How Do Car Park Barriers Work?

A car park barrier is a motorised arm, or boom, that lifts to let a vehicle through and lowers behind it. The arm is driven by a motor and gearbox in a steel cabinet at the side of the lane, usually with a spring that balances the weight of the arm so the motor only has to move it, not hold it. A barrier controller in the same cabinet decides when to open and close.

Every automatic barrier follows the same basic cycle, whether the car park uses tickets, permits or number plate recognition.

Most barriers detect vehicles with inductive loops: a few turns of wire laid in a saw cut in the road surface and connected to a loop detector in the cabinet. The detector measures the loop’s inductance, which changes when a large metal object such as a car sits above it. A typical lane has up to three loops.

The presence or arming loop in front of the barrier tells the system a vehicle is waiting, so a ticket machine or ALPR read is only accepted when a car is actually there. The safety loop directly under the arm holds the barrier open while any part of the vehicle is over it. A closing or exit loop beyond the barrier can trigger the arm to close as soon as the vehicle has passed.

Where cutting the surface is not practical, or to cover motorcycles and high vehicles, barriers use photocells, radar or 3D sensors instead of, or as well as, loops.

Barriers are chosen by the width of the lane, the headroom and how much traffic they handle.

The barrier itself does not decide who is allowed in. It opens when the access control or parking system closes a contact or sends a command. The most common ways in are a ticket from a pay station, a proximity card or tag for staff and permit holders, an intercom to a control room, and automatic license plate recognition.

With ALPR, a camera reads the plate as the vehicle stops on the arming loop. A computer at the lane checks it against a list of permits, bookings or season tickets and then tells the barrier controller to open, usually through a relay output or a serial connection such as RS-232 or RS-485. Our RS-232 vs RS-485 FAQ covers the serial links used for barrier control. The same computer can switch lane lights, update signs and report every entry and exit to the back office. Our ALPR FAQ explains how plate reading works.

A barrier arm is a powered machine moving near people and vehicles, so installers fit safety devices to suit the site: safety loops, photocells across the lane, and controllers that sense an obstruction and reverse the arm. Breakaway arms limit damage if a vehicle hits the boom. In a power cut, most barriers can be released and raised by hand, and many have a battery backup or a setting to open automatically so vehicles are not trapped.

For the lane computer, the same thinking applies: it sits in an outdoor kiosk or cabinet, so a fanless design with a wide temperature range and isolated I/O keeps it reliable. Our PoE computers power the ALPR cameras from the same box, and the ALPR barrier control case study shows a working example. For how barriers fit into the wider site, see our car park management system FAQ.

We supply ALPR and lane control computers for barrier and parking integrators as standard, with the camera software, serial and relay I/O configured and tested before the system ships.