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Road Worrier: With meters and zippers, NCDOT separates Early Mergers from Late Mergers

Traffic merges on to I-40 from Miami Boulevard at exit 281. Ramp meters with stop-go lights will be installed at some Interstate 540 interchanges in northern Wake County to smooth on-ramp flow.
Traffic merges on to I-40 from Miami Boulevard at exit 281. Ramp meters with stop-go lights will be installed at some Interstate 540 interchanges in northern Wake County to smooth on-ramp flow. 2013 News & Observer file photo

What’s the best way for drivers in two lanes to merge into one lane? Ask any two traffic engineers – or any two experienced drivers, for that matter – and you’ll get three or four answers strung together with: “It depends.”

The state Department of Transportation this year will install traffic signals called ramp meters at a few interchanges on Interstate 540 in northern Wake County, to see if they can spread out the flow of drivers merging onto the freeway.

And DOT engineers are considering an I-85 overhaul project in Warren County, where two lanes will squeeze into one for weeks at a time, for an experiment in “dynamic” message signs – flashing instructions that vary with changes in traffic conditions – to help drivers merge together smoothly, like a zipper.

Zippers and meters represent efforts by DOT to reduce congestion on freeways that are getting more hectic every year – by easing the hassles that come with merging.

Smooth merging sometimes is negotiated in a quick mind-meld between drivers who are calm and attentive, generous and almost telepathic: Yes I’m going to slip into that gap ahead of you / Yes I’m going to let you slip ahead of me. It’s like ... highway magic.

But when it’s not like magic, somebody has to hit the brakes to avoid a side-swipe or rear-ender. Not smooth. Other drivers have to brake, too.

That’s what can happen when five cars barrel down the on-ramp together, trying to merge onto the freeway all at once.

Ramp meters are signals that switch back and forth quickly between red light and green light. The signal lets one on-ramp driver go now and tells the next driver to wait a moment, so the cars enter the freeway one at a time.

“It’s the freeway accepting a drip, drip – as opposed to the full faucet,” said Jim Dunlop, a DOT congestion management traffic engineer.

Ramp meters are credited with speeding up the urban rush hour flow in other states. But it’s not guaranteed that they’ll work here. Drivers might get confused. The on-ramp queues might back up onto the side streets.

If they do prove helpful on I-540, we’ll see more ramp meters in coming years at busy interchanges on I-40 and I-440, and in Charlotte. (Learn more online at ncdot.gov/projects/rampMetering/.)

The other kind of merging comes when your road drops from two lanes to one lane. Usually there’s a sign to warn you: Right lane ends in one mile; merge left.

This is where it gets tricky.

Should you be an obedient Early Merger, moving immediately into the other lane? Or should you stay in your lane until it ends, and then weasel your way in front of somebody in that other lane – a Late Merger?

It depends, Dunlop said.

If traffic is moving freely, be an Early Merger.

Don’t slow down if you don’t have to. Start looking for an opportunity to move into that other lane.

“If it’s not congested, once you see that a lane is ending, you merge in at speed,” Dunlop said. “Pick a reasonable point so you don’t slow up the other drivers. Everything runs smoothly.”

Because if you wait until your lane has ended, he explained, you may not be able to merge without forcing drivers in the other lane to slow down.

Dunlop’s advice is just the opposite in cases where traffic is clogged and slow: Be a Late Merger.

Yes. The most efficient way is to merge like a jerk.

Jim Dunlop, NCDOT congestion management traffic engineer

Drive all the way to the end of your lane and then do the zipper-merge – a car from the left lane alternates with a car from the right lane – to combine your two lanes. It’s easier in this case because everybody is moving slowly. And you’re making full use of both lanes.

That can mean other drivers will be backed up in the left lane while you speed past them in the right lane – making them hate you.

It’s still the right thing to do, Dunlop explained to members of the state Board of Transportation.

“I was asked a the board meeting: So the best thing to do is go to the end of the line, where everybody’s calling you a jerk?” Dunlop said. “Yes. The most efficient way is to merge like a jerk. Speed past all the people in the lane that’s not ending.”

This was hard enough to explain to board members and to the Road Worrier. Now Dunlop and his DOT colleagues are trying to figure out how to make this clear for drivers affected by that I-85 project in Warren County.

They aim to combine traffic radar with message signs to tell I-85 drivers when they should be Early Mergers, when they should be Late Mergers – and how to act like zippers.

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