Zipper merging doesn't actually work because it is a fundamentally flawed theory. It requires the traffic speed after the merge to be faster than the speed before the merge in order to handle the increased volume moving through a narrower path. Ideally you'd incrementally step the pre-merge speed down to 35-40 and then increase the limit of the merged single lane to something like 55, this way traffic is hitting the merge and accelerating instead of slowing down.