Zipper merging works great as long as everyone on both sides is willing to alternate (or if strong enforcement requires them to do so). Otherwise, letting "late mergers" scream past fifty "early mergers" often creates a situation worse than simple early merge. There's plenty of blame to go around.
* The vast majority of the people blasting down the late-merge lane clearly have no interest in alternating or merging into the largest/safest gap at the end. They'll squeeze into the smallest latest gap that they think they can possibly get away with.
* Quite a few of the early mergers feel they've already done their part. Whether they're competitive, territorial, or sincerely concerned with traffic flow/safety, they're not much interested in alternating with the late mergers either.
So how does this play out every effing day at construction projects and toll booths? You end up with a few late mergers playing chicken with a few early mergers at the chokepoint, jockeying for position inch by inch and degree by degree as they try to cut each other off. That's where all the "vigilantes" come from - they're people who are already stuck in the aftermath, they know how it happened, and they have decided that they'd rather settle for forced early merge than botched zipper merge.
Without enforcing proper merge behavior on both sides, zipper merge just becomes a big Prisoners' Dilemma game with a few defectors screwing all of the cooperators.
