With his narrow upset victory this week over 23-year Board of County Supervisors veteran Kathleen K. Seefeldt (D), GOP political novice Sean Connaughton appeared to tap voters' mounting concerns over the everyday effects of Prince William's rapid growth.

Bumper-to-bumper traffic, schools spilling into 140 trailers and the still-highest county real estate tax rate in the commonwealth--these are the pocketbook and quality-of-life issues that routed a board chairman who presided over two decades of development, yet may have failed to heed voters' calls to curb it and address its effects soon enough.

"I strongly supported putting the brakes on residential development," Seefeldt, 64, said last week, reflecting on her 695-vote loss to Connaughton on Tuesday.

A prominent figure in regional transportation planning, Seefeldt had campaigned on her record of building new schools and roads and luring Internet giant America Online Inc. to put a data center in Prince William.

"But people wanted a change," she acknowledged. "There comes a time when it's time to go, and my time came."

Advocates of curbing the county's build-up did not make a clean sweep. In Gainesville, Supervisor Edgar S. Wilbourn III (R) beat back a challenge from Democrat and slow-growth activist Gary C. Friedman, who had attacked the incumbent for his ties to the building industry and his opposition to the slow-growth Comprehensive Plan supervisors approved last year.

Supervisor Hilda M. Barg (D-Woodbridge) also won an easy victory against GOP challenger Lester Gabriel in a low-key race.

One thing seems clear--the county's increasingly young electorate is looking to younger leadership to reflect its experience. Voters want their taxes lowered, value restored to homes devalued by overdevelopment, children out of classroom trailers and new roads built--fast.

"I didn't just say growth was the issue and let's stop it," said Connaughton, who won't take office until in January but already has been asked to speak to a prominent regional builders association this month.

"I was talking about the issues people were most concerned with at a household level, school overcrowding or the fact that people owned town houses they couldn't sell, or their taxes are too high," Connaughton said.

He took 17,786 votes to Seefeldt's 17,091, triumphing in five of the county's seven magisterial districts, with his biggest showing in the western end of the county. The underdog, who spent $45,000 of his own money in the campaign's waning weeks, beat Seefeldt in her home district of Occoquan, where she was first elected supervisor in 1975. Independent, first-time candidate Robert K. McBride, who campaigned on a Libertarian platform, received 1,925 votes.

Connaughton, a Triangle lawyer, pledged to work harder than his predecessor to lure high-tech and high-end commercial business to Prince William to offset the county's real estate tax rate of $1.36 per $100 of assessed value.

And he said the board must focus its land-use policy on the 40,000 homes long ago approved for development. To wipe some of those projects off county planning maps, Connaughton proposes offering developers incentives such as tax breaks or relief from sewer or road costs to reduce the number of homes allowed on each acre of land or to switch to commercial projects.

"They put a tremendous burden on county services," he said of the unbuilt projects.

Just 27 percent of Prince William's registered voters went to the polls Tuesday, down from a 45 percent turnout for local races in 1995.

At 38, with two young children and a daily commute to the District, the rookie chairman may have more in common with the young, increasingly Republican families that have nearly doubled the county's population since 1980. And with the election last year of 37-year-old Ruth T. Griggs (R-Occoquan), a self-described soccer mom and slow-growth advocate, the board's leadership appears to be shifting to younger members.

"We're very demographically similar to a lot of the county's population," said Griggs, who ran unopposed last week. "There was a growing movement of people who were recognizing the impact of the way we had been growing and said we needed a new vision that was different from the one that always allowed things to happen to us instead of making them happen."

Connaughton, who moved to the county 12 years ago, promised to shake up a board he said often fails to debate issues in public, and to scrutinize how the county and school system spend money.

"You'll see a different style at board meetings," he said. "I'd like to see things opened up a little bit."

Seefeldt's departure leaves just two Democrats, Barg and Supervisor John D. Jenkins of Neabsco, on the board. Although the board is not known for party-line votes, it's unclear how Connaughton will be received by Seefeldt loyalists, both Democrats and Republicans.

"I've told Sean I'll do everything possible to help," said Jenkins, who also was unopposed in Tuesday's election. Jenkins will become the board's senior member, with 18 years in office. "But I don't think we're going to undo any of our policies."

Although the job is full time for Seefeldt and several supervisors, Connaughton said he'll continue--but scale back--his law practice as a contract partner with the D.C. firm of Eckert Seamans Cherin & Mellott.

Seefeldt served a total of 12 years as board chairman. She said she plans to spend more time with her grandson and has two more grandchildren on the way. Her legacy includes many things visible and invisible--construction of 11 elementary schools, a new jail and courthouse, and dozens of roads, the biggest of them the county-built Prince William Parkway.

Seefeldt said she's most proud of helping establish the county's first university presence, a satellite campus for George Mason University, which now comprises two academic buildings and a newly opened recreation center.

She also cited her emphasis on involving ordinary residents in county government with numerous task forces convened to study issues from tourism to dirt bikes.

While Seefeldt campaigned on a record of promoting the county's quality of life and curbing development with her vote for the Comprehensive Plan, in the end she may have failed to reach voters who struggle every day with the after-effects of two decades of explosive construction.

"For me, it was the traffic situation," said Joe McMahon, 44, a lifelong county resident and Republican who said he voted for Connaughton and Friedman. "It's on the verge of becoming a nightmare. I realize we have to grow, but we need better planning."