How to zipper merge

While getting over early may seem like the safest and fastest way to merge, it may not be the best way to get through.
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While getting over early may seem like the safest and fastest way to merge, it may not be the best way to get through.

Back-Seat Driver

Tony Bizjak writes about traffic and travel in the Sacramento region

Back-Seat Driver

Should you merge early or late when your freeway lane ends?

By Tony Bizjak

November 20, 2017 03:55 AM

You’re driving on the freeway and you see a sign that says “Lane ends ahead, merge left.” It may be because there is construction work going on or a crash. Or it may simply be because the freeway has fewer lanes ahead.

Should you merge right away? Or should you drive as far as you can before merging, possibly passing dozens of slower moving cars in the clogged lane next to you?

It’s a question Sacramento drivers may face more frequently these days, with all the construction going on, and particularly this holiday season on the crowded freeways.

Most of us prefer to merge early, figuring it is the polite and orderly thing to do. We’re playing nice with our fellow travelers on this asphalt path we call life, right?

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But there are others who scoot ahead as far as they can in the more open lane before they push their way into the line at the last minute. They’re called “side zoomers.”

Are they late for something? Do they know they’re probably saving themselves only a handful of seconds?

At the same time, they risk inviting some road wrath. We’ve seen some indignant drivers pull part way into the other lane to block side zoomers from getting ahead.

But, wait. Is it possible that side zooming has some social merit?

Some smart people say yes, especially with congested freeway conditions like those Sacramento drivers have experienced lately.

The theory is that it is more efficient for cars to fill up as much available lane space as possible, and that means staying in your lane until the actual spot where the lane finally squeezes down and disappears. Once you get there, ideally, you participate in an elegant and cordial “zipper merge,” where cars in each lane take turns sliding in, like the teeth of jacket zipper mesh neatly together.

If everyone does that right, the freeway supposedly moves a bit faster overall.

“Merging late, that purported symbol of individual greed, actually makes things better for everyone,” Tom Vanderbilt wrote in the book, “Traffic: Why We Drive the Way We Do (And What It Says About Us).” (The book, by the way, is full of fascinating stuff for traffic geeks.)

His point of view is largely based on “late merge” experiments in Pennsylvania, where state highway officials encouraged drivers to stay in their lanes until the last minute. It appeared to have improved traffic flow by 15 percent. Analysts say it may have reduced road anger as well, because everyone was following the same rules. To make that work, the state put up signs more than a mile in advance, telling drivers to stay in their lanes until they arrive at the actual spot where one lane squeezes over.

However, some University of Nebraska-Lincoln researchers say it may depend on the situation. They say the late merge may be best on congested urban freeways, but the early merge is the better way to go on rural freeways where speeds are high. That way, drivers aren’t hitting the brakes at the last minute and causing crashes.

In California, there doesn’t appear to be any vehicle code section that directly addresses this, although there is a section that says any lane change needs to be a safe one.

For its part, Caltrans, our state’s transportation agency and a national policy leader, is a believer in the early merge.

“Caltrans continues to recommend that drivers merge well before they reach the lane closure,” agency officials said in an email to The Bee. “Drivers who cut in at the last minute cause sudden stopping and lane changes which cause direct collisions as well as delayed-reaction collisions by drivers further back in the queue who may not be paying attention or expecting traffic speed to suddenly change.”

What do you think? I may include some answers in a later column. Thanks.

Tony Bizjak: 916-321-1059, @TonyBizjak