자료: Wikipedia, http://encyclopedia.thefreedictionary.com/cattle+guard
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Cattle grid in remote Western Australia.
A cattle grid or cattle guard, also known as a vehicle pass, is a type of obstacle used to prevent hoofed animals, such as sheep or cows, from passing. It consists of a grid of bars or tubes, usually made of metal, firmly fixed on the ground so that the holes are wide enough for animals legs to fall through, but narrow enough that a vehicle's wheels will not. A cattle grid will allow wheeled vehicles to pass through the entrance, but will contain horses, cattle or other wildlife within the enclosure because they will refuse to step on the gate.
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Cattle guard on California Highway 76 near Lake Henshaw.
These grids are usually installed over countryside roads where they cross a fence. The alternative is to create a gate that would need to be manually or automatically opened when a vehicle comes. They are common where roads cross open moorland or common land maintained by grazing, but where segregation of fields is impractical, such as in the Scottish Highlands or the National Parks of England and Wales. They are also common throughout the Western United States and Canada, where they are sometimes referred to as "Texas gates." Cattle grids are also used when otherwise unfenced railways cross a fenceline.
While these barriers are usually effective, they can fail due to ingenious animals. Sheep have been known to roll on their backs or sides over grids as wide as 8 feet (2.4m), traversing them in order to find more and better food or water.
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Virtual grid near Lone Pine, California
Portable texas gates suspend the gate by springs so that it lowers to the ground when a vehicle passes over then returns to a position 6 inches above the ground.
"Virtual" cattle grids can also be used. These look like cattle grids, but are only painted lines on the highway. The light-dark pattern of lines and pavement resembles a true cattle guard to animals. Animals see more intense contrasts of light and dark because their night vision is so much better than ours. Animals see the sharp contrast of the cattle guard on the ground as a false visual cliff; they act as if the dark spots are deeper than the light spots. Using a virtual cattle guard is cheaper than a true cattle guard, and can be used on higher-speed roads due to its smooth surface.
See also
External links
- "Crafty sheep conquer cattle grids", BBC News Online, 30 July 2004.
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