I think the amount of ash deposited would depend largely on wind direction. Irrespective of where it was deposited, heavy ash fall would preclude surface transportation. Street, highways and probably even railroads would be clogged and impassable. We are seeing that currently with the LaPalma eruption and, as these things go, LaPalma is a small eruption.
I don't think the entire US military has the resources to supply huge numbers of refugees/victims by air.
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