WTH VISION as well as restraint, Fairfax County has taken the first big step toward its most important transportation decision since participation in the Metro subway system. In a unanimous vote Monday, the board of supervisors agreed to send a bond issue of nearly $135 million -- largest in the county's history -- on to a public hearing, a final board vote on Aug. 5 and a referendum on Nov. 5. If residents hold any hopes of unclogging the traffic jams that now regularly impede both private and public transportation, they should support this carefully balanced package as approved.

Not every bottleneck in the county would be cleared by approval of the bond issue. But the supervisors have agreed on a fair, maximum combination of road projects, under a financial ceiling. Until this week's vote there was a danger that supervisors might be tempted to push for pork-barrel projects that would inflate this considerable dollar total and the whole roads agenda to a politically unacceptable degree. Though their restraint means that certain pet projects will have to wait for another day, the bond issue as voted is a good one.

Money is included for two segments of the cross- county Springfield Bypass, as well as for 12 more road construction and widening projects designed to ease severe congestion. These roads would provide critical links to Metro stations and key arteries. Along with what has turned out to be about $100 million in privately financed overpasses and connector roads, the proposed bond-issue projects would ease countywide traffic snarls.

Board members have recognized that about 50 percent of the working residents have jobs in the county. Chairman John F. Herrity notes that this figure should increase to about percent by 1990. Roads are not built in a day; now is the time to begin. The temptation to load down the package with additional projects should be resisted as it moves along to referendum, where it merits strong public support.