I thought undertaking was technically only "bad" if you change into the left lane to purposely pass, and almost start weave through traffic. If you're in the left lane and staying in the lane, but it happens to be going quicker than the right, then there's nothing wrong. Same goes with slip roads - you're in a different lane/queue to the left-most lane of the motorway, so you can go quicker (as long as you don't pull back onto the main carriageway, when it would be undertaking). But, that doesn't mean it's a safe/intelligent way to pass other road users - especially as people arguably check their left mirror less than their right when changing lane as they naturally don't expect cars there.
I understand why people do it, but isn't it causing more problems than it solves?
I've seen it a few times when there's a slow/dumb car in the outside lane and people start to undertake to get past, which then stops this car from being able to pull over... How can he move into the left lane, to allow people to pass (arguably correctly) when there's now cars flying up his inside?
If dozy people that sit in the right lane start getting undertaken and traffic still flows around then, then surely they're never going to learn/realize they're a problem? No, I don't mean tailgating and flashing lights to get them to move over, but just the notion that the system is adapting to the problem driver instead of addressing the crux of the issue. And, if they ever get pulled for it then all they'll say is that it's unsafe to move into the slow lane due to cars undertaking, so they get away with it.
Yes, I'm one of those people that sits in the left lane when I can and when I'm about to overtake a middle-lane hog, I change 2 lanes to go right of them and then swing back 2 lanes into the left lane. Yes, it's pedantic and frustrating, but 8 out of 10 times the middle lane hog tends to pull over into the left lane too, which is good, instead of just roaring past them on the inside when they'll almost stay in the middle lane as it will seem safer to them compared to changing lane.