Traffic barriers (sometimes called crash barriers, or Armco barriers, also known in North America as guardrails or guard rails and in Britain as crash barriers) keep vehicles within their roadway and prevent vehicles from colliding with dangerous obstacles such as boulders, buildings, walls or large storm drains. Traffic barriers are also installed at the roadside to prevent errant vehicles from traversing steep (non-recoverable) slopes or entering deep water. Traffic barriers are installed within medians of divided highways to prevent errant vehicles from entering the opposing carriageway of traffic and help to reduce head-on collisions. Some of these barriers, designed to be struck from either side, are called median barriers. Crash barriers can also be used to protect vulnerable areas like school yards, pedestrian zones or fuel tanks from errant vehicles. (source)