American Trucking Associations' advanced seasonally adjusted For-Hire Truck Tonnage Index increased 1.1% in May, following a revised loss of 1.4% during April. In May, the index equaled 132.1 (2000=100). The all-time high is 135.8, reached in January 2015.

Compared with May 2014, the SA index increased just 1.8%, which was well below the 2.7% gain in April and the smallest year-over-year gain since February 2013 (-4.3%). Year-to-date through May, compared with the same period last year, tonnage was up 3.7%.

The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 133.1 in May which was 0.3% above the previous month (132.7).

"The good news is that truck tonnage increased in May," said ATA Chief Economist Bob Costello. "But tonnage is certainly not strong at the moment as factory output is soft and there is an inventory reduction occurring throughout the supply chain."

Costello noted that truck tonnage is off 2.7% from the high in January.

"I believe the inventory correction should end this summer and truck freight, helped by better personal consumption, will accelerate," he said, "which is good because I think it is unlikely factory output will boost truck tonnage much until later this year or next year."

Trucking serves as a barometer of the U.S. economy, representing 68.8% of tonnage carried by all modes of domestic freight transportation, including manufactured and retail goods. Trucks hauled just under 10 billion tons of freight in 2014. Motor carriers collected $700.4 billion, or 80.3% of total revenue earned by all transport modes.