American Trucking Associations' advanced seasonally adjusted For-Hire Truck Tonnage Index decreased 2.1% in April, following a 4.4% drop during March. In April, the index equaled 134.8 (2000=100), down from 137.6 in March. The all-time high was 144 in February.

Compared with April 2015, the SA index was up 2%, which was down from March's 2.2% year-over-year gain. Year-to-date, compared with the same period in 2015, tonnage was up 3.5% on February's strength. Excluding February, the index was up just 1.8% over the same months in 2015.

The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 135.1 in April, which was 5% below the previous month (142.2).

"After having an abnormally large seasonally adjusted gain in February, tonnage fell in April, in addition to the large drop in March," said ATA Chief Economist Bob Costello. "However, while freight remained soft in April, based on other economic indicators, the outlook for tonnage is a little better than just a couple of months ago.

"With that said, there is still an inventory correction transpiring throughout the supply chain that will keep a lid on truck freight volumes in the near term," he said. "As a result, we are still likely to experience lackluster tonnage numbers in the next few months."

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.