I have a GPS unit that I mount to my bike and use whenever I measure, and I can see on it when I am getting close to a mile mark. I have even estimated the mile mark locations ahead of time with google earth and downloaded them to the unit before heading out to measure.
But one time out of ten I forget to look down to check it as I'm approaching a mile mark. If there was a audio cue 100m before I get to the mile mark, I'd never miss one again!