Natural gas powers more than 12 million vehicles on the road today. Unfortunately, only about 250,000 of these are being used in the U.S., according to GE. The average growth rate in the U.S. shows a 3.7% increase per year since 2000, as contrasted with a booming global growth rate of 30.6% per year.
http://www.cngnow.com/what-is-cng/Pages/default.aspx
I heard that it is very popular in Thailand. I suppose it happens that a particular fuel tends to capture the majority of the market and then is difficult to change because of the infrastructure cost. Gasoline and diesel pack a big punch per gallon anyway.
I see the same website says natural gas is half the price of gasoline. I wonder if that includes the 100% fuel tax?