Q: I regularly drive through a construction project that often has one of the lanes on the freeway closed. Drivers merge in a single lane a mile ahead and often get angry when people pass them in the unused lane. I even got behind a driver who was straddling the line to prevent anyone from getting ahead in either lane. It makes sense to use both lanes and merge at the orange cones, but is that the law?

A: First, a message for the driver that was blocking both lanes: You’re the problem. The law requires drivers to drive within a single lane. Okay, now let’s talk about the zipper merge.

Many drivers, when they see a “lane closed ahead” sign, move over right away, doing the long-established early merge. But just because we’ve been doing something for a long time doesn’t mean it’s the best idea, and many transportation departments around the country, including Washington DOT, encourage drivers to late merge, or zipper merge, when traffic is heavy. Utah has even made it the law.

If you’re approaching a lane closure when traffic is light (a fictional scenario in some parts of the state) and speeds are relatively fast, keep doing the early merge. Find a good gap to fit into before you run out of pavement.

But when traffic is building up to the point of congestion, it’s time to zipper merge. Drivers in the closing lane should remain in that lane until it begins to taper and then (this part requires some cooperation from drivers in the other lane) merge at a one-to-one ratio with the drivers in the continuing lane, like the teeth of a zipper.

I can hear the early mergers complaining it’s not fair that they wait in a long line while drivers in the closing lane zoom past them and merge at the last moment. You’re right, that’s not fair. But if all the early mergers decided to zipper merge, the closing lane would be as full of traffic as the continuing lane and neither lane would have an advantage. That’s a secondary benefit of the zipper merge; when both lanes are full it eliminates the sense of injustice when someone zooms past a mile-long line of cars to late merge.

The main benefit though, is safety. Instead of drivers trying to guess the best place to merge somewhere along the route, all drivers merge at the same location, reducing conflict points throughout the backup.

It also shortens the length of congested areas. If you take a mile-long one-lane backup and split it into two lanes you now have a half-mile backup. (It’s often closer to a 40 percent reduction rather than 50 percent because we’re not perfect drivers.) That reduces the likelihood of congestion creating problems with the freeway interchange upstream of the merge area.

And the zipper merge eliminates the speed differential between the lanes. When both lanes are similarly slow, if there is a crash it’s more likely to be a fender bender than a serious injury. That leads to some advice: the zipper merge hasn’t fully caught on yet, so when you choose to stay in the merging lane and no one else does, don’t race past the other cars. That defeats a big safety feature of the zipper merge. Keep your speed close to the cars next to you.

Our merging law doesn’t specify where a merge happens; it says don’t move out of your lane unless you can do it safely. Washington law doesn’t mandate the zipper merge, but it’s encouraged as a safer and better practice.

Doug Dahl is with the Traffic Safety Commission and writes a weekly column for this newspaper.