Early merge vs zipper merge
Two highways, identical traffic, one rule difference. On the left, every driver moves into the open lane as soon as the “merge ahead” sign appears. On the right, drivers stay in their lane and alternate at the cone. Push the arrival rate above bottleneck capacity and watch what happens.
Early merge
Drivers move left as soon as the sign appears
Zipper merge
Use both lanes; alternate at the cone
The model
Each car follows a simple car-following rule: maintain a safe gap behind the leader and slow proportionally if the gap shrinks. A bottleneck gate at the cone enforces a maximum throughput (cars/sec) — the rate two lanes can physically funnel into one. Trucks travel slower and take up more space.
The only behavioral difference between the two highways is when drivers leave the closing lane. Early-mergers consolidate early, leaving the right lane empty for most of its remaining length. Zipper-mergers use the full right lane and alternate at the cone.
What the metrics mean
- Throughput — completed cars per minute, measured over a 30-second sliding window.
- Avg travel time — wall-clock seconds from spawn to completion, averaged over all completed cars.
- Max queue length — most cars stopped or near-stopped before the cone at any one time.