The main parts
A boom barrier has a housing with a motor and gearbox, a pivoting arm, a controller and safety devices. The motor turns the arm up and down through a balance mechanism so the movement is smooth.
Who triggers it
The barrier can be triggered by a guard, a remote, a card or tag reader, a ticket machine or a camera. Vehicle loops in the road tell the controller that a car is under the arm.
Arm styles
Straight arms suit standard lanes. Folding and telescopic arms help where height or width is limited. Fence-style arms keep the opening closed to pedestrians as well.