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.