Cell Phone Warning System

I remember when cars went from having two rear brake lights to three, the third one being added to the rear dash. I feel it's time to add another light to each and every car. It would light up "ON CELL PHONE"
This sign lighting up would say to you 'hey, avoid coming close to my car now as I may sway into another lane or drive the speed limit'
Perhaps if people protested, we could add an optional 'H' to the end to indicate that at least this driver is using a hands free system...just a thought.