What Is a Zipper Merge?

You’ve already moved into the open lane, but cars in the lane that’s ending keep passing on their way toward the merge point. That can look like they’re skipping a long line of traffic. In a zipper merge, though, both lanes stay in use until they come together. At the merge point, vehicles from each lane alternate into the remaining lane, joining together like the two sides of a zipper.

The empty lane is still usable road

When most cars move into the open lane well before a closure, traffic can form one long line while the ending lane beside it sits nearly empty. Even though that lane eventually disappears, the pavement before the closure is still available.

Cars continue in both lanes until they get close to where one lane ends. Instead of forming a single line far ahead of the closure, the two lines meet at the narrowing point.

The road still goes from two lanes to one, so the bottleneck itself remains. With cars waiting in both lanes instead of one, the backup can take up less distance along the road.

Why staying in the ending lane can look like cutting

If most cars have moved over early, the ending lane may have plenty of open space. A vehicle traveling through that space can pass a long row of slower or stopped cars before reaching the closure.

From the open lane, that can look like someone driving to the front of the line. In a zipper merge, the two lanes remain separate until they reach the merge point. It only looks like that lane has an advantage because most of the cars moved out of it earlier.

Zipper merges are most noticeable in heavy traffic

When traffic is light, cars may have enough room to move into the continuing lane without creating two noticeable lines. There may be no long backup and little reason for the alternating pattern to stand out.