To concur with the others, yeah, sometimes there is not a lot you can do. Luckily many criminals aren't too intelligent, but you don't want to underestimate them. Some can be rather tactful. This was seen in the assassinations of Texas prosecutors this past summer. I recall reading about assassinations of LEOs in their patrol cars this year, and there were the four officers that were gunned down in the Washington coffee house a few years ago. Not to mention incidents like the D.C./Metro Snipers.
So what can we do? The key is being armed as much as possible, and making mental preparations, such as exit strategies or visualized responses to threats of violence. You can't be in condition yellow all the time, but if you plan you can effectively snap into condition red when needed.
Slow is Smooth. Smooth is Fast.