A closely watched monthly index of domestic shipment activity fell last month to the lowest level seen since October since 2011, according to the firm that publishes the index.

Shipment volumes last month declined 4.7 percent sequentially and 5.3 percent year-over-year, according to Cass Information Systems Inc., a freight-audit and -payment firm that processes more than $26 billion in annual payables. Freight expenditures fell 2.2 percent over September—the third decline in four months—and 8.7 percent year-over-year, Cass said. The steep year-over-year drop mirrored the fall-off in volumes and also reflected continued weakness in noncontractual, or spot, rates, it added. The truckload market remains awash in capacity, which lessens the need for shippers and their freight brokers to tap the spot market for space. As a result, spot rates remain depressed.

According to DAT Solutions, a consultancy that tracks the spot market, loads in October fell 9.3 percent from September and are off 42 percent year-over-year. Truck capacity rose 6 percent sequentially and 16 percent year-over-year, according to DAT. For the week ending Nov. 7, the firm's load-to-truck ratio for dry vans—the most common form of truckload transport—stood at 1.6, meaning there were 1.6 available loads for every truck posted on the DAT network. At this time last year, the ratio stood at 2.8.

Last year was an extraordinary period for the spot market, as bad winter weather at the front end and labor turmoil at West Coast ports later in the year forced many shippers to seek capacity their contracted carriers couldn't or wouldn't provide. In addition, dramatically falling diesel-fuel prices have allowed many struggling truckers to stay in the game when they might otherwise have been forced to exit the market, according to DAT. Another factor was that shippers, burned by their experiences in 2014, paid up this year to secure more contracted capacity, which lessened the need for spot-market services, according to DAT.

The spot market accounts for between 20 and 25 percent of all truckload activity. However, the direction of spot prices often dictates the outlook for upcoming contract rates. Rosalyn Wilson, who authors the monthly report as well as the annual "State of Logistics Report," presented by Penske Logistics, said truckload carriers are getting 2016 rate increases of only 2 to 3 percent, though pricing for dedicated services, where a customer commits to exclusive use of equipment and drivers for a multiyear period and pays based on round trips, will rise, on average, about 3 to 4 percent. Carriers "are unwilling to lose a good customer over a few percentage points," she said in a narrative accompanying the data.

Wilson said the October declines recorded in the survey were "much sharper" than in recent years and were due to bloated inventory levels, which have led businesses to cut back on new orders during the past three or four months. The results have been reduced import volumes, faltering industrial-production activity, and less freight to move, she said. Consumer spending has been a bright spot, with spending up more than 3 percent over each of the past two quarters. Fortunately for the U.S. economy, consumer spending accounts for more than two-thirds of overall economic activity.

The nation's inventory-to-sales ratio—which tracks the value of inventories relative to monthly overall sales—spiked late last year and into 2015. It has remained elevated throughout the year, with the current ratio of 1.37 sitting above the 1.30 level in October 2014, according to the U.S. Census Bureau.

The inventory bloat was due in part to the impact of the labor strife at the West Coast ports, which resulted in ship delays, cargo backlogs, and fewer sales. After labor and management agreed in late February to a new five-year contract, goods began to flow again. However, much of the delayed merchandise was supposed to be in stores during the first quarter for early spring sales. By the time the goods got to retailers, much of the buying season had passed, and retailers were stuck with oversupplies of product that few consumers wanted at that point.

Wilson said the combination of high inventory levels and the possibility of an interest-rate hike by the Federal Reserve as early as December will cause a significant increase in inventory-carrying costs. It also will result in an inventory drawdown similar to that felt in 2009 and 2010, which included the recessionary period when the inventory-to-sales ratio soared because of sharply contracting economic activity, Wilson said. The analyst said that the November and December months, which are historically weak because most holiday merchandise is already in place and companies have exhausted their annual budgets, will likely be just as subpar this year. That's because retailers have ample supply of product on hand, she said.

Though not as strong as September, which is one of the best freight months of the year, October had held its own during the September-to-November period until the 2008-2009 financial crisis and subsequent recession, according to a chart of the Cass index dating back to 1999.

Correction: An earlier version of this story was based on incorrect data tables that reversed the year-over-year and month-to-month figures. The correct data follows. The numbers have been corrected.