The amount of truck tonnage hauled in the U.S. during September was unchanged from the month before remaining at a record high level, according to new American Trucking Associations figures.

Its seasonally adjusted For-Hire Truck Tonnage Index remained at 132.6 for the month, following a 1.6% gain in August.

Compared with September 2013, the seasonally adjusted index increased 3.7%, down from August's 4.5% year-over-year gain. Year-to-date, compared with the same period last year, tonnage is up 3.2%.

The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 135.8 in September, 1.7% above the August level.

"September data was a mixed bag, with retail sales falling while factory output increased nicely," said ATA Chief Economist Bob Costello. "As a result, I'm not too surprised that truck tonnage split both of those readings and remained unchanged."

"During the third quarter, truck tonnage jumped 2.4% from the second quarter and surged 4% from the same period last year," he said. Costello also noted that the third quarter average was the highest on record.

According to ATA, trucking serves as a barometer of the U.S. economy, representing 69.1% of tonnage carried by all modes of domestic freight transportation, including manufactured and retail goods. ATA calculates the tonnage index based on surveys from its membership