You guys forgot the biker groups.
Here in B.C. where mountain driving and road conditions can suck at the best of times, we get the groups of ten to twenty or more bikers strung out two by two moseying along at ten to twenty miles per hour below the speed limit.
Usually they end up with a long line of hostile cagers behind them and I have to admit that this is one of a very few things that has got me pissed at other bikers.
Scan sounds like a professional driver. When ever I train a new driver I really stress to always be checking whats happening up to two blocks ahead of you so there are no surprises. Also to be aware that all those people beside you, behind you and in front of you may not be aware of you so give yourself enough space to react to there actions.