The deterrence effect of punishment relies on two factors: severity of the punishment when caught and the probability of getting caught. You can only up the former so much before a single stupid mistake ruins your life. But we could increase deterrence a lot by increasing the second factor. Just think about it: You a $5 fine stop you from using your phone? Right now probably not. But if it cost you $5 EVERY TIME you used your phone in the car you'd stop rather quickly.