The 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 was up 3.4%. That's 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."

He predicts GDP will continue to increase at a rate over 3% for the rest of the year. The year overall will not be quite as good because the first quarter was so tough, thanks in part to the severe weather experienced in most of the country.

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.

Bob Costello talks more about this month's results: