Zipper Merge Simulator
A two-lane road narrows to one, and every driver faces the same moral dilemma: merge early like a good citizen, or use the emptying lane to the taper and zipper in. This simulator settles it with car-following physics — vehicles accelerate and brake by the Intelligent Driver Model, so shockwaves and stop-and-go jams emerge on their own. A slider sets the fraction of drivers who merge early versus at the merge point, a courtesy toggle makes cars alternate strictly at the taper, and the readouts track throughput in cars per minute and average delay per car. Colour shows speed, so you can watch the queue grow long and thin under early merging or short and dense under the zipper — and measure which one actually moves more metal.
Runs 100% in your browser — simulations are computed locally on your device.
Read the full guide to this tool
Notes
- The late (zipper) merge uses both lanes as queue storage, roughly halving queue length; traffic authorities in Germany and several US states now sign it explicitly in congestion.
- In free-flowing traffic the capacity of the bottleneck itself is what limits throughput — merging style mostly redistributes delay, but panic braking at a messy merge can collapse the bottleneck’s capacity.
- The Intelligent Driver Model reproduces real traffic phenomena — shockwaves that travel backwards, capacity drop after breakdown — from just a handful of parameters per driver.
- Alternating one-for-one at the taper (courtesy zipper) keeps merge speed high and prevents the lane-blocking standoffs that make late merging look bad.
- Runs 100% in your browser — simulations are computed locally on your device.
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