People not indicating at roundabouts are probably the most irritating to me as they stifle the flow of traffic. Most dangerous thing I see regularly here is people going the wrong way down the 2 lane one way system and (vans) doing a right turn from the left lane of the one way system. Been a few nasty manglings here as a result of those. This one I view the other way round ... when everybody merges too early into one lane, they turn two lanes of traffic into a double length single lane tailback. If everyone would flow up both lanes and merge in turn properly, the tailback would be half the length.