Today, Huey perpetuated the yippy small dog myth.

Sorry to everyone with well trained dogs.
He has always been territorial at home, meaning if someone is in the yard or even in the street in front of the yard, he barks at them. Over the last few years, he has become territorial when in the car; he barks at anyone close to his car, the drive thru people, cars next to us at stop lights, etc. Until today, he has always been a complete angel as soon as he is out of the car. I have taken him into all kinds of stores and he doesn't make a peep. I always take him when I get my oil changed. All the mechanics know him and they have a little waiting room where we "hold court" until the car is ready. Today, that waiting room also became "HIS". He barked at the mechanics on the other side of the glass door, at customers in the parking lot, and generally made a massive nuisance of himself. I absolutely could not get him to stop barking until he was ready. Now, when people came in to love on him, he was quiet and excited to be petted.
Any ideas? The drive through is annoying enough, but I can't take him out in public if he is going to be a butt. He doesn't get out much as it is, and I enjoy taking him out and about to see people, and he seems to enjoy it. He was just out of control today, and I was embarrassed - I felt like one of those ineffective parents who say "hush, Timmy" but don't make little Timmy stop. The difference is, if it was a skin kid, I could bust a bottom. I obviously can't do that with Huey. Suggestions?