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The bad guy has the initiative.
If someone has a gun on you and he's right there, all you can do is hope he makes a mistake that might give you an opportunity, because at that point, he holds all the cards. Having the biggest, baddest gun under your shirt, under your seat, or back at home will do you no good unless he screws up and gives you a chance to do something.

Competitive shooting such as USPSA or IDPA can tell you exactly how fast you can draw and put a round on target. Lots of people can do it in under a second from a surrender position (wrists above shoulders, draw and put a round more or less where it's aimed). Drawing from concealment, you better call it a second and a half in the best of circumstances for someone who's practiced a lot. Even then, you're hoping he'll do something dumb to give you that second or two.

Situational awareness, as others have mentioned, whether armed or unarmed, will help you avoid the confrontation most of the time.
You can even play little games with yourself in your mind. If this guy came up and pulled out a gun, right NOW, what would I do? There's cover, there's a lighted area, there's people, my car is x distance away, what are my options? 10 seconds later, ok if something happened, right NOW, what would you do? You can run through scenarios in your mind all day using your surroundings for inspiration. Just like defensive driving: If that guy pulled out in front of me from that driveway over there right now, do I have time to stop or do I have time to steer around him? What are my options. Same sort of deal. You don't have to do it all the time. But eventually, you'll sort of subconsciously register who's around you, where is cover, where could I run, where might I go if I did run, without putting much thought into it. Having some outline of a plan before a situation develops might make the difference.