I know it is politically incorrect to suggest this BUT, there is almost never a good reason to be wearing a seat belt on a tractor. There is only one kind of circumstance where it helps protect you and that is where you need the ROPS to prevent being crushed in a roll-over. In that rare event, you should wear the seat belt, only when you are in danger of rolling the tractor over. In my opinion I should NEVER be operating my tractor where there is significant likelihood of rolling the machine. I have never seen any plausibility argument, much less "proof," that you should wear a seat belt on a tractor. In a car or truck where many high speed objects are around you all the time -- absolutely wear the belt ! On a tractor, NO. I also hold the opinion that a seat belt on a tractor is predominantly a butt covering exercise demanded by lawyers working for the manufacturers.
It doesn't take much to roll over. You can do it on flat ground with an off center load in the bucket that shifts or by letting off of the clutch too fast in the wrong gear or with a heavy load on the back.
Per the Australian Centre for Agricultural Health and Safety:
http://farmsafewa.org/media/1377/Tractor%20Rollovers.pdf - Page 10 said:
It is possible to roll or flip a tractor over on any terrain carrying out any
task. In contrast to popular belief, 90% of tractor accidents occur at speeds
less than 8 km/hr. In addition, 60% of all accidents occur on slopes less
than 5 degrees. Thus, there is always an element of risk involved with
operating a tractor. However, certain environmental conditions pose more
of a risk than others. For example, broken ground such as gullies and
stumps hidden in long grass represent a significant risk despite the terrain
being flat
Here are some excellent videos of a tractor rolling over on a slight slope:
Tractor Overturn, H.J. Sommer III
It doesn't take a big hole or a nasty slope, it could be a slight dip on one side and a rock on the other, or just a rock and a slight slope, missing a gear on a slope (been there, done that with a garden tractor), or having unlocked turning brakes at the wrong time.
Here is a news story showing the aftermath of a rollover on a golf course:
Maintenance worker killed in country club rollover accident - CBS46 News
Also, when pushing snow with a plow or the loader, scraping with a backblade, box blade, loader, etc, if you find something in the ground (like a rock, stump, root, etc) it can stop the tractor dead in its tracks and if you aren't buckled in, it can throw you into the windshield of a cab tractor or completely off of an open station tractor.
Aaron Z