Signposts should be designed to be frangible, meaning they fracture, break away, or bend upon impact to reduce damage to vehicles and the risk of injury to occupants. Smaller signs use posts that deform easily, while larger signposts are equipped with mechanisms that yield in a controlled way.
The standard AS/NZS 3845.2: 2017 requires crash testing with an 1100 kg vehicle at 30 km/h, ensuring safety at various speeds. Burbridge et al. (other TMR QLD legends) (2018) found that circular hollow section posts of 76.1 mm diameter are the frangibility limit for small signs under this standard.
Importantly, if a post fractures, it should not pose a risk to vehicle occupants. Studies indicate that if the centre of gravity is more than 2.8 m above the lower end, the risk of the pole striking a vehicle roof is reduced.
Important note: while frangible posts are safer for cars, they still pose a hazard to motorcyclists. Check out our post (excuse the pun) about this from a few years ago: https://safesystemsolutions.com.au/safe-system-snippet-171-galvanised-steel-poles/
The research: Burbridge, A, Spathonis, J, Kung, L, Douglas, J & Moir, L 2018, ‘Exploring the frangibility of steel circular hollow section small sign support posts’, Journal of the Australasian College of Road Safety – Volume 29 No. 1 2018.
Video credit: Midwest Roadside Safety Facility
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