ARLINGTON, Va. -- American trucks aren't as productive as some in the rest of the world, says the American Trucking Associations, which is once again urging regulators to allow "higher productivity vehicle models" on U.S. highways.

"U.S. trucks would be more fuel efficient and emit less greenhouse gases if they were more productive like trucks in Europe, Canada and Australia," according to the preliminary results of a multi-nation study on long combination vehicles.

"As countries around the world prepare for increasing freight volumes, many have already instituted programs that increase truck size and weight and have seen great productivity and environmental gains, with continuing improvements in safety."

At a recent conference hosted by the University of Michigan Transportation Research Institute, truck industry experts noted that the U.S. is lagging in several truck productivity and sustainability categories when compared with international counterparts.

"Current U.S. truck size and weight regulations serve as a limiting factor, preventing trucks from utilizing the full potential of our infrastructure. Increasing truck size and weight standards to align more closely with international standards would improve truck productivity and the ability to reduce greenhouse gas emissions and carbon output," the study says.

ATA supports allowing States to authorize more productive vehicles to operate on national highways, specifically a program to allow 6-axle vehicles to carry 97,000 pounds in states that agree to permit them; allow states to permit 33-foot trailer combinations; harmonize the use of longer combination vehicles in western states and expand their use where appropriate; and allow a 10 percent increase in auto hauler weights to account for today's heavier vehicles.