When you approach a bottleneck in traffic — whether it’s due to construction or a permanent reduction in the number of lanes — what do you do?

Are you the driver who gets into the long line of cars in the lane that will stay open — or do you go along in the open lane as far as you can (getting in front of a lot of other people), and then merge just before the road barrier or closure?

freeway-traffic-merge-cars

Waiting to take your turn vs taking turns

If you’re like a lot of women, you do what seems the most polite: you get into line well ahead of the merge… and mutter under your breath as you watch those thoughtless other guys cut into the front of the line.

Here’s the thing: Those other guys? Whether they are acting out of selfishness or have a good understanding of automobile traffic dynamics, they’re actually doing the right thing.

Get to know the zipper theory

Several state transportation boards — along with the governments of several other countries — now recommend that both lanes of traffic stay full, then merge at the end. The key is that the cars in each lane need to alternate during the merge, much like how the teeth on a zipper come together. (This is also the same principle enforced by on-ramp traffic metering lights.)

Benefits of the zipper
  • Reduces differences in speeds between two lanes.
  • Reduces overall length of traffic backup by as much as 40%.
  • Reduces congestion on freeway interchanges.
  • Sense of fairness that all lanes are moving at the same rate.

Source: Minnesota Department of Transportation

Most drivers start to merge in construction zones as soon as they see warning signs and learn which lane ahead is closed, says the the Minnesota Department of Transportation. This driving behavior, called “early merge,” can lead to dangerous lane switching, inconsistent driving speeds that cause crashes, long back-ups that block interchanges, and road rage.

The MDT goes on to say that zipper merging 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 a “zipper” fashion into the open lane.

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