American Trucking Associations' advanced seasonally adjusted For-Hire Truck Tonnage Index decreased 4.5% in March, following a 7.2% surge during February. In March, the index equaled 137.6 (2000=100), down from 144 in February. February's level is an all-time high.

Compared with March 2015, the SA index was up 2.2%, which was down from February's 8.6% year-over-year gain. Year-to-date, compared with the same period in 2015, tonnage was up 3.9%.

The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 142.1 in March, which was 10.2% above the previous month (129).

"As expected, tonnage came back to earth in March from the jump in February," said ATA Chief Economist Bob Costello. "These things tend to correct, and March took back more than half of the surprisingly large gain in February.

"The freight economy continues to be mixed, with housing and consumer spending generally giving support to tonnage, while new fracking activity and factory output being drags," he said. "In addition, freight volumes are softer than the overall economy because of the current inventory overhang throughout the supply chain."

The decrease is the largest monthly contraction for the index since September 2012 (-5.3%).

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.