Rising truck shipments show the U.S. economic expansion is intact, even amid concerns that a slowdown in retail sales and Europe’s sovereign-debt crisis could stall growth.

Two measures of trucking activity signal the industry remains steady and has even “firmed up” since mid-May, according to Ben Hartford, an analyst in Milwaukee with Robert W. Baird & Co. The data complement anecdotal information from carriers that freight demand ended May on a strong note after more weakness than anticipated earlier in the month, he said.

“Trucking trends are reflective of an economic environment that is stable, not deteriorating,” Hartford said.

The for-hire truck-tonnage index rose 2.8 percent in April from a year earlier, up from 0.2 percent the prior month, marking 29 months of growth, based on data from the American Trucking Associations.

The economy has never contracted without tonnage turning negative first, so the truck figures are a leading indicator, providing the “first signal” of a slump, said Thom Albrecht, an analyst with BB&T Capital Markets.

Another index that tracks the movement of goods between manufacturers and consumers also is a “good barometer” of the economy, said Jonathan Starks, director of transportation analysis at FTR Associates. FTR’s index of U.S. truck loadings increased 3 percent to 115.9 in April from a year earlier, the highest since 2008, based on data from the transportation-forecasting company in Nashville, Ind.

April’s improvement suggests the economy is expanding.

Starks said annual gains above 5 percent would suggest robust activity. Index growth exceeded 5 percent between July 2010 and March 2011, the data show, while gross domestic product expanded an average 2.9 percent year-over-year in the same period.

Contacts at trucking companies describe a “seasonally stable demand environment,” Hartford said.