Loud whistles have consistently filled the job of scaring people away, or alerting others to allow them to understand that you were in danger. The trouble with police whistles, nevertheless, is you have to have the ability to get it into your mouth to work with it, and you will need to continue to blow it till help arrives. Which may not regularly be possible? Your own alarm is far more dependable.
Another good thing about personal alarms is the pitch of the noise is really high that it also scares away dogs and wildlife.