Barrier clarity

There’s a difference between a barrier designed to stop a person falling and one designed to stop a moving vehicle. As engineers, we frequently encounter residential driveways, car decks and garages where this distinction has been blurred. We see pedestrian balustrades on driveway walls or lightweight framing at the end of a garage backing onto a bedroom. The assumption is often that because the setting is residential, the risks are lower, or that a simple wheel stop will suffice.

Physics does not adjust its parameters for private land, and a vehicle impact on a structure designed only for pedestrians can cause it to fail. The confusion often stems from the Building Code’s different sections. Clause F4 addresses pedestrian safety from falling, typically requiring a barrier where a fall of 1m or more is possible. Clause B1 addresses structure and requires resistance to impact loads likely to be experienced. Meeting the requirements of F4 does not automatically meet the requirements of B1. The difference between vehicular and pedestrian loading is vast. A system designed solely for human loads cannot withstand vehicular impact.

People often argue that a specific area is not “intended” for vehicles, or that the homeowner will be careful. Ministry of Building, Innovation and Employment Determination 2017/077 clarifies that barriers are required based on foreseeable use rather than the owner’s intentions. If an area is physically accessible to a vehicle and a fall hazard exists, the risk of accidental impact is considered a “reasonable probability” rather than a remote possibility. This foreseeable use principle means we must design for the worst scenario.

There is a common misconception in vehicle barrier design that wheel stops are a safety barrier, but they are designed to position a car, not to stop a moving vehicle (see AS/NZS 2890.1). Standard kerbs and wheel stops can be counter-productive at higher speeds. A barrier kerb, for example, can act as a fulcrum, causing a vehicle to overturn or become airborne if struck at speed.

Accepting that a vehicle barrier is necessary, the design challenge becomes the load path. It’s not sufficient to select a post and bolt that can withstand 30kN in isolation. Structures are systems: we need to visualise where that energy goes and follow the load path. For example, a deliberate collector strategy that accounts for stiffness and strength, like a continuous edge beam, is required. Otherwise that force can be concentrated on a single member or connection. This can tear the connection out or split the timber grain before the load can be shared across adjacent members. On a timber car deck, the impact load is applied 0.5m above the deck, but the connections sit at joist/bearer level. This creates a significant moment and high loads from the post to the foundations. The barrier is part of a system and every link in that chain, from the rail to the joists and down to the foundation – including all the connections – must be considered and verified.

There are situations where the full load on a vehicle barrier may be reduced. For example, highway barriers function by deflecting vehicles impacting at shallow angles. While AS/NZS 1170.1 prescribes fixed impact loads, a design could be treated as an Alternative Solution if the geometry constrains the vehicle. If a residential driveway is narrow and prevents a vehicle from turning to hit the barrier perpendicularly, we can calculate the reduced force based on the angle of impact. The reduced loading must be supported by calculations and diagrams that show your assumptions.

An often-overlooked issue is garage end walls. If a garage wall backs onto a bedroom or a steep bank, that wall is acting as a vehicle barrier. It may require a Specific Engineering Design to ensure it can withstand the impact actions. Ultimately, vehicle barriers on residential land require the same rigour we apply to all structures. By understanding the true magnitude of vehicle loads, and designing complete load paths to the ground, we ensure that our designs protect both the structure and the people around it. A new guidance document is now available in the Guidelines and Templates section of Engineering New Zealand’s membership portal to help navigate these decisions.

Martin Pratchett MEngNZ CPEng is Engineering Practice Manager at Te Ao Rangahau.

This article was first published in the March 2026 issue of EG magazine.