The index grew 0.4 percent during March, landing at 109.2 (2000=100), its highest level since November 2008.
Compared with March 2009, tonnage jumped 7.5 percent, the fourth straight year-over-year gain and the largest increase since January 2005. For the first quarter of 2010, tonnage was up 4.9 percent compared with the same period last year.
The news made Bob Costello, ATA chief economist, optimistic about the industry's recovery.
"Freight is moving in the right direction and I continue to hear from motor carriers that both the demand and supply situations are steadily improving," he said. Costello attributed the first-quarter improvement in tonnage to the growing economy and to a slight inventory build after some sectors slashed inventories by too much in 2009.
"For most fleets, freight volumes feel better than reported tonnage because the supply situation, particularly in the truckload sector, is turning quickly."
The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, was at 116.4 in March, up 19.1 percent from the previous month.
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.