exit ramps of the Interstate Highway System in rural areas. Variations Almost all cattle grids are built around a grid. Most include a pit dug along a fence line, a base for the grid to rest on, and wings to connect the guard to the fence. Since many guards were or are home-made and to some degree idiosyncratic, a wide variety of designs exist. Designs may vary with local conditions. Materials used for construction of a cattle grid depend partly on the weight it must bear. A study of the bars of traditional cattle grids in the Flint Hills of Kansas found that 80 percent were made of pipe, while smaller percentages were made of railroad tracks, I-beams, planks, and other materials. The size of the bars varied from ; the spaces between bars varied from ; the number of bars per grid varied from 4 to 22. Grids differed in length from and in width from , while the pits beneath grids were deep. Cattle grids, as they are called in Great Britain, Ireland, and South Africa, are known by a wide variety of other names in other parts of the world. In the United States, they are cattle guards. Mata burro ("donkey killer") is the preferred name in Brazil and Venezuela, while guarda ganado ("cattle keeper") is what they are called in Argentina. Alternatives in the United States include car crossing, auto gate, corduroy gate, stock gap, cattle pass, run-over, and many others. Canadians use pit gate, vehicle pass, and Texas gate, as well as cattle guard, which in Canada refers mainly to guards at railway lines. Concrete Cattle grids made entirely or mostly of concrete have existed since the 1940s. Individual ranchers have often constructed their own, sometimes using plans developed in the 1940s. In the 21st century, a set of plans for do-it-yourself guards made of wood and concrete are available via the web site of the Missouri Alternatives Center at the University of Missouri in the US. Commercial precast concrete versions are also available; Smith Cattleguard Company, based in Virginia, sold more than 15,000 of them between 1960 and 1980. Manufacturers also produce commercial polyethylene forms with reinforcing rods. Placed in or on the ground and filled, a finished cattle grid with concrete reinforced with fiberglass (GFRP) rebar can support vehicle loads of up to per axle. Steel University Lands, which manages land and mineral