American Trucking Associations’ advanced seasonally adjusted (SA) For-Hire Truck Tonnage Index fell 0.4% in June after rising 0.4% in May. In June, the index equaled 113 (2015=100), down from 113.4 in May.

ATA revised the May increase from the originally reported 0.7% to 0.4%.

Compared with June 2017, the SA index increased 7.8%, up from May’s 7.4% year-over-year increase. Year-to-date, compared with the first half of last year, tonnage increased 7.9%, far outpacing the annual gain of 3.8% in 2017.

The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 116.3 in June, which was 1.1% below the previous month (117.6).

“In the second quarter, we saw the tonnage index jump 1.8% from the previous quarter and 8.4% from a year earlier,” said ATA Chief Economist Bob Costello. “This robust growth fits with what is likely to be a very strong GDP reading for the second quarter. I expect the growth in tonnage to moderate, but remain at very high levels in the months ahead.”

Trucking serves as a barometer of the U.S. economy, representing 70.6% of tonnage carried by all modes of domestic freight transportation, including manufactured and retail goods. Trucks hauled nearly 10.5 billion tons of freight in 2016. Motor carriers collected $676.2 billion, or 79.8% 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.