I guess if someone had the time and energy to drive some distance behind a cycling friend and "shadow him", you might get lucky at trying to catch a thief or two, but realistically, what's going to happen?
If you see, for instance, two guys starting to attack your friend up ahead, you scream up in the car, get out (presumably you'd want to be armed otherwise you're just going to lose the car too) and you point a gun at them. They split and then what? You shoot them as they run away? From what I've heard, that's not going to do you any favours in court. Or they have guns and shoot at you? (possibly unlikely, I don't know how often these guys actually have loaded guns?) or they aren't armed and they don't run, so you cover them with your gun and call the police. How long before the police arrive to take them away? How many hours before they are out again but now they know what you are up too?
It might be tempting to want to beat the thief to the point where he never considers robbing anyone ever again, but I don't see the law being on your side either.
So what is the answer? No more riding? Turbo trainer in your garage behind an electric fence? Even guys riding in groups have been attacked, as we've seen on here.
Sad days man.
