These structures were first required by legislation in Sweden in 1959. Before they 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 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 to the ground, in both cases causing the tractor to pivot around the rear axle. ROPS were first required in the United States in 1986, but this requirement did not retroactively apply to tractors produced before this year; therefore, adoption of ROPS has been incomplete in the farming community. To combat this problem, CROPS (cost-effective roll-over protection structures) have been developed to encourage farmers to retrofit older tractors.