Undertaken? How far out in the road are you riding? I've never undertaken a fellow cyclist in my life, out on the open road, and I can't think that I've ever seen it done.
Quite common on roundabouts or when parked cars are on my inside; I've seen too many people nearly killed by the sudden opening of a car door into their path. On roundabouts I try to take up the whole lane so drivers know what you are doing and don't assume you are turning off.
I could probably write a book about defensive cycling; the amount of times drivers would have turned into me if I wasn't predicting it is probably over 50... it's the most common way to be killed.
