Story time...
Quite a while ago, my only ghetto friend, after threatening suicide, went and collapsed in the street, screaming he was going to let a car run him over, as it was in the dead of night. One of my white belts and I, we had been sitting on my driveway at the time, ran over to him.
Although he resisted I pulled him off the ground, more worried than anything, and got him up on his feet. In a nervous stupor he stepped towards me, pulled back his hand and threw a few haymakers that I saw coming a mile away. I blocked it with a karate upward block, closed the distance, which made his other punches simply miss me, grabbed his collar and stepped in to throw him with o soto gari. He went to the concrete, but not hard, I didn't want to hurt him. We rolled for a bit, him clawing at my face and eyes, until I went for a RNC and he turtled up basically, so I did what we had been practicing earlier the day, which is attack the turtle with an armbar. He struggled and was trying to grab my groin with his other hand, which I did not appreciate. I locked in the armbar, barely, and he started screaming and crying.
My white belt ran over to us wide eyed. Cars were beginning to stop in the road and ask questions, but we managed to pull my friend away and walk him home. Moral of the story, don't use an armbar to subdue someone. Use a choke, or a pin. But any submission will work in a street fight. The martial arts, even BJJ and Judo, at their very base, are combat systems, not sports. People seem to forget that they were invented to cause harm to people.