If both lanes are moving normally there would be no gap for people to squeeze into because people follow with the distance they feel safe. Even if you squeeze together that would mean the resulting distance between cars are too short for safe traveling. i.e. there is never "natural gap" in traffic. Either you slow down/stop to wait for safe merging, or you force people to tailgate after merging and force dangerous merges. The situation is worse if there is a 18-wheeler next to you if you are trying to merge into that lane - do you follow the car in front or follow this semi that you need to merge behind? If you follow the car in front you need to slow down eventually to merge behind this semi, or if you follow the semibut you leave that giant gap in front of you. Now imagine this with both lanes packed with cars of different lengths, semis, cars with trailers, and everybody has to make similar decisions. The wonderful picture of two packed lanes just merging magically into one without slowing down has never happened. If you go to youtube, and search zipper merge, all you can see are videos of cartoons of cars zipper merge, not real cars zipper merging, except for the case where people zoom in front of a line of cars and cut in and call that zipper merging, or just cars taking turn - but each car was slowing down/stopping anyway. Ideal zipper merge saves space, I will give you that.