In traffic engineering, the late merge or zipper method is a convention for merging traffic into a reduced number of lanes. Drivers in merging lanes are expected to use both lanes to advance to the lane reduction point and merge at that location, alternating turns.
The late merge method contrasts with the early merge method. A related scheme is the dynamic late merge.
The late merge method has not been found to increase throughput (throughput is the number of vehicles that pass through a point in a given period of time). However, it considerably reduces queue ("backup") length (because drivers use the ending lane until its end) and reduces speed differences between the two lanes, increasing safety. In the case of Interstate 77 in North Carolina, where signs directed people to use the zipper merge, the length of the backup was reduced from eight miles to two.Governments hold campaigns to promote the late merge method because irritation and aggression are common among drivers who are not educated about the benefits of the technique, sometimes including straddling lanes to block late mergers. Often drivers who change lanes too early do not like to see other drivers continue until the end of the drop-away lane, even though this late merging is encouraged by the authorities. In most countries, a driver can be penalized for not using the late merge method, but in some countries only where a traffic sign so indicates.
Most states in the United States require merging traffic to yield to through traffic which already exists in the lane they wish to enter. This further complicates the common understanding of proper merging protocol, as even though zipper merging is widely encouraged, those doing so are still legally required to yield, and those who choose not to let them merge are not doing anything wrong from a legal standpoint. Traffic already in the lane being merged into has the right of way over the merging traffic from the lane that will disappear.