American Trucking Associations' advanced seasonally adjusted For-Hire Truck Tonnage Index increased 0.7% in September, following a decrease of 0.9% during August. In September, the index equaled 135.1 (2000=100), up from 134.1 in August. The all-time high of 135.8 was reached in January 2015.

Compared with September 2014, the SA index increased 3.1%, which was above the year-over-year increase of 2.1% in August. Year-to-date through September, compared with the same period last year, tonnage was up 3.3%.

The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 138.9 in September, which was 1.4% above the previous month (136.9).

"The see-saw pattern in truck freight tonnage continued again in September, except that the gain didn't fully wipe out August's decline," said ATA Chief Economist Bob Costello. "However, over the last few months tonnage has snapped back from softness this past spring and early summer, and is approaching the record high.

"I remain concerned about the high level of inventories throughout the supply chain. We recently learned that inventories throughout the supply chain and relative to sales rose slightly in August, which is not a good sign. This could have a negative impact on truck freight volumes over the next few months," he said.

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.

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.