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Well sometimes they have the help of the police. Show on UK TV this week had air ambulance landing in area thoughtfully taped off by police right next to incident. Only problem was that the taped off area was a crime scene not a landing area. Pilot - was that FOD? - doctor no, I think it was evidence
Over here all medivac landings are pre-determined and agreed to by all involved to insure safety. And it may or may not be directly near the actual incident depending on terrain/location. Typically the closest parking lot or open field.
I think the mode of operation is different in the USA, to UK. In the UK the helicopter is used as a fast response critical care service carrying skilled doctors and advanced parmedics. They often perform advanced treatments, even up to opening the chest to carry out direct heart massage, at the scene. The helicoptor carries advanced equipment. Thus it is important to get them on scene ASAP. Pre-arranging a landing site would cause too much delay resulting in loss of life.
Many medical flights in the USA are just patient transfers with a nurse or paramedic.
Oh and in the UK they are free, so they can't sort out a landing site while waiting for the insurance company to authorise payment.