A Boom Barrier is a bar, or pole pivoted to allow the boom to block vehicular access through a controlled point. Typically the tip of a boom gate rises in a vertical arc to a near vertical position. Boom gates are often counterweighted, so the pole is easily tipped. Boom gates are often paired either end to end, or offset appropriately to block traffic in both directions. Some boom gates also have a second arm, which pivots on links that allow the second arm to hang 300 to 400 mm below the upper arm when it descends into the horizontal position, to increase approach visibility.