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OLIVERWhat is a “zipper merge,” and where would it be used in Lake Oswego?


(A Lake Oswego police officer answers readers’ questions each week in this space. To submit a question, call staff reporter Cliff Newell at 503-636-1281 ext. 105 or send an email to This email address is being protected from spambots. You need JavaScript enabled to view it..)

In traffic engineering, the late merge or “zipper” method is a conventional term for merging traffic into a reduced number of lanes. Drivers in merging lanes are expected to use both travel lanes to merge into one lane, alternating turns.

When a lane is closed in a construction zone, for example, motorists should use both lanes of traffic until reaching the defined merge area, and then alternate in zipper fashion into the open lane. Although it may not feel like it at times, zipper merging does reduce the overall length of traffic backups by half.

One mistake I see drivers make is when they see a “lane closed ahead” sign in a work zone. They slow too quickly and move to the lane that will continue through the construction area. This driving behavior can lead to unexpected and dangerous lane switching, serious crashes and road rage.

Zipper merging, however, benefits individual drivers as well as the public at large. Research shows that these dangers decrease when motorists use both lanes until reaching the defined merge area and then alternate in zipper fashion into the open lane.

So drivers are supposed to merge late?

Yes! As you see the “lane closed ahead” sign and traffic backing up, stay in your current lane up to the point of merge. Then take turns with other drivers to safely and smoothly ease into the remaining lane. When traffic is heavy and slow, it is much safer for motorists to remain in their current lane until the point where traffic can orderly take turns merging.

However, there is an exception: When traffic is moving at highway speeds and there are no backups, it makes sense to move sooner to the lane that will remain open through construction. The bottom line is to merge when it is safe to do so.

The zipper's catch, of course, is that every driver has to be aware of — and believe in — the style of merging before it reaches maximum efficiency. Here in Lake Oswego, you’ll find a zipper merge at McVey Avenue and State Street; as long as enough drivers don't fill both lanes or intentionally block the soon-to-end lane in the form of ‘vigilante car justice,’ it is a useful traffic control tool that can help save you time.

— Motor Officer Jeff Oliver

If you have a specific request or see a violation occur, the Lake Oswego Police Department encourages all citizens to contact the non-emergency line at 503-635-0238 and an officer will be dispatched as soon as available.