Rules of the Road

The best way to zip into a single lane? Use the ‘zipper merge’

Traffic merges in 2009 at East Holly and North Forest streets for construction. Most states encourage drivers to utilize the “zipper merge” and wait until reaching the cones to merge.
Traffic merges in 2009 at East Holly and North Forest streets for construction. Most states encourage drivers to utilize the “zipper merge” and wait until reaching the cones to merge. Bellingham Herald file

Q: How are drivers supposed to merge onto the freeway? It seems like most drivers in Bellingham expect to merge into traffic immediately when they enter the onramp instead of getting up to speed and “zipper-merging” near the end of the onramp lane. Entering drivers seem to think it’s their right to merge into traffic before getting up to speed and expect other drivers to slow down or change lanes to make room for them, creating hazardous conditions for all drivers in their area. What is the law?

A: With increased summertime traffic, along with more freeway lane closures as road crews maximize the good weather, summer is the season of merging frustration. Although, as traffic has increased in the Northwest, any day can include merge-induced headaches. We might be able to alleviate some of those headaches if we could all agree on the best way to merge. Whether it’s merging onto the freeway from an onramp, as this question poses, or merging due to a lane closure, research shows that there really is a best way.

Let’s start with the least controversial merge; the onramp. The key to a successful onramp merge is speed. More specifically, reaching a speed similar to the vehicles already on the freeway. Use the full length of the onramp to gain speed and look for a gap in traffic. Even though it’s the only option, signal intent to merge. And note that activating the turn signal doesn’t automatically give a merging driver the right to move left. According to state law, the merging vehicle has the primary responsibility of making sure that movement is safe. For drivers already on the freeway, that doesn’t mean you should ignore the needs of the merging cars. Even though the freeway traffic has the right of way, consider merging a team sport and give the incoming driver some room if possible.

Now for the controversy. How many of you hate it when you come to a “lane closed ahead” sign on the freeway and, as a respectful motorist, you promptly move out of the closing lane, only to have other cars zoom past you, waiting to merge until the cones block the lane? Maybe you don’t swear at them out loud, but you wish upon them a flat tire as you grumble about how they should obey the rules. If that’s you, I have some bad news for you: It’s legal. But wait, there’s more. If we could convince more drivers to stay in the lane until the actual closure, it would improve traffic flow.

Transportation officials from an increasing number of states, including Washington, have been advocating the “zipper merge.” The “zipper merge” is when drivers in the merging lane alternate with cars in the continuing lane, every other one, like the teeth of a zipper. This is sometimes also referred to as a “late merge.” That’s because, when done properly, a zipper merge occurs as late as safely possible in the ending lane. Zipper merging increases driver predictability (it’s easier to anticipate the merge location) and it uses more of the pavement, so the backup isn’t as long. In contrast, merging early creates small choke points, increasing congestion.

Not everyone likes the zipper merge. I’ve seen drivers straddle two lanes as self-appointed merge enforcers, preventing other cars from continuing in the merging lane. I’d like to make two points about this: First, it’s dangerous. Putting a slow-moving car partially into a lane of faster vehicles is, well, it’s obvious so I won’t insult your intelligence by overexplaining. Second, it’s illegal. RCW 46.61.140 states that vehicles shall be driven entirely within a single lane. If staying in the merging lane right up to the cones seems like cheating, just realize that if we could get everyone to do it, no one would have an advantage. Then we’d reduce the merge area to one spot, increasing flow, decreasing the length of the backup and reducing crashes.

For a visual example of the zipper merge, the Kansas Department of Transportation has developed a video (narrated by two annoying traffic cones) that shows how it works. Next time you encounter a lane closure on the freeway, give it a try. If you get some dirty looks from people who don’t understand the zipper merge, just know that you’re doing your part to reduce congestion and increase safety on the freeway.

Road Rules is a regular column on road laws, safe driving habits and general police practices. Doug Dahl is the Target Zero Manager for the Whatcom County Traffic Safety Task Force. Target Zero is Washington’s vision to reduce traffic fatalities and serious injuries to zero by 2030. For more traffic safety information visit Ask a question.