WASHINGTON — U.S. trucking industry shipments declined for the second consecutive month in March, a trade group said Thursday.

The American Trucking Associations said its seasonally adjusted truck tonnage index fell by 3.2 percent in March versus the previous month. In February the index declined by 3.6 percent, according to revised figures.

Because more than two-thirds of all manufactured and retail goods in the U.S. are carried by truck, the industry is considered an important economic bellwether.

The Alexandria, Va.-based trucking group said its tonnage index stood at 110.1 in March, the lowest level since November 2003 and 2.6 percent below year ago levels. The index, which stood at 100 in 2000, measures the weight of freight hauled by U.S. truckers, based on surveys from its membership.

Truckers have seen their fuel costs surge in the past few years, though unlike airlines they have been able to pass most of those extra expenses through to customers.

"Anecdotally, nobody's out there saying freight is really strong, we're not hearing that," said Bob Costello, the association's chief economist. "But we're also not hearing 'We're dying.' Hopefully this will be short-lived."

Earlier this week, the trade group projected that the U.S. trucking industry would spend $94.3 billion on fuel throughout 2006, an increase of $6.6 billion from the year before. Nationwide, diesel prices average $2.88 a gallon.