The American Trucking Associations' advanced seasonally adjusted (SA) For-Hire Truck Tonnage Index contracted 0.9% in August after increasing 0.4% in July. (July's gain was better than the no change ATA reported on 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 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 126.8 in August, 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," ATA Chief Economist Bob Costello said. "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, adding 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.

Note on the impact of trucking company failures on the index: Each month, ATA asks its membership the amount of tonnage each carrier hauled, including all types of freight.  The indexes are calculated based on those responses. The sample includes an array of trucking companies, ranging from small fleets to multi-billion dollar carriers. When a company in the sample fails, we include its final month of operation and zero it out for the following month, with the assumption that the remaining carriers pick up that freight. As a result, it is close to a net wash and does not end up in a false increase. Nevertheless, some carriers are picking up freight from failures and it may have boosted the index. Due to our correction mentioned above however, it should be limited.

Trucking serves as a barometer of the U.S. 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.

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.