The same rational and logical processes are used to determine appropriate speed at any speed. Logic does not take a U turn at some arbitrary speed limit, nor does rationality. The only effect the speed limit has is to bring into play another hazard factor to be included in the calculation, that of the possible presence of Mr Plod (which, indeed, may be no minor factor, but it is simply part of the same universal process).
EDIT: Coming back to the original question, having a radar detector helps ameliorate the risk of possible Plod-Presence, thus reducing (though not eliminating) one hazard factor. They may therefore be considered a safety device.
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