I think it is only heavy truck fleets and they are doing it to get away from the expense, maintenance, and down time, of crappy Diesel Particulate Filter systems.
It's only short-haul vehicles. Fleets that operate in a specific metro area are the poster child, which is why you mostly see CNG buses. It provides poor energy density compared to diesel fuel, and the engines are gasoline engines so they provide poor torque compared to diesel fuel as well unless you spend more on gearing.
Since we have the technology to make biodiesel from algae and run that in our existing diesel fleets, that would be the "greenest" solution available with current technology. No fracking involved.