On a seasonally adjusted basis, the tonnage index declined to a six-month low of 110.6 (2000 = 100) in May from 112.1 the previous month. Compared with a year earlier, tonnage was down 3.6 percent, the largest year-over-year drop since January 2007.
The not seasonally adjusted index increased 6.8 percent from April to 117.9.
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 said.
ATA's index underscored the Federal Reserve's June 13 Beige Book, which stated that declines were widely reported in trucking volumes from mid-April through the end of May. The Beige Book is an anecdotal account of economic activity.
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 percent 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 percent from April's minus 2.7 percent 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."
Trucking serves as a barometer of the U.S. economy because it represents nearly 70 percent of tonnage carried by all modes of domestic freight transportation, including manufactured and retail goods.
Trucks hauled 10.7 billion tons of freight in 2005. Motor carriers collected $623 billion, or 84.3 percent of total revenue earned by all transport modes.
ATA calculates the tonnage index based on surveys from its membership and has been doing so since the 1970s. This is a preliminary figure and subject to change in the final report issued around the 10th day of the month. The report includes month-to-month and year-over-year results, relevant economic comparisons, and key financial indicators.