A cattle grid or cattle guard – also known as a vehicle pass or, in New Zealand, a cattle stop – is a type of obstacle used to prevent hooved animals, such as sheep or cows, from passing along a road which penetrates the fencing surrounding an an enclosed piece of land. It consists of a depression in the road covered by a transverse grid of bars or tubes, normally made of metal and firmly fixed to the ground on either side of the depression, such that the gaps between them are wide enough for animals' legs to fall through, but sufficiently narrow not to impede a wheeled vehicle. They rely for their effect (of barring passage to animals but not to wheeled vehicles) upon animals' reluctance to set foot upon them.
Cattle grids are usually installed over countryside roads where they cross a fenceline, often at a boundary between public and private lands. They are an alternative to the erection of gates that would need to be opened and closed every time a vehicle passed, and 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, particularly on BLM and Forest Service land, where they are usually called a cattle guard or, occasionally, a Texas gate. 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 jump or run along the side of grids as wide as , traversing them in order to find more and better food or water. Some animals, particularly wildlife, can jump across them, and animals with particularly large feet, such as American bison or even particularly large bovine bulls, can walk across them without slipping between the bars. In areas with heavy snowfall and long periods without a thaw, snow can fill up under a grid and allow animals to walk across it.
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 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. Many animals see a more intense contrast between light and dark because their night vision is much better than humans'. 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.
There is a British Standard for cattle grids: BS4008:2006. The US standards are put forth by The American Association of State Highway Transportation Officials (AASHTO). AASHTO provides load rating guidelines for cattle guards that are used on public roads. All cattle guards used on public roads must be certified by a qualified engineer that the guard meets AASHTO guidelines.