How pratical are hydrogen fuel cell vechicles?

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?

What impact could hydrogen fuel cell powered vehicles have on the environment?

Wouldn't the water vapor released from tens of millions of fuel cell powered vehicles create massive amounts of humidity to the point that in 50 years the whole earth will be covered in a massive humidity blanket which could raise temperatures and destroy ecosystems