Out of curiosity, I did a web search on ROPS. There were a lot of hits.
It's pretty clear that it is a fairly complex regulatory area, with rules being specified for lots of different machinery. Most of it seems to come from OSHA and refers to a couple of different SAE standards. That isn't surprising, since regulation by reference is a common federal practice.
I found the following on a Kansas State Univerisity page:
<font color="red">1. All two- or four-wheel drive type vehicles, or track vehicles of more than 20 hp engines that are designed to furnish the power to pull, carry, propel, or drive agricultural implements must be equipped with ROPS. Older vehicles not having ROPS should be retrofitted when operated in conditions where roll-over potential is high. Self-propelled implements are excluded.</font>
In the above statement, must had been lined through and replaced with should. I don't know why.
I tried searching the OSHA web site, but gave up when it asked for a user name and password. I'm not sure if it was due to server misconfiguration, or that I wandered into an area that isn't open to the public.
SnowRidge