A sign is shown for a natural gas station in San Diego, California.
A sign is shown for a natural gas station in San Diego, California November 3, 2015. Sales of trucks powered by natural gas are sputtering and growth will be far weaker this year than last as tumbling diesel prices prevent drivers from switching over to the cleaner-burning fuel even though it is cheaper than it has been in years. Picture taken November 3, 2015. REUTERS/Mike Blake