When Don Keith talks, people listen.

Usually, they don't have much choice because Keith, the Virginia Highway Department's top official in Fairfax County, has a voice as large as a 600-horsepower diesel truck. The voice starts as a low rumble, and by the time Keith launches full-swing into a discussion, has acquired teacup-rattling resonance.

What Keith has to say, so loud and clear, is bad news for Fairfax Countians who are counting on more and better roads to take them to work, to shopping centers and the scores of other places in this spread-out, suburban county.

"The 2 cent increase in the state gasoline tax (from 9 to 11 cents) means we will almost equal last year in improvements," Keith says. "But it's going to be downhill from here on out."

Keith has just given the Fairfax Board of Supervisors a revised six-year spending plan for the county's 1,700 miles of secondary roads, and he estimates there will be less than half the money that was calculated two years ago. Instead of $48.4 million, there will be about $21.2 million, he says, and that figure could shrink even more.

The biggest dose of bad news Keith has this year is for Springfield-Burke residents who commute on Backlick and Burke Land roads -- narrow, two-lane roads that were not intended for the heavy traffic they now carry.

The word from Keith is that there is no money in the six-year plan for the long-promised widening of the two roads.

Nor is there money, Keith says, for two other long-planned projects -- the widening of Franconia Road and North Kings Highway in the southeastern part of the county. North Kings is a vital access road to Fairfax County's first Metro stop, Huntington Avenue, which is expected to open in the summer of 1982, but there is no money allocated for improvements by then.

When it comes to deciding which Fairfax roads will be repaired, widened or built from scratch, there is no official in the county more powerful than Keith. The supervisors can declaim loudly and pass all sorts of resolutions, but they do not have the authority to fill so much as one pothole. rUnder Virginia law, the state has absolute authority over county roads.

Keith and his staff of engineers and planners occupy an inconspicuous red brick building near routes 123 and 50 in Fairfax City. Keith's office is at the end of cinderblock hallway. On his desk is a sign: "To err is human, to forgive is not my policy."

Though Keith is the highway department personified in Fairfax, he says that very few of the 10,000 or so calls the Fairfax office gets each year involve complaints about inadequate roads. Most of the calls, he says, involve weather-related problems like potholes, fallen tree limbs and poor drainage.

"I very seldom get a request for a road improvement," he says. "The idea seems to be, if you leave the road narrow, maybe nobody else will come."

One of the most controversial projects in years has been the proposed Springfield Bypass, which would arc across the county from Rte. 7 in the north to Rte. 1 in the south. The bypass would cost $90 million to $140 million, according to some estimates.

Residents in western Fairfax, primarily in the area between routes 50 and 123, don't want the road -- or at least not the section that would straddle their neighborhoods.

Theoretically, the highway department is waiting for a consultant's report before it makes its decision on the road -- to build the entire bypass to build only sections of it or to build no road at all.

But Keith has some definite notions about the subject.

"Let's say the cost is $100 million," he says. "'Oh, that's a terrible sum of money,' some people say. But look at the number of people it will serve every day of the year. And the amount of money is not as much as the Metro deficit in Northern Virginia -- and Metro doesn't serve as many people as the Springfield Bypass would."

The Northern Virginia Metro deficit for the fiscal year beginning July 1 is expected to be about $38 million.

Keith says transportation is becoming the No. 1 issue -- at least as far as the Board of Supervisors is concerned.

If the county decides to seek a city charter and succeeds, the question arises: Should it take over road responsibilities from the state, as it would be permitted to do?

Keith has a ready answer: "I don't think a local highway department could, given the same amount of dollars, give as good services as a state highway department."

But Keith's view is disputed in some quarters. The Fairfax transportation office, headed by Shiva K. Pant, a former state highway engineer, argued last year that the state department, through poor management, had held up a number of key projects in the county, and thus alowed costs to escalate.

While most of the criticism was aimed not at Keith but the bureaucracy in Richmond and at district headquarters in Culpeper, Keith answered: "What's important is not when roads are approved, but when they are funded. That's the nitty-gritty."

It's teacup-rattling time again.