Arm type is not a preference. It is determined by the width of the lane, the clearance above it and what sits beside it — and it is the decision that most often has to be undone on site because nobody measured first.
The arm follows the lane, not the catalogue
A barrier arm sweeps through a vertical arc. That arc needs space: clear height above the pivot for the raised arm, and clear ground for the arm to rest across when lowered. Three measurements decide everything:
- Lane width — the distance the arm must actually cover when down.
- Clear height above the barrier housing — the canopy, the beam, the overhead pipe or the cable run that a raised arm will meet.
- Side clearance for the housing itself, out of the path of a turning vehicle.
Get those three right and the arm type follows almost automatically.
Straight arms
The default, and correct for most lanes with open sky above. Simplest mechanism, fewest moving parts, longest service life, lowest cost. If your entry has a clear overhead and the lane is within a standard span, this is the answer and anything else is over-specification.
The limit is height. A straight arm raised vertically needs its full length in clear space above the pivot, so a canopy, an overhead beam or a low gate arch rules it out — and this is exactly the case in a great many society entrances where the guard cabin sits under a projecting slab.
Telescopic arms
A telescopic arm retracts into itself as it rises, so it needs materially less overhead clearance than a straight arm of the same span. This is the answer where a canopy or beam sits above the entry, which describes most basement ramps and many society gates.
The trade-offs are honest: more moving parts, so more to maintain, and a slightly slower cycle. Neither is a reason to avoid it where clearance demands it — but it is a reason not to fit one where a straight arm would have worked.
Curved and folding arms
A curved arm angles upward along its length, achieving something similar to telescopic in a different way, and it suits entries where the obstruction is partial rather than a full canopy. A folding arm hinges midway, and is used in the tightest overhead conditions — typically low basement entries where neither a straight nor a telescopic arm clears.
Both are specialised. If a quote proposes one, ask which measurement drove the choice. There should be a specific number behind it.
What the survey has to establish before quoting
The arm is one line in a decision that has several. A survey that only measures lane width has not done the job:
- Foundation. A barrier is bolted to a concrete base, and the base is set before the barrier arrives. Getting it wrong means breaking it out. What is under your lane surface matters — a basement ramp slab and an open driveway are different problems.
- Power routing to the barrier position, and whether it sits on the building’s backup supply so the gate still works during an outage.
- What triggers it. Guard push-button, access card or tag, vehicle-tag reader, or number-plate recognition — and where the reader physically sits relative to a driver’s window.
- Safety detection. Loop or sensor to prevent the arm descending onto a vehicle. Non-negotiable, and the item most often left off a cheap quote.
- Pedestrian separation. A barrier controls vehicles. If pedestrians share the lane, that is a design problem the barrier does not solve.
- Cycle rate. A gate handling a few hundred movements a day needs a mechanism rated for it. Domestic-duty barriers on a busy society entrance fail early and predictably.
Ask for the measurements on the quote. An arm type stated without the clearance figure behind it is a guess that gets discovered on installation day.
Common questions
How long should a boom barrier take to open?
Fast enough that a queue does not form, slow enough to be safe. On a busy society gate, cycle speed matters more than most people expect, because a slow barrier at peak hour produces exactly the pressure on the guard that the system was meant to remove. Ask for the cycle time and compare it against your peak movements per hour.
Will the barrier work during a power cut?
Only if it is on a backup supply, and this needs deciding at the design stage. Most barriers can also be released manually so vehicles are not trapped - confirm how, and make sure the guard has been shown, because discovering it during an outage with a queue building is not the moment to learn.
Can one barrier handle both entry and exit?
On a single shared lane, yes, and many societies run that way. It costs you throughput at peak times and it makes the entry and exit records harder to separate. Where the gate has room for two lanes, separate barriers give you cleaner records and no queueing conflict.