This is something my wife and I instituted at the beginning of junior high and extended all the way through high school graduation. If the kids ever find themselves in an uncomfortable situation that they want to get out of, they can call home and we will come get them with no questions asked. We tell them we can park a few blocks away, if needed, to be discreet. Basically, whatever it takes to make them comfortable exercising this offer if in a bad situation. We make it clear that we won’t interrogate them into telling us the reason for the call if they would rather us not know. The important thing is that they know this safety net is there.
Be careful with this offer unless you are genuine. Think about some of the possible situations. Your kid is of driving age and instead of going to the friend’s house they told you, they went to a party somewhere else. They had some alcohol to drink and are nervous about driving home. They exercise the penalty-free call home and when you arrive you can tell it’s a serious party, surely with no parents in sight. Can you resist the interrogation?