Had to look up ROPS... thanks for the tip :)
Tractor rollovers are the single deadliest type of injury incident on farms in the United States. The latest figures from the National Institute of Occupational Safety and Health (NIOSH) suggest there are approximately 250 tractor rollover fatalities per year. NIOSH estimates that there are approximately 4.7 million tractors in use on U.S. farms; one-half of them are without rollover protection for the operator. This fact sheet will closely examine tractor rollover protection issues.
What are ROPS?
Rollover Protection Structures (ROPS) are roll bars or roll cages designed for wheel- and track-type agricultural tractors. ROPS are designed to create a protective zone around the operator when a rollover occurs. When used with a seat belt, the ROPS will prevent the operator from being thrown from the protective zone and crushed from an overturning tractor or from equipment mounted or hooked to the tractor (see Figure 1).
http://www.nasdonline.org/document/113/d001656/rollover-protection-for-farm-tractor-operators.html