A whistle, in an urban setting, is a very effective signaling device. However, it is not specifically for summoning HELP, it is for summoning ATTENTION. This is why traffic cops and crossing guards routinely use them. I have effectively used a whistle in urban emergencies to stop traffic, help control crowds and to alert pedestrians to danger.
It's also good to cause one's free-range recombinant DNA to look around and perhaps be receptive to parental instruction.
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