Although the transportation sector is mainly fueled by petroleum, the amount of natural gas used for vehicle fuel has grown to 43.4 billion cubic feet (bcf) in 2017. From 1990 to 2017 the average annual rate of growth was 20.7%.
According to the Energy Information Administration’s Annual Energy Outlook, more than half (52%) of natural gas vehicle fuel was used by heavy trucks in 2017. Buses used 27% and light-duty vehicles, typically in fleets, accounted for 20% of the vehicle fuel used. Locomotives used only 1%.
Energy Information Administration, September Monthly Energy Review, Table 4.3.