If a majority of our automobiles were replaced with vehicles which operate on hydrogen powered fuel cells, how could massive amounts of hydrogen be stored safely for refueling? How safe are these cars when the fuel tank ruptures due to collision compared to current gasoline/diesel ones currently in use? How many miles will the vehicles be able to go before refueling?
Let's think outside the box for a moment shall we? You don't specify the form that the hydrogen needs to stored in to use this system. How about we assume hydrogen is created by combining to perfectly stable elements in a catalyzing method to generate the hydrogen.
Then we don't have to worry about storage safety. These cars would then be perfectly safe if there is a rupture in the tank because we have two stable catalyst agents and we only have to worry about making sure they don't combine.
Miles before refueling is of course determined by how much fuel we have in the vehicle. The only limiting parameters would be storage weight and bulk of the fuel in relation to the vehicle, so we could theoretically have a vehicle go thousands of miles before refueling.
I'm not saying this will be the system that is created, I'm just saying it's possible as we learn to develop new technologies. Let's think BIG!