Funny how people complain about the dangers of driving around with a tank of combustible hydrogen in the trunk, when they're already driving around with a tank of combustible petroleum in the trunk.
A more practicable alternative would be a hydrogen fuel cell with the hydrogen being produced on board from a traditional hydrocarbon source. Honda and others are researching that technology today.
There are significant differences between these two fuels. First, hydrogen is a very small molecule which diffuses through fuel tanks into the area surrounding the vehicle. Secondly, hydrogen ignites at a much lower temperature than does gasoline. Third, hydrogen combusts a great deal faster, and thus more explosively than does gasoline. Fourth, a hydrogen flame is invisible in daylight ... you'll walk into a very hot flame before you know it is present.
There is a very large sign at the liquid hydrogen storage facility at the Redstone Arsenal: "Danger, no smoking. Violators will be decimated."
Hydrogen is not a safe fuel. Besides, what is your source of hydrogen? Fossil fuel? Water? It takes more energy to break water down into hydrogen and oxygen that you get from the combustion of that fuel.
Here's a little experiment for you. Take a cup and fill it with gas. Light a match. Throw match into cup. Watch match go out. Happens every time.
It's not so much the combustibility that's the problem, it's the pressure. The pressure required to condense enough hydrogen to drive to the grocery store and back (and yet fit in the truck of your car) is enormous.
Combine the energy required to compress the hydrogen along with the initial energy required to separate it from whatever it's bonded to, and H2 is a big, big loser.
It would be cheaper (in dollars and lives lost) to just invade Iran.
H2 under high pressure will autoignite if it leaks with a high pressure drop. I'll take my chances with petroleum.