American Trucking Associations' advanced seasonally adjusted For-Hire Truck Tonnage Index decreased 0.8% in September, sliding to 111.8 from August's mark of 112.7.

"Truck freight slowed at the end of the third quarter," said ATA Chief Economist Bob Costello. "As anticipated, the year-over-year gains have slowed on strength a year earlier, but there is no doubt that freight softened in August and September. Despite the decreases late in the quarter, based on July's strength, third quarter tonnage rose 0.1% from the second quarter and 5.2% from the same period in 2017."

August's change over the previous month was revised down to -2.0% (-1.8% was originally reported in our press release on September 18).

Compared with September 2017, the SA index rose 2.9%, down from August's 4.2% year-over-year increase. Year-to-date, compared with the same period last year, tonnage increased 7.0%.

The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 110.2 in September, which was 8.2% below the previous month (120.1). In calculating the index, 100 represents 2015.

Trucking serves as a barometer of the U.S. economy, representing 70.2% of tonnage carried by all modes of domestic freight transportation, including manufactured and retail goods. Trucks hauled 10.77 billion tons of freight in 2017. Motor carriers collected $700.1 billion, or 79.3% of total revenue earned by all transport modes.