Maybe, maybe not.
I agree, there would have been some signs in the case that was caught on video. They had just walked out of the store. Unlike most people, Id assume most here walk out of a store onto a city street and take a quick look to the left and right and all around to see whats going on. I most certainly do look around and size up everyone. Seeing a thug right there, Id be taking little glances in that direction often as I walked. Somebody walking up on you faster than your pace should be heard or even felt as they get close enough. A combination of turning your head a bit and using your peripheral vision every few seconds will alert you to the presence of somebody.
Contrary to normal thinking on level of danger, desolate city streets are easiest to avoid something like this on. If somebody "comes out of nowhere" when its desolate, you are doing it wrong. No they did not, you failed to see them or failed to use due caution around a possible hiding place somebody could pop out of. Medium foot traffic, not that bad. High foot traffic, if you are watching and are suspicious of absolutely everyone behind you, you are gonna get tired quick and have many "false alarms".
The real solution to this would be for people to not be such sheep and be so scared of everything. We are so god damned soft as a society that we are not willing to fight for whats right even though, whether it be a tyrannical govt or a thug or 2 on the street, "we" far outnumber "them". Case in point a thug knocks somebody out in the middle of a crowded city street and 20+ people see it and fail to run the thug down. If I witness that, I'm going after the person and detaining them until the police arrive. I don't advocate the use of offensive violence but it will hurt and my fingers will be firmly pressed into some very painful pressure point and the idiots face in the ground for the 5 min before the cops arrive. All it takes is one willing party to sack up and chase the offender, 2 or 3 or more who are "on the fence" will jump in once somebody gets the ball rolling. Flight 93 anyone? I'm sure it was 1 or 2 that got the masses moving in sync.