The American Trucking Associations’ advanced seasonally adjusted (SA) For-Hire Truck Tonnage Index fell 0.9% in August 2012 after increasing 0.4% in July. July’s gain was better than the no change ATA reported August 21.

In August, the SA index equaled 118.3 (2000=100). The sequential drop in August, while not erasing the cumulative 1.5% gain in June and July, was significant. Compared with August 2011, the SA index was 3.2% higher. Year-to-date, compared with the same period in 2011, 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 126.8 in August 2012, which was 5.7% above the previous month.

“While there has been acceleration in housing during the last few months, truck tonnage is being weighed down by a flattening in manufacturing output and an unintentional increase in inventories throughout the supply chain,” said Bob Costello, ATA chief economist. “While choppy, tonnage has essentially been flat this year, with August being the second-lowest month of the year.”

Costello also noted that the SA index in August was 0.3% below January 2012 and 1.4% less than the high in March.

“Expect tough year-over-year comparisons to continue through the rest of the year as tonnage grew nicely during the last five months of 2011,” he said. Costello added that the economy isn’t expected to grow much in the second half of the year as manufacturing decelerates and excess inventories are worked off. As a result, tonnage is expected to increase less than 3.5% in 2012.

Trucking serves as a barometer of the US economy, representing 67% of tonnage carried by all modes of domestic freight transportation, including manufactured and retail goods. Trucks hauled 9.2 billion tons of freight in 2011. Motor carriers collected $603.9 billion, or 80.9% of total revenue earned by all transport modes.