No one knows who else is armed unless everybody is naked. (San Francisco may be trying this as a public safety strategy.)
One in a million can "out draw" another person. Try it with a quick-draw experiment. Have a friend point a finger at you while you keep you hands down at your side. Tell your friend to say "bang" when fires an imaginary bullet. He's not allowed to shoot, however, until you begin to raise your finger/gun toward him. The result will be that both say "bang" at the same time or you're even a little bit quicker. Its much worse when both parties have their hands to their sides, like in a typical traffic stop situation. The point is if the cop has to quick draw after the bad guy's gun is motion, the cop will be slower every time. In California I was taught to keep my hand on my gun during night time or limited view stops and in most cases, I had my gun drawn and hidden behind my thigh. It has been a common practice for years. Of course today, allegations of racism are bullying policy makers. Years ago, an incident occurred in Ohio (if memory serves) where an officer had a car door closed on his hand during a traffic stop. The driver sped away intentionally dragging the officer. Fearing for his life the officer fired into the car killing the driver. Riots ensued because the driver was black and the police officer was white. The assumption being that had the driver been white the officer would have been perfectly satisfied to die. Cops touch their guns only to prevent premature death.
Rattle snakes rattle, bears growl, and lions roar. These are all warning signs provided for the general good. When a cop has his hand on his gun don't jump to open your glove box or grab something from the car seat. Take the warning that the officer is concerned about immediate circumstances.
If you wait to draw your gun until the bad guy is armed and taking an affirmative step toward your injury or death, you waited too long.