Shelby, the fastest-growing county in Alabama, grew by 28,172 people - almost 20 percent - in the five years since the previous census. The county added more than 6,000 people from mid-2004 to mid-2005, pushing it past Tuscaloosa County to become the fifth most populous county in Alabama.

Of the seven Birmingham-area counties, only Jefferson and Walker are estimated to have decreased in population since 2000.

Jefferson lost 4,811 residents, while Walker lost 596. Those decreases were less than 1 percent for each county.

St. Clair County had the metro area's second-largest increase, adding 7,588 residents to grow by nearly 12 percent since 2000.

St. Clair's growth can be attributed in part to its proximity to the Honda plant in Lincoln and the willingness of Springville residents to make a nearly hour-long commute to Birmingham, said Brett Isom, senior geographic information systems analyst for the Regional Planning Commission of Greater Birmingham.

The Interstate 20 corridor, which runs through St. Clair, is one of the least developed corridors in Alabama, Isom said, but that should change soon.

"You're going to see a lot of stuff going up on I-20 between Birmingham and Lincoln," Isom said.

The three other counties in the metro area - Bibb, Blount and Chilton - grew between 5 percent and 9 percent.

The Census estimates for cities have yet to be released, but Isom said the fastest-growing cities in Shelby County are Helena and Chelsea, based on information compiled by the RPC.

A comprehensive plan for Helena drafted by the RPC predicts its population will have more than doubled, perhaps growing to 25,000, by the year 2030, Isom said.

Chelsea, off U.S. 280, could continue growing because it has the potential to annex large tracts, he said.

Isom said Shelby County will be busy adding infrastructure during the next couple of decades.

About half of Shelby County's employed population commutes to Jefferson County, Isom said.

As Shelby's population grows, he said, new residents likely will rely on the three major thoroughfares between the two counties - U.S. 280, U.S. 31 and Interstate 65 - to commute.

In addition to work that will be required to maintain those roads in the wake of added traffic, more sewer lines, water lines and schools will be needed to serve the expanded population, Isom said. To keep up with the pace of growth, Shelby County will need to build a school a year, he said.

Transportation is one of the main problems facing Shelby County given the residential growth, said County Commission Chairwoman Lindsey Allison.

"We're pretty much able to afford the growth except for transportation," Allison said.

The increased sales- and property-tax revenue that have followed the residential boom helped offset the expense of water system investments and new schools, but the county is at the mercy of the state and federal governments to maintain highways and interstates, Allison said.