Environmentally friendly hydrogen gas fueled vehicles can dramatically reduce greenhouse gas emissions and lessen the country's dependence on sources of fossil fuel. Though several hydrogen vehicles exist on the market today, there is still much room for improvement in the way that hydrogen is stored on-board the vehicle. With current technologies, hydrogen gas storage tanks have to be as large as or larger than the trunk of a car to carry enough gas to travel only one to two hundred miles. more about how here... http://www.sciencedaily.com/releases/2008/02/080227082835.htm
Since commercial quantities of hydrogen are derived from natural gas, it hardly seems like an "environmentally friendly" alternative. In my opinion, a hydrogen infrastructure is pure fantasy land. The water vapor produced by burning hydrogen is itself a greenhouse gas.
If you look past just filling up your [impractically expensive theoretical] car with hydrogen instead of gasoline--you'll find out that it takes some other fuel to create hydrogen the car can run on. And that fuel is something like oil, coal, or wood. The only solution is to stop using so much energy and hybridize the systems we need. De-centralize energy production--put solar panels up, wind generators, dams, tide generators, use your waste products to convert into fuel, but most importantly--stop using so much.
You need to think more clearly. 1 - You extrasct 1h1 from sea water using solar power. 2 - The water vapor is collected and recycled as liquid. 3 - You recycle H2O using Solar-Electric. There is NO green house gas.
I found the following series interesting, i'm sure somewhere in the series it is claimed we can make hydrogen fuel for less than petrol\gasoline... Hydrogen Fuel - Engine Fundamentals
Hydrogen type of fuel cells are almost on line now, Honda will be introducing some new cars that use that technology within a few years.