American Trucking Associations' advanced seasonally adjusted For-Hire Truck Tonnage Index increased 1% in May, following a revised 0.9% gain the previous month. In May, the index equaled 129.7 (2000=100) versus 128.3 in April. The index is off just 1% from the all-time high in November 2013 (131.0).

Compared with May 2013, the SA index increased 3.4%, down from April's 4.2% year-over-year gain, but is the second largest increase in 2014. Year-to-date, compared with the same period last year, tonnage is up 2.9%.

The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 133 in May which was 1.8% above the previous month (130.7).

"I'm pleased at the direction of freight, highlighted by May's fourth consecutive gain in tonnage totaling 4.5%," said ATA Chief Economist Bob Costello.

"While the year-to-date improvement is running behind last year's robust 6.3% increase, gains this year are more broad-based," he said. "It isn't just heavy freight for sectors like tank truck and flatbed from energy and housing that are improving this year. Now, generic dry van trailer freight is doing better as well, which wasn't the case in 2013. This is a good sign for the economy."

To hear more from Bob Costello on tonnage, click here.

Trucking serves as a barometer of the U.S. economy, representing 68.5% of tonnage carried by all modes of domestic freight transportation, including manufactured and retail goods. Trucks hauled 9.4 billion tons of freight in 2012. Motor carriers collected $642.1 billion, or 80.7% of total revenue earned by all transport modes.