Sensors that pick up movement and alert you to danger in your immediate vicinity, I think I've seen these before, they are called ears, evolved over thousands of years exactly for this purpose. Take the damn earplugs out while walking alone, and on railroad tracks. Replacing intelligence with technology in this case will just get more people killed. 

You can't always think of something bad - you have to be able to walk down a dark road without fear and suspicion. With technology getting cheaper everyday - why don't  we try to prevent shit from happening? There was a murder recently in the district where I'm from - an 18 y/o girl in a dark alley. You have to wonder how things could have been prevented. Sure, she could have listened to the streets and immediately call her parents for help / collecting her. But humans don't think of everything, especially in a stressful situation. 

I don't see how this would replace intelligence, as this would probably only remind people to be careful. OTOH - it will be hard to tell the difference if it is actually okay that people are in your personal space. Sneaking up is one thing, but having people around in everyday situations is something you don't want to be alerted to. So without addressing this, it will probably turn out as a "turn on when alone" device - and you would have to think of it. 

@Dr Croc Dundee  You'd need something that notices a direct collision course - so tracking of objects and their movements, maybe take some pictures once in a while and upload them / notify cell phone when something suspicious is going on. Maybe a kinect to start with? It's a good project in itself, possible and may it not be a solution to make them aware to be vigilant, it's a good prototype/concept for keychains/caps/wearables in 2035. Maybe it inspires someone else to make it smaller or write better algorithms... who knows. Eyes in the back, that's useful.

Like a Google Glass watching your back. A machine vision that recognizes knives, guns, fast moving people would come into my mind...