This app will warn you who is behind you

I was on the bus, and there was a drunk guy standing behind me, almost falling on me. I didn't know that he was behind me until he burped and fell on my shoulder. If I had known he was there, I would've moved in the first place.<br><br>I'm thinking of an app where you can choose to be notified when someone comes close to you. It will be very useful in the public space.