DES MOINES, Iowa (AP) -- The explosive growth of the nation's wind energy industry is prompting double takes and gawking by motorists on Iowa's interstate highways.

About 25 to 40 trucks are cruising Iowa's roads every day with oversized trailers hauling huge wind turbine towers, extraordinarily long blades and other equipment en route to wind energy farms under construction here and in other states, according to the Iowa Department of Transportation.

The surge of wind turbine construction in Iowa and other states has created a major increase in oversized loads on Iowa's roads, although state officials said the loads, which often travel in truck convoys, have caused relatively few traffic problems.

Trucks carrying wind generators can weigh 240,000 pounds, while some rigs loaded with turbine blades extend about 180 feet -- about triple the weight and length of regular semitrailer loads, state officials said.

"They are amazing," said Clint Swank, a meat-cutter from Earlham who has observed the super-sized shipments regularly while driving on Interstate Highway 80.

The United States' wind energy industry is experiencing a construction boom, recently hitting the milestone of 20,000 megawatts of capacity. More wind turbine capacity has been installed in the past two years than in the past two decades, according to the American Wind Energy Association.

Iowa ranks third nationally, with 1,375 megawatts of wind energy capacity, enough to power more than 300,000 homes, and another 1,586 megawatts are under construction here, the association said. Iowa hosts nine major wind farms and five companies manufacturing wind energy components, including newly opened TPI Composites of Newton, which makes wind turbine blades. In addition, Trinity Structural Towers plans to manufacture wind turbine towers in Newton.

"Transportation is a big part of the cost to the customer," said Marcia Scott, a spokeswoman for TPI Composites. "That is one of the reasons why Newton is such an ideal location. It's close to the wind belts where many of the wind farms are being built."

Each oversized load on Iowa's roads requires paperwork for state permits that detail specific truck routes and how many trailer axles are needed to spread weight to avoid damaging road pavement. The trucks also have special traffic safety requirements, such as escort vehicles that often accompany oversized trailers, said Bruce Schuck, director of the Iowa DOT's Office of Motor Carrier Services.

"We try to keep them on the interstate highways," Schuck said. "The blades aren't a big issue on the interstates because they aren't wide. They are just long. But you get tower sections that are fourteen- and fifteen-feet wide and it becomes more difficult during the construction season to use the interstates."

Motorists who encounter wind energy equipment being transported should "be very careful, especially if you have to pass them," said Scott Falb, an Iowa Department of Transportation safety planner.

"With any truck, you are dealing with blind spots. If you can't get by them fairly quickly, then stay behind," Falb said.

MidAmerican Energy, which plans to have 833 wind turbines online statewide by the end of the year, receives truckloads of equipment daily at its Adair wind energy farm under construction near I-80 in western Iowa, said Tom Budler, MidAmerican's wind project manager. This includes wind towers and power units from Denmark that are transported on ships across the Atlantic Ocean, through the Great Lakes to Duluth, Minn., then trucked southbound on Interstate Highway 35 into Iowa. The turbine blades are all trucked from Siemens Power Generation factory in Fort Madison.

The expanding wind turbine industry has been good business for Argee Transport Co. of Des Moines, which has developed a steady source of revenue from the oversized shipments. "I would guess we have done a million dollars this year, just on hauling wind towers," said Brad Kohlwes, Argee's business manager.

Jim Rommann of Dysart was driving an extended trailer along I-80 in Dallas County last week with a 95-foot section of a wind tower. He had picked up the load a day earlier in Clinton, Ill., and was headed to a wind energy farm under construction near Hanna, Wy.

Rommann said he keeps a steady speed of 50 to 55 mph and he has never had problems with other motorists.

"They just kind of look at it and they take their time going around you," he said. "But other than that, they keep right on going."