‘Zipper Merge’ Pushed by Several States to Reduce Construction Crash Risk

Drivers who get angry when other motorists wait until the last possible moment to merge in construction zones need to reconsider their long-held notions of highway courtesy, transportation officials in a growing number of states are concluding.

Transportation departments in Missouri and Kansas have joined Minnesota and Washington in urging drivers to use the “zipper merge” method when approaching lane closures, most often associated with road construction.

Using all available lanes until the last moment, then alternating entry into the open lane, helps reduce accidents by keeping both lanes moving at the same speed, said David Silvester, a Missouri Department of Transportation engineer.

“This isn’t rocket science,” he said. “It’s easy.”

The goal is to change a mindset among drivers whose first instinct is to get in line as soon as they see a sign warning of closed lanes ahead, Silvester said. For those folks, drivers who buzz past in the lane that is ending and crowd back into line at the last second are considered rude or inconsiderate.

Dwight Hennessy, a psychology professor at Buffalo State College in New York who specializes in traffic psychology, said Midwesterners tend to be polite and follow the rules – even unwritten ones – and get upset when others don’t.

“When a rule is being violated by someone else, it frustrates us, it irritates us, it makes us angry,” Hennessy said. “We expect everyone else to follow the rules, and when they don’t and we know they’re getting an advantage, it ticks us off.”

While motorists in other states might be accustomed to using some informal form of the zipper merge, four states have officially championed the technique.

Missouri officials started promoting the idea earlier this year ahead of what was expected to be a heavy road construction season. One of the bigger bottlenecks begins next week, when traffic on Interstate 70 west of Boonville in central Missouri will be reduced to one lane in each direction because of bridge repairs, Silvester said.

While Missouri’s effort mainly is a public relations campaign to change how drivers deal with lane closures, Kansas has taken a more deliberate approach that includes using electronic signs and measuring the pace of traffic flow with Doppler radar. The state will review the results of the pilot project and determine whether it’s worth expanding next year.

Minnesota began promoting the zipper merge in the early 2000s with something it called “dynamic late merge.” That system relies on sensors that activate portable electronic signs when traffic is congested and there are lane closures ahead.

The state later changed the name of that method to “active zipper merge” because the term “late merge” had negative connotations, said Ken Johnson, state work zone engineer.

In late 2007, Minnesota tried using a “passive zipper merge” system that uses permanent signs – but no electronic ones – to tell motorists of an upcoming lane reduction and encourage them to use both lanes. That version was slow to catch on, Johnson said, and in 2011 Minnesota transportation officials launched a campaign to educate drivers about how the merge works.

“We’ve struggled for a long time with what to do with merging behavior during lane reductions,” Johnson said.

Two years ago, Washington state began urging drivers to use the zipper merge, and Missouri and Kansas followed this year. Johnson said other states also have contacted his office about Minnesota’s experience with the traffic method.

Zipper merging is a simple concept that kids seem to understand better than some adult drivers, Silvester said. That point is made in a video MoDOT released last week featuring children reacting to footage of adults using cardboard cars to make a zipper merge.

“When we saw the video, all of us were like, oh, my gosh, this is perfect,” Silvester said.