Transcore's North American Freight Index slipped 3.2 percent in May from the previous month, after rising 25 percent and 44 percent month-to-month in April and March.

The drop may indicate that the surge in spot market freight demand filling trailers since March is slowing, though still strong compared with 2009 and even 2008.

Load volume was the highest for the month of May in the past 10 years, TransCore said. The freight index was up 216 percent over May 2009, and 12 percent over May 2008.

After staying within a 100-point range for six months, the index increased more than 140 percent from January through April, rising from 556 points to 1,355.

Flatbed freight has been "extraordinarily high" since January, contributing to the surge in spot market load volume, TransCore said. It declined by 15 percent in May, however.

TransCore's index measures the availability of freight and equipment on the truckload spot market, often an early indicator of trends in contract freight markets.

The index is based on data from TransCore's DAT load-matching network, which handles more than 60 million loads and trucks per year.

Source: JOC