National Renewable Energy Laboratory (NREL):

So much for switching from diesel fuel to natural gas to protect the environment. A new report concludes that it is best to stick with oil-based fuels because methane leakage from natural gas is much greater than currently estimated.
Feb 18, 2014   The New York Times - U.S.
<p>In the San Francisco Bay Area, two public transit agencies are conducting trials with "Zero Emission Buses", or ZEBs. They are using different types of hydrogen fuel cell technologies, with very different results.</p>
Mar 3, 2008   San Jose Mercury News