Paul Scott wrote:

Its that difficult to understand sign that usually gets referred to as
'Danger - low flying motorbikes' - most car drivers are pretty well
aware of the normal 'No Entry' sign...

The sign would have to be triangular to have the meaning you suggest.
Being circular it must prohibit low-flying motorbikes, like the one that
prohibits drivers of vehicles with sun roofs having yellow afro hairdos.

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