Most of the people who safely use anhydrous ammonia on a regular basis are farmhands, not experienced hazmat workers. The training and equipment that allows them to do that would also be available to e.g. municipal bus lines or delivery companies. Any firm that hires people to refuel its vehicles could afford this. Eventually more fool-proof nozzles and storage tanks might be developed so that regular people with personal cars could safely handle this fuel, but it will be practical for other uses long before then.
Interestingly, the last time I heard an argument like this, it was being put forth in favor of using butane or propane as a replacement for CFC refrigerants in cars. That idea never caught on, but it's not clear if safety concerns in accidents were the reason.
At any rate, unless Farmer Brown regularly drives his fertilizer sprayer around on the farm at 70 MPH, I'm not convinced your analogy holds much more water than the anhydrous ammonia does.
Hydrogen is best thought of as an energy transfer mechanism -- an extremely lossy one at that -- not an energy source, and I don't see that changing anytime soon. It's hard to believe that ammonia-based storage is the missing factor in the hydrogen equation that we've all been searching for.
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.
Side note: common air duster can be used as refrigerant in old R12 systems used in cars until the early 1990s. there's a cheap "clamp" available online that punctures the duster can to tap it for the refrigerant. As far as i know, no modifications are needed to the old R12 system besides vacuuming out which any professional air conditioning technician can do.
