Relative speed


Hi...thought of a geeky app that would be neat if it could be integrated into google maps but wouldnt have to be.
Once a destination is known and you are en route, the app would display your relative speed to your destination. It would be interesting to see one's relative speed when the speedometer says 65mph.
Sometimes the relative speed could be a negative number.
Would like feedback on this...good/dumb idea or how hard it would be.