A key monthly index of U.S. shipping activity continues to show improvement as 2009 progresses. The index, published by freight audit and payment firm Cass Information Systems, is based on the expenditures and shipments of Cass's clients. In June, the freight expenditure index reached 1.489, up from 1.43 in May and 1.39 in April. The shipment index came in at 0.933, up from 0.913 in May and 0.878 in April.

Since the start of 2009, the Cass index has fluctuated wildly. For instance, the February expenditure index was 1.507 and the shipment index stood at 0.938, which are both above June levels. Given the general view that economic weakness was more pronounced in February than in June, Cass executives warn against reading too much into month-over-month numbers, saying the index won't produce a meaningful trend until summer's end.

To put June's numbers in historical context, from the end of 2005 until mid-2008, the expenditure index averaged about 2.0, while the shipment index averaged about 1.2. Both averages are significantly higher than June's figures, a reflection of the rapid decline in the economy and in shipping since last summer.

In 2008, Cass audited about 26 million shipments representing roughly $17.5 billion in shipper expenditures.