Your mobile phone or navigation device alerts you as you approach police speed traps.
Not so useless. A friend and I were building a ramp for my 93 year old mother and the spirit level we had was broken. We calibrated the iPhone's level app with a known level surface, and used it to build the ramp. A week or so later, I bought a replacement spirit level, and checked the levelness of the work done with the iPhone's app. It was right on, perfect.
Then there are apps like ePocrates which my physician friends are finding invaluable. There are literally thousands of very useful apps.
This one? Not so much.
Your mobile phone or navigation device alerts you as you approach police speed traps.