You have inconsistent fears. Presumably you mention 70mph because you're afraid of explosions from vehicle collisions. Yet ammonia is much less flammable (NFPA

You have inconsistent fears. Presumably you mention 70mph because you're afraid of explosions from vehicle collisions. Yet ammonia is much less flammable (NFPA 1) than gasoline (3), and much much less flammable than elemental hydrogen (4). Ammonia is regularly used as a refrigerant and has been for longer than CFCs have existed. It is poisonous, but no more so than many gasoline additives and less so than tetraethyllead, which was a common additive until quite recently. More to the point, when burned in an engine it produces nitrogen and water, which are much safer than ICE exhaust. No one is proposing hydrogen or ammonia as an energy source. There are no underground deposits. They will only make sense once solar generation is efficient enough that electrolysis becomes competitive with fossil fuels. We'll know that's coming soon when hydrogen hype artists are drowned out by engineers talking about ammonia.