emergency button

I believe that there is something like this which exists already. However its purpose is that it stops thieves by way of noise. When a button is pushed (mostly when someone is being mugged or something) the device emits a lound high pitched noise making everyone around turn and look. This deters a theft attempt. However wouldn't it be a good idea to have a small pocket sized device with a button that, when pushed dials an emergency services number, and has a homing becon. Say that you are in a car accident or something in the middle of nowhere, and you can hardly move. You could push this button and it would dial out, alerting the ES to your whereabouts. I thought it was quite a good idea - what do you think?