In 2002, the Chester County Planning Commission, using data from the 2000 U.S. Census, projected county population would be about 483,500 by 2010.

That would mean an increase of about 50,000 people over the 2000 Census Bureau figure of 433,501. Yet a 2005 bureau estimate shows the county has already added 40,526 residents, or nearly 80 percent of the increase projected by 2010.

"It's clear that we have had more growth than anticipated," said county demographer Bob Walker, indicating that the commission might consider revising its estimates, which it doesn't ordinarily do between 10-year census counts.

The county population estimates are projected in five-year increments after each census. Municipalities and school districts use them for planning.

The latest bureau figures show Chester County was the 80th fastest-growing county of 3,141 in the United States from July 1, 2004 to July 1, 2005. The figures for 2005-06 are expected to be available by April 1.

Ron Bailey, executive director of the county Planning Commission, said there were obvious reasons for the county's increasing population.

Will it maintain the recent rate of growth? That will depend on several factors, Bailey said.

"Population growth is never in a straight line, although we have seen some rapid growth in the last couple of years," Bailey said. "What had been driving the surge was jobs in the county and [real estate] investment activity." He said that people were moving here from other areas and buying larger houses.

He noted, though, that "recently, we've seen a slowdown in the real estate market. The rate of [home] purchases has dropped significantly."

Still, there are several thousand new houses planned, with builders seeking approval from local officials.

"We're still creating jobs here. Chester County has become a major employment center," Bailey said. "In some municipalities, their populations could double with what's been approved for development. At the micro level you could see some extreme changes."

But Bailey does not necessarily see the county's dramatic population growth continuing.

Pointing to one factor, he said, "lower-wage workers don't have a range of housing. That could end up forcing them out" of the county.

In the short term, Bailey said, the rate of increase could fall off and then it might accelerate.

"When we get to 2010, if we look back over the past 10 years, we'll probably see less of a population growth rate than we have in the past two or three years. It tends to level out with peaks and valleys.

"I'm comfortable saying that, in the short term, we'll have continued growth along the major routes like the Pennsylvania Turnpike and Routes 202 and 30."