regarding psycho69's cousin's case,
from black's law dictionary:
negligence is the failure to use such care as a resonably prudent and careful person would use under similar circumstances; it is the doing of some act which a person of ordinary prudence would not have done under similar circumstances or failure to do what a person of ordinary prudence would have done under similar circumstances. the term refers only to that legal delinquency which results whenever a man fails to exhibit the care which he ought to exhibit, whether it be slight, ordinary or great.
assumption of risk means legally that a plaintiff may not recover for an injury to which he assents, i.e., that a person may not recover for an injury received when he voluntarily exposes himself to a known and appreciated danger. the requirements for the defense of assumption of risk are that: 1. the plaintiff has knowledge of facts constituting a dangerous situtation, 2. he knows that the condition is dangerous, 3. he appreciates the nature or extent of the danger and 4. he voluntarily exposes himself to the danger.
i don't think anyone can argue that the track owner took even slight care parking the tractor next to the track.
as for saying that he assumed the risk, when you sign up for a race, tractors in the way are not part of what constitutes the hazards of racing. in order to defend himself against a negligence suit, the track owner would have to prove that the rider was aware of the danger and chose to ride anyway. if i was a gambling man, based on what psycho has said, i would bet that the track owner will lose this one (as i believe he should).