for its passage.

As state funds have lagged far behind the transportation problems faced in many urban jurisdictions, bond issues have become an increasingly common way for Virginia localities to make up the difference. Fairfax County voters have passed three road bond referendums, including a $135 million bond issue last fall.

"Chronologically, we are about 12 years behind Fairfax in our development," said Pfitzner. "But in attacking our problems with progressive thinking, we are only three or four years behind. We are learning from the lesson of our more urban brethren."

In other action yesterday, the board unanimously agreed to seek $15.5 million in bonds from the Virginia Public School Authority to replace the Saunders Middle School in Woodbridge, and to install air conditioners in many county schools.