A cattle guard is a series of parallel metal bars which is installed in a roadway over a ditch to prevent ungulates from escaping a fenced area. The parallel bars are designed to be wide enough so that hooves and legs will fall through, while cars can drive over them safely, although they do need to slow down. Most hoofed animals are smart enough to recognize the potential hazard of a cattle guard, and they will avoid them: in some cases, animals will even avoid lines painted on a busy road to resemble a guard. These devices are in use all over the world as a practical alternative to gates, which must be opened and closed every time someone wants to pass through.
The concept of a cattle guard was originally conceived of in the American west by the railroads, which constantly had problems with free ranging cattle getting onto the train tracks and causing accidents. In 1913, an inventor named William J. Hickey recognized the potential uses for a cattle guard, with cars taking over America, and filed a patent for his invention, which was specifically developed for use in roadways. Two years later, the United States Patent and Trade Office approved the patent.
Today, the cattle guard is used all over the world, especially in nations where animals graze public lands which are split by roads. Prior to its introduction, anyone walking or driving across a fence line would have had to open a gate and close it behind them: while this task is not too onerous for walkers, it can be irritating to drivers, especially on long trips through public lands. The guard is designed to survive in the roadway for several years: it is laid flush with the road and treated with anti-rusting agents to prevent it from rusting out or being jostled by cars. Fence inspectors will periodically check the cattle guards as well, to make sure that they are still safe and usable.
Some animals can figure out a way around a cattle guard: sheep, for example, have been known to roll across guards of up to three feet (one meter) across. Horses and cattle sometimes attempt to jump them, although usually the width of the cattle guard is enough to deter this idea. If a farmer does need to move livestock over a guard, a sheet of wood can be thrown down over the bars for the animals to walk over, and if a guard needs to be removed from the roadway altogether, the bars are lifted and the ditch is filled before that section of the road is resurfaced with fresh asphalt.