The American Trucking Associations' advanced seasonally adjusted For-Hire Truck Tonnage Index increased by 3.7% overall in 2017 compared to the year before, marking its largest annual gain since 2013.

While it was up overall, the index stumbled toward the end of the year, falling 5.7% in December. The index equalled 142.9, down from 151.6 in November. Compared to December 2016, the index increased 5.9%.

The not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equaled 141.9 in December, which was 3.4% below the previous month.

“Despite the decline in December, last year was a solid year for truck tonnage, especially during the second half of 2017,” said Bob Costello, ATA Chief Economist . “I remain optimistic for 2018 for a host of reasons, including a pickup in factory activity, better housing construction, solid retail sales, and an expected shot in the arm from the new tax law.”

Trucking serves as a barometer of the U.S. economy, representing 70.6% of tonnage carried by all modes of domestic freight transportation, including manufactured and retail goods. Trucks hauled nearly 10.5 billion tons of freight in 2016. Motor carriers collected $676.2 billion, or 79.8% of total revenue earned by all transport modes.

ATA calculates the tonnage index based on surveys from its membership and has been doing so since the 1970s.