If it's a straight, flat road then a good radar detector should pick up a radar gun from around 2km away - mine does exactly that. If there is other traffic around, then the cop will probably want to 'shoot' other vehicles in front with their radar gun as well, which should, in theory, give the speedster some warning to the presence of the officer.
I have only ever seen cops sitting on the side of the road with a radar gun on busy motorways (busy for NZ standards!), and other major roads where there is plenty of other traffic.
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