It is now possible for at least one white-collar federal worker in every 13 to avoid the worst of rush-hour traffic, take a long lunch or midday break for personal business or arrange child-care coverage, and still work and be paid for an eight-hour day.

The quiet revolution in federal stop-and-go times results from the growing use of flexible work schedules. Civil servants in fields from code breaking to check writing to law enforcement are now setting their own hours.This has virtually eliminated tardiness in some agencies, cut sick-leave use and resulted in longer service hours to the taxpayers.

Although flexitime is still not the official policy of the U.S. government, experts estimate that at least 90,000 civil servants - and maybe as many as 150,000 - already are setting their own hours under the program.

Use of the flexitime concept ranges from conservative (50 employees involved) experiments at the National Archieves to the Geological Survey, which has virtually all its 10,000 workers on the stretched-to-fit work day, and generally loving it.

Flexitime has taken a back seat to the four-day week in getting the media spotlight, but the four-day week (with 10-hour days) is still some time away. It must overcome staunch and unbending opposition from the AFLCIO's aging Washington leadership.

The AFL-CLO opposes a gevernment plan to test the four-day week for a three-year period because it would mean the voluntary waiver of overtime pay on the part of employees who wanted to work an extra two hours a day to qualify for a three-day weekend.

Hearings on legislation that would permit the three-year test of flexitime programs, including the four day week, begin May 24 before the House Employee Ethics and Utilization subcommittee. It is headed by Rep. Patricia Schroeder (D-Colo.).

But if the four-day week is still facing legislative and political stumbling blocks, the set-your-own-hours program is here now, and growing everyday. Thausands of Washington area government employees are involved in it, and other local agencies are planning tests of the system within selected areas of their operations.

Flexitime is a catch-word for many varying work schedules. Typically, an agency will set up a work day of 12 hours, say form 7 a.m. to 7 p.m. During the "core day" period, all workers must be on duty - say form 9 a.m. to 3 p.m. But within those times they can arrange to come in later, leave earlier or take a longer mid-day break, long as they put in the required eight-hour day.

The program is not - as many government workers believe - set up for the convenience of government workers. At least it isn't supposed to be that way. Rather, the idea is to permit workers limited freedom to pick their hours and make it possible to expand hours of service to the public from the old bar-the-doors-at-5 p.m. approach to, in some instances, a 12-hour-day operation.

Biggest use of flexitime in the Washington area is the Social Security Administration in Baltimore and the Environmental Protection Agency. SAA has about 6,000 employees under flexitime and EPA around 3,000. The Library of Congress has nearly 3,000, workers on flexitime: Food and Drug Administration here around 1,100; Treasury's Bureau of Government Finances Operation 1,450 and Law Enforcement Assistance Administration about 600.