Late Merge

A two-lane road narrows to one lane at a construction zone. Drivers face a choice: Early (courtesy) merge: upon seeing the “Lane Ends Ahead” sign, drivers immediately move from the closing lane into the open lane. Late (zipper) merge: drivers use both lanes all the way to the merge point, then alternate — one car from each lane in turn, like a zipper. It turns out that late merging produces higher throughput and shorter queues than early merging, even though it feels less polite. Early merging creates a single long queue that wastes the closing lane’s capacity. Late merging fully utilises both lanes up to the bottleneck, then processes cars at the same rate with a zipper pattern. This result is not merely theoretical: the Minnesota Department of Transportation, the German ADAC, and the UK Highway Code all recommend late merging in slow-moving traffic precisely because it is provably more efficient. The primary benefit of late merging is higher throughput: more cars complete the merge…