The amount of freight moved by the nation’s for-hire transportation industry fell in August after rising four straight months, according to new Transportation Department figures, and reflecting trends in the overall economy.

The Freight Transportation Services Index (TSI) declined 1.8% in August from July to a reading of 122.3. It remains less than 2% below the downwardly revised all-time high level of 124.5 hit this past July, first reported as 124.6. Monthly numbers for January through May were revised down slightly.

The Freight TSI measures the month-to-month changes in freight shipments by mode of transportation in tons and ton-miles, which are combined into one index. It measures the output of the for-hire freight transportation industry and consists of data from for-hire trucking, rail, inland waterways, pipelines and air freight.

The August Freight TSI decrease was due to declines in pipelines, rail carloads, and particularly trucking, while all other modes grew or were stable, according to the department. “The August decrease was driven by declines in the mining, including oil and gas well drilling and servicing, and manufacturing sectors of the economy.”

The Freight TSI drop in August from July was the largest monthly decrease since January 2014. The decrease, following an increase of 1.6% in July, virtually brought the index back to the June level.

In the two-month period, the index is down just 0.2%. However, since the beginning of 2015, the index has been extremely stable as it fluctuated above and below 122. It has deviated only twice by more than 1% from the 122 level twice, in March 2016 when it fell 2% below 122 to a two-year low, and in July 2016 when it rose 2% above 122 to reach an all-time high. The index has risen 29% since the low of 94.8 in April 2009.

Freight shipments measured by the index increased 0.8% in August compared to the end of 2015 while August freight shipments fell 0.3% from the same time a year ago.

According to the Transportation Department, its research shows a clear relationship between economic cycles and freight as well as its separate passenger index, which measures activity from air, local transit and intercity rail.