That's assuming the hydrogen is just loose in the area, like it'd been released from a balloon in a chemistry classroom. That amount of hydrogen is extremely small, from an energy standpoint. Equivalent to a teaspoon of gasoline or so.

If you assume a realistic fuel capacity for a hydrogen vehicle, the hydrogen tank will be both much larger than a gas tank and the hydrogen will be under extreme pressure. A tank like that in your car would be extremely dangerous even if it were filled only with inert gas.

Hydrogen mixed with air has a very wide range of concentrations where it is explosive. It accumulates inside containers or just the roof of the car… where the passengers are. It takes just one lit cigarette for it to go boom.