Of course it's worth being overcautious.
It comes down to the same old thing, people are impatient, lack concentration and attention to the act of driving and more often than not incapable of accurately reading such a dynamic environment that is our roads.
It's the impatience that annoys me though, people just do not care how they can get 10 yards further up the road and it's quite pathetic, if I had a quid for every near-miss I've seen caused by impatient twonks over the last few years I'd be able to retire tomorrow.
When confronted, all you get is - 'I'm making progress'.... no you're not, you are being selfish and unnecessarily endangering other road users so your usually pointless journey progresses a little further up the road at any give time than it would if you were driving sensibly.