Zipper is solid in a situation where an entire lane merges into another. There are different situations where it doesn’t make sense. I’m a big believer in zippering where it works, but then there’s the 6-lane highway where one exit lane is backing up into the through lanes. Then someone blows past that line in the other through lanes and cuts into the very front of the line. I’m sure some of those people are saying “zipper!” as they do it, but if everyone conventionalized a zipper point at the end of an exit lane, we would impede the through lanes, too. The point of the zipper is to smooth traffic flow and use the full capacity of the roadway; applying it broadly to highway lanes mixed with through traffic is antithetical.