Do you do the zipper merge? The zipper merge, in which you use up all the space in a lane that's closing and then take turns merging in at the very end, is both safer and better for traffic flow, traffic experts agree. The problem is that many motorists consider it very rude. If you think that yourself, you might want to change your attitude. Experts say the zipper merge keeps traffic flowing better and reduces accidents. And at least one state has made it the law.

It's a familiar situation. You see that the lane you're in will close in a half mile or so. Traffic is heavy, so to be polite, you move over into the through lane as soon as you can. Other polite drivers do the same, and now the through lane is crawling along, while the lane that's ending is virtually empty. Then a car sails up to the end of that lane and puts on a turn signal, asking to merge in front of you. If you're like most people, including me before I learned better, this makes you fighting mad. You inch as close as you can to the car ahead of you to make sure this selfish person can't cut in front of you or the thoughtful drivers behind you.

The problem is, that rude driver is right and you're wrong. That's not just an opinion. Research has shown that in heavy traffic, things go better when drivers in a lane that's ending drive to the end of that lane and then take turns, or zipper merge, with the drivers in the through lane. Traffic overall flows much more quickly and everyone gets where they're going sooner. Not only that, there are fewer accidents. That's because if drivers fill up both lanes, they'll move at roughly the same speed. And it's safer and easier to change lanes when that's the case than when you're going from a fast-moving lane to a slow one, or vice versa.

Nathan Silberhorn, a project engineer and highway designer with more than 20 years of experience, provides a clear for the zipper merge and its benefits in a blog post at the Ayres Associates website.

Your state wants you to zipper.

Many states are trying to get the word out about the zipper merge, with videos explaining the maneuver, with signage, and with catchy phrases like, "Merge late, cooperate!" For both a detailed explanation of the technique and for general corniness, you can't beat this video from the Kansas Department of Transportation in which animated traffic cones discuss the issue in depth.

Some states are taking it a step farther, mandating that the zipper merge be included in driver education materials. And Illinois has passed a law imposing a fine on drivers who refuse to take turns when a merging lane ends. A similar law has passed in the North Carolina House and will head to the state Senate.

In the meantime, North Carolina has deployed dynamic signage that encourages drivers to zipper merge when traffic slows to a crawl. A first sign instructs drivers to "Use Both Lanes/To Merge Point." And a second, at the end of the merging lane, says, "Merge Here/Take Turns." That should at least settle any arguments inside the car about the right thing to do in this situation.

Highway engineers are hoping all this will be enough to turn the zipper merge into common practice. But if it isn't, they have one more--somewhat unpleasant--tactic they can try. "States promoting the zipper merge have one grand secret to success: Don't tell which lane will close!" Silberhorn writes. "When drivers see signs alerting to an eventual lane closure, they become more alert to changing conditions but use both lanes as long as possible." Drivers may gripe about signs that say a lane is ending without specifying which one, he admits. But in fact, those complaining drivers are much better off than if they did know. "Traffic will move much better through the bottleneck," he writes.