The Virginia Supreme Court, in a heavy blow to localities trying to cope with growth, has ruled that residential developers cannot be compelled to make improvements to primary and secondary roads.

Traditionally localities, especially in rapidly developing areas of Northern Virginia, have relied on developers to finance millions of dollars' worth of improvements. The court decision forces localities to rely wholly on the Virginia Department of Highways and Transportation, which has been able to provide only a fraction of what urbanizing jurisdictions say they need.

"This is going to cost us a lot," said Deputy County Attorney John H. Foote of Prince William County, which lost of 6-to-0 decision. "I think the decision will have a substantial impact."

In Dale City alone, the decision frees Hylton Enterprises, Inc., which sued the county in the case, from the necessity of providing a little more than $2 million worth of secondary road improvements, according to county officials.

Although the court's decision places the burden on the highway and transportation department, that agency has allocated only $3 million to all of Prince William for both new secondary construction and maintenance in the current fiscal year.

Officials in rapidly developing Fairfax County were uncertain about the decision's impact. Traditionally the county has depended on developers to make million of dollars of improvements.

According to attorney Marc E. Bettius, who represented Hylton in the case, Fairfax developers will be freed of responsibility for financing from $70 million to $80 million in road improvements because of the decision, for which there is no appeal.

"we haven't ripped off the people," Bettius said. "The costs for these improvements should not be passed on to people who need a place to live."

Bettius said the highway department is the "sole funder for highways." He acknowledged "we may have to sue as a region" to get more money allocated to Northern Virginia.

There could be substantial impact, too, in Londoun County, which is trying to get a developer seeking a major rezoning to contribute to a cloverleaf interchange at the intersection of Rtes. 7 and 28.

"I think [the decision] will affect quite a bit of development throughout the state," said John V. Walvius, vice president of Hylton. "The obligation to make improvements rests with the departments of highways."

If developers don't have to contribute to expensive improvements and if there aren't enough state funds to fill the gap, where will the money come from?

"The most probable answer," according to Prince William's Foote, "is state legislation to compel developers to make improvements, or for the highway department to allocate more money in Northern Virginia."

Any reallocation would siphon money from other sections of the state, and that would be strenuously resisted by those areas. "The more screams and howls from other parts of the state, the more likely we will get the legislation we need," reasoned Foote.

One reason localities like Fairfax have turned to developers for road improvements is that they have found that the highway department's timetables for construction tend to get stretched out into the hazy future.

Fairfax Supervisor James M. Scott (D-Providence) complained yesterday that long-promised project -- both major and minor -- have been repeatedly put off by the department.

The state Supreme Court's decision follows a pattern of others in recent years in which localities have been the losers and developers the winners.

In what are considered the most important decisions, localities were told they could not halt or slow down growth because there were not enough public facilities to support it. When growth comes, the facilities follow naturally, the court said.