What are the luminous little pieces of plastic attached to the wheel nuts on large lorries for?

I've seen this a few times now and was wondering what purpose these plastic pointers serve. The tips of the plastic pieces are tapered towards one another usualy as well

Answers:
These were designed to indicate if the wheel nuts were becoming lose after a spate of regular road deaths and accidents some years ago through badly maintained HGV's. If the arrows have moved from their position, then this indicates that the wheel nuts are loosening.
To show if the wheel nuts move (i.e. get loose) and get unsafe.
indicates any movement or loosening of the nuts without having to check them with a torque wrench
this is wheel gaurds used to tell u if nut is coming loose if pointer dont line up nut may be loose time to check
hi the plastic markers are there to show if the wheel nuts become loose they point at each other but move apart if a wheel nut comes loose.
means you are able to see and keep the distance
A quick visual indication to check on wheel nut tightness. This has now become part of the checks professional drivers have to do before taking a vehicle out - much easier than using a wrench.
hi i drive a hgv , all the answers are right , but bear in mind i average 100,000 + miles a year ,more miles=more wear and tear , they are handy for doing a quick visual check on wheels

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