Morse code with our headlights..... but since the car ahead never pays attention to the one behind, it never seems to work.
We mostly rely on cell phones but find that the majority of time nobody can hear it ringing when driving, so it isn't until we make a stop that we realize someone is missing. Eventually I want to get a CB and hopefully convince others that it is a good idea. In the mean time my family uses hand held radios, they are really only good for line of sight but they have been a great asset when we used them. Those of you who have CB's, do you get a lot of feedback from the ignition?