Merge early or merge late?
Road works ahead and two lanes become one. Some drivers move over as soon as they see the sign. Others drive to the cones and take turns. Who's right? Run a traffic simulator and see what happens to the queue.
Tip: tap the road to drop in a rude driver who races down the free lane and pushes in at the cones.
Both versions run side by side with exactly the same cars arriving. Queue means road where traffic crawls below 20 km/h.
Why taking turns works
The zipper doesn't make the road works any wider. It just uses the road you already have more sensibly.
If everyone squeezes into one lane early, the other lane sits empty and the queue stretches back twice as far. Using both lanes to the end keeps it shorter, so it's less likely to block junctions and exits behind it.
Only so many cars per minute fit through the narrow part. That's true whichever way people merge, so the zipper doesn't get you home much sooner. It mainly saves space and stress.
One car from each lane, like the teeth of a zip. When everyone takes turns, nobody gains by racing ahead, and there's no empty lane tempting people to push in.
When traffic is moving at normal speed with gaps, move over early and smoothly. The zipper is for heavy, slow traffic. Try the volume slider to see both.
How to zip, step by step
For slow, heavy traffic where a lane is closed. Always follow the signs and any instructions at the road works.
- Stay in your lane while the traffic is slow. Don't keep switching.
- Use both lanes all the way up to the merge point.
- At the cones, take turns: one car from each lane.
- Leave a gap for the car next to you and let them in. Then it's your turn.
- No racing, no blocking, no pushing in out of turn.
- Keep your speed low and your eyes up: workers may be close to the road.
Sources: Minnesota Department of Transportation (MnDOT) zipper merge guidance: in heavy, slow traffic stay in your lane until the merge point and take turns, which can reduce the length of the backup by as much as 40 percent; when traffic is moving at highway speed with no backup, it makes sense to merge sooner. Other US state transport departments give similar "late merge" advice. UK Highway Code rule 134: "Merging in turn is recommended but only if safe and appropriate when vehicles are travelling at a very low speed, e.g. when approaching road works or a road traffic incident. It is not recommended at high speed." The simulator uses the Intelligent Driver Model (Treiber, Hennecke and Helbing, 2000), a standard car-following model, with a slower single lane through the works. It is a simplified illustration, not a prediction for any real road. In the UAE, follow the signs, the road works instructions and any police directions. In an emergency call 999.