All true.. however.. none of it relieves liability from either party. If the tractor did not have proper sinage or lighting.. thent he tractor operator contributed tot he accident by creating an unsafe condition.. If it was a crest of a hill, then I still feel the driver was at fault. The law says you have to be in controll of your vehicle 100% of the time. If that meanse you have to slow down on hills and curves.. or when driving at night so as not to overdrive yourheadlamps.. then so be it. I've heard of a ticket for exceding a safe speed, or for going to fast for 'condition's, even though the person was not exceeding the posted speed limit. Obviously this means it is up to the driver to choose a speed at or under the posted speed limit that lets him drive safely, and maintain control of his vehicle, and not run into them because he is going to fast.. etc.
There may be other mitigating circumstances.. such as visual obstructions caused by the municipality or private owners partially blocking visibility of traffic ahead thru billboards, illegal sinage, parjed cars ont he row.. etc..
Soundguy