The Zipper Merge—Coming to a Construction Zone near You: NMA E-Newsletter #499

From Montana Member Bob Morrow

The most controversial, or more accurately the least understood, highway driving maneuver sanctioned by transportation experts is the zipper merge. It is a method of combining two streams of traffic into one when there is a lane restriction ahead, such as in a construction zone. Invariably some drivers think that others are trying to take advantage of them by zipper merging, causing tempers to sometimes flare.

You know what happens in construction zones, where people see signs saying “Left Lane Closed Ahead 1 Mile” and everyone moves to the right lane, so that there’s one mile of traffic in the right lane and not much in the left? That’s what the zipper merge is designed to deal with. It allows for greater number of vehicles in a shorter overall distance and gives a sense of fairness in that all lanes are moving at about the same speed. And, since speeds are similar, crashes (or their severity) may be reduced.

Yes, it means you do not merge until the very last minute and then do so alternately with vehicles already in the merged lane. That’s why it’s called a zipper merge.

Here in Montana the state is doing a construction project on US 12 between Helena and East Helena. It’s a 5-lane road with a center turn lane and a 55-mph speed limit. They’re putting islands in the center turn lane so that means the left lanes of east- and westbound traffic will be closed for a few weeks. The zipper merge is being tried here for the first time in the state. I think it is also important that the Montana Department of Transportation’s main office is less than 5 miles away, so they can easily monitor this. Helena (population 30,000) is the capitol of Montana.

Another possible reason for this is that the west end of the construction zone is about a half mile from an interchange between Interstate 15 and US 12. US 12 goes through Helena itself and is a major cross-town route. The congestion caused by the construction zone would cause problems at the interchange and west of it into Helena.

What’s different?

For one, there are no signs saying the left lane is closed 1 mile ahead. Instead, about one-half mile out is an electronic message board saying “Zipper Merge Ahead/Use Both Lanes”. Closer to the merge, there are orange signs asking drivers to stay in their current lane and to use both lanes. About 30 yards from the merge point, there is sign that says “Left Lane Closed Ahead.” Finally, about 10 yards before the merge is an orange sign stating “Take Turns Merge Here”. At the merge point is an electronic arrow board pointing traffic to the right lane.

I think this is a great way to manage a construction zone merge. Time will tell if it actually promotes the merging of traffic at the end.

One problem that might skew the results: since this is not an interstate, it carries mostly local traffic and drivers might just stay in the right lane before they get close to the construction zone.  I also think the arrow board should be turned off in the day, but it is probably needed at night.

For more details, the Montana Department of Transportation has a web page on the zipper merge: https://www.mdt.mt.gov/pubinvolve/ehelenaviaduct/zippermerge.shtml

Editor’s Note: The opinions expressed in this newsletter are those of the author.

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