This is an idea I had a while ago. For people who live in rough neighborhoods, on your cellphone, you put it into 'dead-man' switch mode and it will automatically dial a list of people if released before the cancel button combination (and maybe a fake "duress" combination where it executes silently). If it has GPS, it could start texting the coordinates to the police or a website that updates your position using google maps. It might have siren that starts going off as well. I was told 'dead-man' switch was a little morbid so it would need a catchier name.