hydrogen powered cars has been a technology that is available for years. go to this link to see one of the early pioneers
Billings Energy Corporation The post office had a fleet of vehicles for delivering the mail for quite a while to see the feasability of doing that. When you burn hydrogen in a internal combustion engine you get almost no emissions. Billings made a fuel cell that used aromatic hydrocarbons to absorb the hydrogen so you eliminated the danger of it blowing up in an accident. The biggest problem I see is range. The postal vehicles had most of the space in the vehicle taken up by tanks and it only had a range of 110 miles.