The American Trucking Associations' advance seasonally adjusted For-Hire Truck Tonnage Index fell 0.6 percent in May, the first month-to-month drop since February of this year.

However, tonnage was up 7.2 percent from May 2009, the sixth consecutive year-over-year gain. In April, the year-over-year increase was 9.5 percent.

"Despite the month-to-month drop in May, the trend line is still solid," said Bob Costello, chief economist of ATA. "There is no way that freight can increase every month, and we should expect periodic decreases. This doesn't take away from the fact that freight volumes are quite good, especially considering the reduction in truck supply over the last couple of years."

This puts the index at 109.6 (2000=100). Year to date, the tonnage index has grown a total of 6.2 percent compared with the same period of 2009.

The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 108.3 in May, down 2.8 percent from the previous month.