NASHVILLE (AP) - A national traffic study shows congestion in Nashville has nearly doubled over the last three decades.

The 2012 Urban Mobility Report issued by the Texas A&M Transportation Institute says the congestion costs commuters money in lost time and extra fuel, as well as pollutes the region's air.

The institute estimates congestion in 2011 cost Nashville commuters $801 million.

Chip Knauf, the city's chief traffic engineer, told The Tennessean (http://tnne.ws/17GPK5s) synchronized traffic lights on key corridors is one option in dealing with the congestion.

Knauf said adding traffic lanes can be prohibitively expensive and there sometimes is not enough room to do that.

Adding lanes also is always an option, but Knauf said that can prove expensive and there isn't always room.

"It frankly would probably be a waste of everyone's money," added Michael Skipper, executive director of the Nashville Area Metrop olitan Planning Organization.

Planners say more than 1 million vehicles pass through each of the city's 20 busiest intersections daily. In the next 20 years, that figure could expand 15 percent. That would happen as Middle Tennessee adds an expected 1 million residents in the next two decades.

The figures came to light during planning for a proposed bus rapid transit system dubbed The Amp. The buses would run in designated lanes from West Nashville, through downtown, to the Five Points neighborhood on the east side of the Cumberland River.

A tool that Mount Juliet will be the first to use in Middle Tennessee should help. Rather than simply retiming traffic signals every few years, a computer program changes the signal intervals, based on current traffic flow.

"This is only new technology to our area," Skipper said. "It has proven successful."

The solutions must extend well beyond Nashville's city limits. The nine surrounding counties send large p ercentage of their residents to Nashville each day to work. From neighboring Cheatham County, 55 percent of the workforce makes the daily trip to the Capital City. One metro area county, Maury, has fewer than 10 percent of its workers not commuting to Nashville.

City planning decisions can help, too. Nashville has rezoned some midtown areas to encourage development so more people live near downtown. Skipper said the market will drive that effort over time.