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zipper merge

The Zipper Merge

Whenever a traffic lane ends, drivers must merge into the remaining lane(s).Choosing the incorrect merging strategy will generate speed differentials between open and closed lanes. This may result in aggressive driving manoeuvers, an increase in the probability of collisions and road rage.

The speed differential is, in part, a result of the traffic volumes and average road speeds. Traditionally, drivers slow down and move into the lane that will continue through the area as soon as they see the first lane closure sign. This is not always the most efficient and safe way for traffic to merge. The best traffic merging strategy is based on the pre-existing traffic conditions. Two potential strategies are outlined below:

Zipper Merge (Late Merge) Strategy

The zipper merge is a late merge strategy where all available lanes of traffic are used right up to the lane closure. Drivers then alternate into the open lane. The zipper merge strategy is most effective when there are high traffic volumes on the road, combined with low average speeds due to congestion.

Potential benefits of the implementing a late merge strategy include the following:

  • Increased traffic capacity through the reduced lane zone;
  • Reduces the overall length of traffic backup;
  • Decreases number of collisions; and
  • Creates a sense of fairness and equity.

Early Merge Strategy

With the early merge strategy, drivers move out of the closed lane well before the forced merge point, and before traffic starts to backup. The early merge strategy is most effective when there are low traffic volumes on the road, combined with high average speeds.


Share the Merge
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