The University of Massachusetts Amherst
University of Massachusetts Amherst

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Knodler and Muriel Interviewed by Public Radio on the Enigmatic “Zipper Merge”

highway with a "road narrows" caution sign

In August New England Public Radio (NEPR) posted a long article on the often misunderstood “zipper merge” in traffic. NEPR interviewed a number of experts, including two College of Engineering faculty members: Civil and Environmental Engineering Department Professor Michael Knodler, director of MassSafe; and Mechanical and Industrial Engineering Department Associate Professor Ana Muriel, who is also the associate department head. See also: Maine Public.

We have all confronted the zipper merge when encountering lane closures on highways and we are called upon to take turns merging cooperatively into the open lane or lanes. In such situations, according to the NEPR piece, drivers should use the zipper merge technique in a cooperative way in which drivers keep to all open lanes as long as possible and then take turns at the merge point.

As the NEPR piece explained, Knodler and other traffic engineers insist that the zipper merge, used properly and considerately, is safer and keeps traffic flowing better than when drivers compete with each other.

NEPR said that traffic engineers recommend the zipper merge not just on super highways, but also on roads like Route 9 in Massachusetts. On a recent morning in Amherst, Knodler stood by the side of the road next to a yellow merge sign.

“We have a regular joke,” said Knodler, “which is you can't be an outstanding traffic engineer unless you're out, standing in traffic.”  

Knodler watched cars proceed through a traffic light and merge from two lanes to one. “This group seems to alternate merging, and it worked perfectly because they did sort of sequence — it was 1:1,” Knodler said. He meant that drivers willingly cooperated with each other in an abbreviated zipper merge. Knodler and other engineers say the maneuver has the potential to improve roadway safety and efficiency.

Later in the segment, NEPR quoted Muriel, who commutes on I-91 between Longmeadow and Amherst and is a logistics researcher “who knows a lot about efficiency,” as NEPR said. 

"My major focus is on production environments and also healthcare," Muriel said. "So this was my application to transportation, just because I was sitting there and suffering through it."

Even before she learned the zipper merge was a thing, stuck in traffic around Springfield and Boston, Muriel knew merging late made better sense.

Earlier this year, Muriel wrote a letter about the zipper merge to The New York Times. It didn't get published, but she is still trying to get the word out to anyone who will listen, and she’s seen the zipper merge work elsewhere, including Spain, where she come from originally. (September 2019)