In rapidly urbanizing Fairfax County, traffic congestion is spreading as rapidly as crabgress. To help stop the relentless spread, Fairfax's political leaders are trying a solution that, only a few years ago, would have been politically unthinkable: Local financing of highway improvements through voter-approved bond referendum.

For decades, the county relied on the state to finance roads, but the Virginia Department of Highways and Transportation is so short of money that it expects to run a $27 million deficit for the just-ended fiscal year. So on Nov. 3, county voters will vote on a $30 million bond issue to finance secondary road improvements that, with political care, cover every part of the country.

The ballot will be crowded with other bond questions, the biggest of which is $57.2 million for schools. Voters also will be asked to approve $11.9 million for public safety, including enlargement of the jail.

To win support for the road bond issue, the supervisors sought -- and got -- support from the Fairfax County Federation of Citizens Associations. Federation president Lilla D. Richards acknowledged that the $30 million figure "is a drop in the bucket" compared to the county's road needs.

The state highway department has estimated that Fairfax needs secondary road improvements that would cost $700 million, said William B. Wrench, commissioner for Northern Virginia.

Fairfax's transportation director, Shiva K. Pant, has listed a series of "immediate needs" that, altogether, would cost from $150 million to $175 million.

Under the recently enacted state legislation that it had sought, Fairfax cannot ask voters to approve more than $10 million worth of bond financing for roads per year (the $30 million November referendum covers three years). At that rate it would take Fairfax 10 to 15 years to meet its immediate needs with bond financing, although that is unlikely because it could raise the county's debt level and threaten its favorable bond rating. The hard-pressed state can give Fairfax only about $2.5 million annually for secondary construction.

Although the bond referendum is principle has picked up support from the citizens federation, Richards has complained that the supervisors did not follow up on an important promise -- creating a countrywide commission to access road needs.

One of the biggest bottlenecks in the county -- the intersection of Braddock and Port Royal roads near the Capital Beltway -- is slated to be improved if the $30 million bond referendum is approved. Pant acknowledges that even if the money is available, there may be not improvement. a

"I don't know what can be done," Pant said about the problem, which has perplexed road engineers for years. The only sure solution would be a massive interchange, but that would eat up half or more of the $30 million.

Voters who are hoping the $30 million will result in solutions to some other major bottlenecks may be disappointed. For example, the traffic mess at Tysons Corner will not get a single dollar if the bond issue is approved.

Problems in the Tysons area will have to wait on construction of additional lanes on the Dulles Access Highway and of a freeway-style interchange at Rte. 123 and International ydrive;

Nor is any section of the Springfield Bypass, the road that would extend from the Herndon-Reston area in the northwest to Fort Belvoir in the southeast, included in the bond issue. The bypass, which has been endorsed by the supervisors, will have to be funded by the state and/or federal government.

Many of the proposed improvements are quite modest. For example, the only two that would be made in the fast-growing Centreville district (which includes Reston) would be the widening of a one-lane bridge on Vale Road at Difficult Run, straightening a curve at the same place and building another lane in the intersection of Hunter Mill Road and Sunrise Valley Drive. That's all.