 I and others I've talked to thought the satisfying merge of every other car that happens during traffic was also zipper merging. And while the technique is similar, it's not a true zipper merge. When you come upon a zipper merge, it's when traffic is being reduced to one lane in a construction zone. And you'll see very specific signage that says this. Use both lanes during backups and take turns at merge. So those are the very specific zipper merge signs.