A modern tractor with roll protection

Modern tractors have rollover protection systems (ROPS) to prevent an operator from being crushed if the tractor rolls over. This is especially important in open-air tractors, where the ROPS is a steel beam that extends above the operator's seat. For tractors with operator cabs, the ROPS is part of the frame of the cab. Before ROPS were required, some farmers died when their tractors rolled on top of them. Row-crop tractors, before ROPS, were particularly dangerous because of their 'tricycle' design with the two front wheels spaced close together and angled inward toward the ground. Some farmers were killed by rollovers while operating tractors along steep slopes. Others have been killed while attempting to tow or pull an excessive load from above axle height, or when cold weather caused the tires to freeze down, in both cases causing the tractor to pivot around the rear axle. ROPS were first required by legislation in New Zealand in the 1960s.


http://en.wikipedia.org/wiki/Tractor