American Trucking Associations' advanced seasonally adjusted For-Hire Truck Tonnage Index rose 0.7% in May after rising 2.7% in April. In May, the index equaled 113.8 (2015=100), up from 113 in April.

ATA revised the April increase from the originally reported 2.2% to 2.7%.  

Compared with May 2017, the SA index increased 7.8%, down from April's 9.9% year-over-year increase. Year-to-date, compared with the same five months last year, tonnage increased 8%, 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 118 in May, which was 7.6% above the previous month (109.7).

"This continues to be one of the best, if not the best, truck freight markets we have ever seen," said ATA Chief Economist Bob Costello. "May's increases, both sequentially and year-over-year, not only exhibit a robust freight market, but what is likely to be a very strong GDP reading for the second quarter. However, in the near-term, look for moderating growth rates for freight simply due to more difficult year-over-year comparisons, not from falling tonnage levels."

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. 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.