I turned around and did a quick half run so I have a video showing approx 250m warning from the rear (it alerted as soon as I turned back onto Whitfords). I couldn't be bothered going all the way back to Marmion Av in peak hour though.

There was nothing unusual about the way it was set up. I'd say that it was reflecting the signal back either from the oncoming traffic or from something else (sign, bus stop, light pole etc).

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MY99 WRX (Sold)
MY06 WRX 241hp 430nm atw (Sold)
MY09 BMW 135i 247kw atw (Current)