‘Zipper merge’ please: To keep traffic moving, Colorado wants you to be late when merging

Thomas Phippen
Glenwood Springs Post Independent

Zooming past a long line of cars to merge just before the lane closes may seem impolite, but it’s actually more efficient, according to transportation officials.

For several months this summer, Glenwood Springs drivers had to navigate several major construction projects.

Roads were reduced from four to two lanes, or from two to one, or were closed entirely, creating long queues of stand-still traffic, but the congestion could be reduced by one simple thing: the “zipper merge.”

“If you are coming up on an area of the roadway that is merging into one lane, the last thing we want is for one lane, one queue traffic to be backed up with tens, dozens or hundreds of cars. That’s a very inefficient way to move traffic,” said Lisa Schwantes of the Colorado Department of Transportation.

Instead of merging early, CDOT and other agencies want drivers to use the “zipper merge,” or wait until the lane actually closes to move to the other lane.

“It’s much quicker, much smoother if you use both lanes all the way up to the merge point,” Schwantes said.

Colorado and several other states have been pushing the zipper merge for years. CDOT even produced a video several years ago based on the idea that taking turns, rather than waiting in line, is the polite thing to do.

So if it’s more efficient and the state advocates for it, why haven’t more drivers adopted the late merge?

“This is counter to what we have been doing for the past many years. People think it’s rude,” Schwantes said. “We learn from a very early age, as school children, not to cut in line,” she said.

Some may think the nice thing to do is merge early and wait, but that creates enormously long single lines of cars.

When the zipper merge works properly, cars merge at a single point, which allows even heavy traffic to move through at a consistent speed.

“It can reduce delays by up to 40% when traffic is heavy and congested,” she said.

The state has been pushing for the zipper merge for years. But how do you change a driving etiquette for greater traffic efficiency?

“It’s a matter of habit, it’s a matter of what you become accustomed to,” Schwantes said.

Some construction projects have “use both lanes” and  “merge here” signs.

Two years ago, Durango had a months-long construction project that required lane closures, and Schwantes said she noticed drivers eventually accepting the late merge.

“It took a good month or two for local folks to keep both lanes moving up to the merge point,” Schwantes said.

But that was two years ago, and when Durango had another long construction project on a bridge, drivers had to relearn the zipper merge all over again.

“We’re having to reeducate people again,” Schwantes said.