Market conditions are growing more difficult for the nation's motor freight shippers.

A monthly index released Friday by consultancy FTR Associates fell to -11.4 in May, a dramatic decline from the -7.7 reading posted in April. Any reading below zero is considered unfavorable for shippers. The index, known as the Shippers' Condition Index (SCI), is a compilation of "market influences" that affect shippers, the Nashville, Ind.-based consultancy said.

The May figures reflect the double whammy of rapidly tightening truck capacity and soaring transportation costs, FTR said. The most visible cost factor is the price of diesel fuel, which, on average nationwide, reached $4.10 a gallon as of May 9, an increase of more than $1 a gallon from year-earlier levels.

Shippers may see some relief in the short term, the consultancy said. "Looking ahead, with the slower recent growth in the economy, we are expecting the SCI to begin to stabilize and would not be surprised to even see some modest improvement in the next few months," said Larry Gross, a senior consultant at FTR, in a statement. "Such improvement would not, however, change our basic outlook for a difficult environment for shippers through the end of this year and well beyond."

FTR launched the index in 2010.