A key measure of truckload capacity shot up 5.9 percent, or 9.1 points, indicating that truckload capacity tightened in the first week of April as freight demand increased.
The Longbow Research Truckload Barometer rose to 164.2 from 155.1, reversing a 5.7 percent drop in the index in the month of March.
Since January, the index has risen 49.4 percent. The index measures available freight against available equipment, climbing as capacity contracts.
Year-over-year, the barometer is up 51.7 percent, Longbow said April 7.
Strong produce demand in the Southeast and improvements in manufacturing and retailing are constricting the availability of tractor-trailers, Longbow said.
Truckload shipping volume rose 4 to 6 percent in March, on average, the investment research firm said, and many truckload carriers are overbooked.
The Cass Freight Index showed freight shipments up 6.9 percent in March from February, with tonnage increasing across transportation modes.
Longbow said some truckload carriers are operating at 110 to 115 percent of capacity and looking for 5 to 10 percent rate increases in contract negotiations.
Spot truck rates jumped 15 to 30 percent, Longbow said, leading some trucking companies to "reserve" capacity for more profitable spot market freight.
Even when rate increases are more modest, carriers are beginning to be compensated for deadhead or empty miles and repositioning tractors.
That alone would increase shipper transportation costs even if rates stay flat.