An echo is the repetition of sound produced by the reflection of sound waves from an obstructing surface.
The image above shows the bloom application for an iPod, the application will play the notes which have been stuck repeatedly whilst fading them away like an echo.
This is the concept which I wish to apply to my final solution; an echo of a past event which plays out repeatedly for a period of time, allowing people to see whether others have travelled down that route recently.