American truckers in the last three months are hauling more freight and spending more time on the road as fuel prices stabilize and more materials are getting transported, industry indices show.

The Ceridian-UCLA Pulse of Commerce Index, measuring fuel consumption for trucks, increased 0.8 percent in May after increasing 0.1 percent in April and 0.3 percent in March, according to data released Wednesday. The group's May index was 94.9, up from 93.2 in January.

North American freight shipments and spending increased in May for the fourth consecutive month, according to the Cass Freight Index Report. North American freight shipments and expenditures both posted increases for the fourth consecutive month, with spending growth outpacing the rise in shipment volume, said the Cass Report, released Wednesday.

The number of freight shipments has risen at a very steady pace in 2012 but is still well below the most recent high point in September 2011.

Cumulatively for the year, freight dollars are up 7.8 percent year over year, while shipments volume has increased by 8.5 percent since  2011.

The gains have come as fuel costs have stabilized, but labor costs have continued to rise, especially for drivers, according to the Cass report. Still, the transportation industry has been shielded from the nation's weak job market, with more railroad, truck, warehouse and distribution jobs added in the last several months.

Continued growth is uncertain, as imports and exports have fallen amid the eurozone's recession and slowing growth in China. A resultant slowdown in manufacturing would led to lower volume and possible layoffs for truck companies.

The economy is still growing at its very slow rate, and what looked like a resurgence during this second quarter may fizzle out in the next few months, said the Cass report.