Are hydrogen cars likely to blow up your garage?
Probably not. Very many years ago, I spent my fourth year at university working on a project which involved cooling an infrared transparent window to 20 K. This was done by means of hydrogen: a day's experiment involves the use of quite a large cylinder (about 5 foot tall) filled with hydrogen at some rather high pressure. The hydrogen would first be pre-cooled with liquid nitrogen before being allowed to expand and therefore cool further. After about an hour of this, the infrared window would reach 20 K.
What did we do with the hydrogen once it had been expanded and cooled? There was a tube which ran out to the window. That was it. During the course of the day's experiment, a full cylinder of hydrogen would be vented to the air outside. I looked at this, and asked, what's the chance of this blowing up? The answer I was given is that concentration of hydrogen in the air had to be greater than 4% in order for it to ignite. There's an article about it here:
https://en.wikipedia.org/wiki/Hydrogen_safety
The other problem is that if the system springs a leak, and the hydrogen is ignited, the flame is totally colourless (no incandescent articles of carbon). This can also be a hazard since you can't see it.
At the moment, hydrogen powered cars are something of a rarity:
https://en.wikipedia.org/wiki/Hydrogen_vehicle
PS – sorry Chris, for feeding your monster, but in this, the year of the plague, there's not a lot else to do.