Trucking activity in the U.S. saw a slight increase in July after two months of declines but remains relatively flat since March, according to industry group data released Tuesday.

Truck freight tonnage rose 0.6% in July, according to the American Trucking Associations' advanced seasonally adjusted For-Hire Truck Tonnage Index. That follows a 0.7% decline in June and a 0.1% slide in May.

"July truck tonnage increased sequentially, but did not erase the 0.7% decline in June," said Bob Costello, the trucking group's chief economist. "Since March, truck tonnage has been in a tight range. The good news is truck freight volumes haven't fallen much over that period, but we are not seeing many increases either."

Costello said July increases in tonnage due to increased housing starts and retail sales were counterbalanced by flat-to-weaker manufacturing output.

Year to date, the seasonally adjusted index is unchanged from the same period last year, the trade group data show.

ATA said its not seasonally adjusted index, which calculates raw changes in tonnage hauled, equaled 116.8 in July. That's a 1.9% increase from June.

ATA says that trucking serves as a barometer of U.S. economic activity, as the industry carries 72.7% of tonnage carried by U.S. freight transportation.

The ATA index is dominated by contract freight, as opposed to traditional spot market freight. The trade group compiles the tonnage index based on surveys of its 37,000 members.

This content was created by Oil Price Information Service, which is operated by Dow Jones & Co. OPIS is run independently from Dow Jones Newswires and The Wall Street Journal.

--Reporting by Steve Cronin, [email protected]; Editing by Michael Kelly, [email protected]