First off NEVER, NEVER, NEVER, NEVER, NEVER, NEVER, NEVER fire a warning shot. Warning shots are indeed "deadly force" in nearly every jurisdiction I'm aware of. Warning shots are almost always considered evidence that the shooter wasn't really in legitamite, reasonable fear of death or great bodily harm. Some sort of verbal warning before, during or after clearing leather is fine, and may well prevent you having to fire. But for goodness sake, don't crank one off into the dirt as a warning.
Now, lets say essentially the same thing happened to you next week while you had your grandkids in the car. All else was the same. You get out of your car, then the nutjob drives up and blocks you in. While I don't believe you have a legal duty to retreat you have a few options. First is, take the blame and try to diffuse the situation. Appologise, and offer to buy him a cup of coffee or something.
If that doesn't work, or you don't have time to try it, if possible make a tactical retreat. That doesn't mean running from the situation. That means putting enough space between yourself and the road rager to give you more time to react and possibly provide cover and/or draw him away from the little ones. During all this, continue to attempt to get him to back down without resorting to violence on either side. If he continues to advance give a warning that you will protect yourself if he comes closer, and at this point preparing to draw may be in order.
Don't clear leather until it is absolutly necessary, but if it comes to that point, try to give him one last chance to back down either through some verbal warning as you bring your gun to low ready, or just by taking an aggressive stance and clearly point your gun at him with your finger on the trigger but hesitate just long enough for him to register the situation.
If all attempts to get him to back down fail, the fire 2 COM. Evaluate if he's still a threat, and if so fire 1 to the head. After that, if he's still a threat, fire to slide lock. Once he's no longer a threat, de-cock (if applicable) and reholster.
Call 911 and await the police. If you have a number handy, call a lawyer. Otherwise, call your wife, or someone else you trust, and have them get a lawyer for you. When the cops arrive keep your hands in plain site and follow all instruction, except those related to obtaining any statement beyond your identity and that you were in fear for your life and that of your grandkids and that you fired in defense of same.
Even if you don't need to shoot the guy, if you ever say "I have a gun", or show it to him holstered, but especially if you brandish it, you should call 911 as soon as it's over. The first person to call 911 is the "victim" and is usually accorded more credibility in the ensuing investigation. In fact, if you can call 911 at the start of the altercation, that is probably the best since they will get an audio recording of everything which may be critical to your defense should charges (criminal or civil) ever come about.