what happens if a visitor to the USA, without insurance, is involved in a major accident?

Any hospital in the US that participates in either Medicare or Medicaid (and that’s almost all of them) is required to give a medical screening exam to all people for whom one is requested, and to give stabilizing treatment for any emergent condition, regardless of ability to pay. The bill may be huge, so travel insurance is a good idea for visitors.
Oh, and the law that requires hospitals to treat does not fund the requirement; it’s just there, for hospitals to find the money wherever they can.