The U.S. has barely dipped its toe in the water when it comes to using natural gas to power transportation. A shift from diesel and gasoline to natural gas for fleet vehicles and heavy-duty trucks would yield significant emission reductions. Clean Energy Fuels ( CLNE 5.16% ) is leading the charge with America's Natural Gas Highway project. With abundant domestic natural gas reserves and tremendous advances in engine technology, Clean Energy Fuels sees huge opportunity, particularly in the heavy-duty market.