I like the emoji idea. So simple and practical both idea and the system as well
If you already need an app on your phone to use the system... why not vibrate the phone? The user is likely holding in their hands to read something (or listening to music with it). I'm not looking at the display for an emoji...
Often, a cute interface idea can break a user out of their usual notification habits, which could be pretty important for something like catching a bus!