Natural Gas and Propane
Many heavy-duty and mass-transit vehicles in the US already run off either propane or natural gas. These fuels have lower emissions of nitrogen oxides and carbon monoxide than gasoline, leading to environmental benefits versus traditional fossil fuels. Natural gas and propane are still fossil fuels, however, so they introduce carbon into the atmosphere that was not there previously. Additionally, like oil, there are limited natural gas and propane reservoirs available for use, making them a short term solution at best.