The natural-gas-powered vehicle market has slowed as diesel fuel prices continue to drop, according to recent ACT Research's study of fuel trends in the heavy duty truck market.

Ensuring driver safety is the biggest priority of fleet managers, according to a survey conducted at the 2014 NAFA Institute and Expo by GE Capital Fleet Services. With more than a third (35 percent) of fleet managers identifying it as their main concern, driver safety outpaced cost-savings goals and workforce productivity (27 percent and 22 percent, respectively) as the top concern for the second year in a row.

Natural gas has started to challenge oil as the dominant transport fuel with companies building gas-powered ships and installing networks of service stations on water and land.