From SCDigest's perspective, data about US trucking volumes, rates and more is very difficult to get a handle on.

Frankly, the data from the American Trucking Associations, other sources such as Cass Information Systems, carriers themselves during earnings reports and more never seem to all line up. For example, for the past year at least there has been quite a discrepancy between the ATA's freight tonnage index and the Cass shipments index.

While tonnage and shipments are not the same thing, one would think they would generally move in tandem, but recently they have not, with the ATA index reaching record highs, and the Cass shipment index largely flat. Cass does say that weight per shipment has been rising a bit, but we don't believe enough to explain most of the variance between shipment and tonnage growth.

Nevertheless,  below is one of our web charts that compares the ATA tonnage index against the the Cass linehaul price index. The latter measure tracks per mile US truckload rates, before accessorials, fuel surcharge, etc. Since Cass pays some $23 billion in shipper freight bills each year, its data in this regard has to be pretty darn good. (The ATA index uses a baseline year of 2000, and the Cass index 2005.)

It is interesting to use the chart tools to compare different time periods, but let's focus on the past two years here.

During that time, the ATA measure has risen from 118 to 131.7 in December - that's a rise of 11.6% in 24 months - very healthy volume growth, with the index at an all-time high.

Meanwhile, over that same period, the Cass Linenhaul index is up from 110 to 116 - that's a rise of just 5.4%, meaning truckload rates have been rising 2.7% or so over the past two years. That's sounds about right, and consistent with carrier reports.

If the ATA is correct about freight tonnage growth, in a period where clearly carriers have been very disciplined about capacity, we would expect to see both capacity and price pressures in 2014, like more in the 3-4% range for rate increases - but as always that depends on the economy, which as usual is again sending mixed signals in this new normal.

Any Feedback on our Supply Chain Graphic of the Week? Let us know your thoughts at the Feedback section below.