The 400' restriction within 3 miles of an airport makes since, since full-scale aircraft may be close to 400' AGL when landing or taking off. However, unless an aircraft is landing or taking off, or is experiencing an emergency, the aircraft must remain at least 500' from all people, vehicles, and structures (or higher, is many areas.) Model aircraft pilots do theoretically need to be aware of the "emergency" provision, though. If an full-scale is attempting an emergency landing in a field near a flying site (even on the model aircraft runway itself,) I think the FAA and AMA would agree that all model aircraft must yield to the full-scale aircraft, even if this means intentionally crashing a destroying a model aircraft. In the end, model aircraft pilots need to "see and avoid" full scale aircraft, just as full-scale aircraft pilots do under VFR conditions. A little common sense goes a long way. If I hear a full-scale aircraft in the vicinity while I am flying, I tend to descend a bit until the aircraft is no longer in the area. At our site, when full-scale aircraft are nearby, people who are not flying keep an eye on the aircraft, just in case the aircraft gets a little too close for comfort. One of our members even keeps a hand-held air band transceiver at the field (tuned to the nearby airport's traffic frequency) so we can hear transmissions from (and even communicate with) any aircraft in the area.