The U.S. Department of Transportation is calling its Transportation Services Indexes, especially the freight index, leading indicators of economic cycles.

The Bureau of Transportation Statistics released new research on the topic. In the report, "Transportation Services Index and the Economy – Revisited," BTS found that the freight index showed turning points in the health of the economy. When comparing the transportation index to the economy going back to 1979, the index often indicated the direction of the economy as much as four months in advance.

“When the accelerations and decelerations of the freight TSI are compared are compared to the growth cycles of the economy, the freight measure leads by an average of approximately four months,” said BTS.

These turning points in the freight index can often lead a major economic change such as a recession or recovery, according the DOT.

BTS has issued the Freight TSI monthly since 2004. The index measures month to month changes in freight shipments by transportation type, tons and ton-miles. It gathers data from for-hire trucking, rail, inland waterways, pipelines and air freight.

In the most recent index, the Freight TSI hit a record high, marking the third consecutive monthly gain. Freight shipments were up as much as 2.1% over the previous year.