My commute to work has 20,30,40 and 60 in an 8 mile journey, and 40 does seem to be a popular choice no matter what the limit is!
I can come up behind someone in the 30, stay behind in the 40, lose them in the 20 or 30, catch them up again in the 60, lose them in the next 30, etc etc.
Though what annoys me more is the person who sits right up my arse when I'm behind one of these drivers, like touching my bumper is going to somehow change what the car in front is doing. I will already give a car driving like that a bit of extra room, because I've learned they're more likely to do something stupid or suddenly and without warning like stopping dead for no apparent reason, and in one memorable case stopping and attempting to reverse back to their missed junction!
When you've left zero braking distance between you and me, I slow even more, because I'd rather not hit them up the arse as well when you hit me up the arse if we all have to brake suddenly.
I can't control other people's driving, but I can respond to it in a way that makes the situation safer for me.
I'll overtake if I can, but safely, and that's not always possible, but it's always my decision and my responsibility if I do. Of course it's frustrating, but frustration doesn't mean that the responsibility for my decision is someone else's.
I've been the person facing lights coming towards me in my lane suddenly around a corner and having to stand on my brakes to avoid a head on, as the overtaking car zips between me and the car they're overtaking just in time. I've done sod all in that situation to cause any frustration yet I'm the one going to pay for two people's shit decisions.