ALEXANDRIA, Va. — The American Trucking Associations’ advanced seasonally adjusted For-Hire Truck Tonnage Index decreased 1.3% in May, marking the second consecutive month-to-month drop.

In April, tonnage fell 2.2% and on a seasonally adjusted basis, the tonnage index declined to a six-month low of 110.6 in May from 112.1 the previous month.

Compared with a year earlier, tonnage was down 3.6%, the largest year-over-year drop since January 2007.

ATA Chief Economist Bob Costello said that while May’s numbers were disappointing, they fit with the anecdotal reports from many fleets.

“Most carriers indicated that volumes were soft and spotty in May, which was clearly reflected in our index,” he noted.

Costello said one of the primary reasons for the continued year-over-year index contractions is that manufacturing activity by weight, not value, fell again in May. Manufacturing activity declined 1.4% when compared with the same month in 2006. This marked the seventh consecutive month that this weight-based manufacturing measure fell.

“The fact that the year-over-year comparison for truck tonnage worsened to a negative 3.6% from April’s minus 2.7% is troubling,” said Costello. “We fully anticipated a contraction, but the fact that it deteriorated may mean that more volatility is in store. Unfortunately, we won’t have a good feel for the direction of volumes until we see June’s data.”