|This article does not cite any references or sources. (December 2009)|
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.
Boom gates are typically found at level (rail) crossings, drawbridges, parking facilities, checkpoints and entrances to restricted areas. They are also the usual method for controlling passage through toll booths, and can also be found on some freeway entrance ramps which are automatically controlled to drop to restrict traffic in the event of accident cleanup or road closures without the need to dispatch road workers or law enforcement to use a vehicle to block the way.
Some boom gates are automatic and powered, others are manually operated. Manual gates are sometimes hung in the manner of a normal gate (i.e. hinged horizontally).
In some places, boom gates are installed across suburban streets as a traffic calming measure, preventing through traffic, while allowing authorised vehicles such as emergency services and buses to take advantage of the shorter and more direct route.