FTR’s Trucking Conditions Index fell to 4.22 in March, down nearly 50 percent from the previous month, the industry research firm said Monday.

A sluggish U.S. economy during the first quarter of the year pushed the index lower.

“The freight markets have slowed significantly over the last year with March volumes just 1.5 percent above year ago levels,” said Jonathan Starks, FTR’s chief operating officer.

The March index is the lowest it has been since 2011.

“Manufacturing output will remain weak during most of 2016, and consumer spending hasn’t grown strong enough to make up the difference,” Starks said. “Add in the serious inventory glut that has persisted since early 2015, and there is little to call for a significant change in the carriers operating environment for 2016.”

Starks said the weak economy has encouraged shippers to ask carriers for discounts.

Manufacturing appears to be growing again but output is expected to remain weak during most of the year and consumers are not spending enough to make up the difference, FTR reported.

Looking forward, Starks said “weak rate growth will persist but not at the negative level that we are currently seeing, especially once we hit 2017.”