| Rush hour traffic is backed up along Interstate 66 near the Vienna Metro station. (Nikki Kahn/The Washington Post)|
By Robert Thomson, Linda Mooring, Fairfax and Steve Fahey, Kensington
VIENNA, VA. - I’m doing this for your own good. What with the inauguration and the Women’s March and various Metrorail problems, it’s been a while since I’ve let drivers vent their frustrations. And I’m sure it’s better that they steam away here than behind the wheel.
Dear Dr. Gridlock:
I am frustrated that you printed Camilla Stroud’s letter [Dr. Gridlock, Sept. 28] bemoaning people who wait until their lane ends before merging. I am more frustrated that you seem to understand nothing about traffic engineering principles and that you agreed with her.
When traffic is flowing, it makes sense for people driving in the lane that will end to merge as soon as safely possible when they see that their lane will end soon.
But traffic around here does not usually flow, and a remarkable amount of simple traffic engineering evidence shows that merging early only slows traffic more.
Close your eyes and imagine a crowded freeway merge, perhaps Interstate 66 westbound at Nutley at 5 p.m. Watch from the Metro overpass.
Let’s imagine everyone down there is a perfect, polite driver, and let’s imagine cars fill both the merge lane and the freeway lane. As traffic in the freeway lane starts to move, imagine the car at the front gently merging. Oh dear, that means the car behind must slow just a little bit, but in our perfect, polite world, the driver in the merge lane sees it slowing, and slows pace to match.
This next driver in line lets one car go by and then merges behind that car. But — oh, darn — that slows the car in the freeway lane. That car must slow down to let the other car merge. But the next polite driver in the merge lane slows to match.
Slowly, the cars in the merge lane no longer are creeping all the way up to the intersection of the two lanes. Uh oh! What happens if our polite drivers keep using Camilla’s proposed solution?
Because you are giving Camilla voice, people continue to botch this whole thing up. Eventually, too much space opens up in the merge line.
Please research the zipper merge. I’ve seen this work in person. There was a need for major repairs on Interstate 17 in California. Traffic had to be reduced from two lanes to one.
Traffic engineers made a huge effort to educate drivers about the zipper merge. They constructed signs and had an ad campaign. I remember that highway patrol officers stood at the merge mark and ticketed people who did not zipper merge. Don’t block mergers; allow one from one lane each. It worked extremely well.
You are so very wrong about merging. Please stop confusing people and advocating driving practices that just don’t work.
Linda Mooring, Fairfax
DG: Like many traffic engineers, I endorse the zipper merge in exactly the same circumstances that Mooring describes. In heavy, slow traffic, when the rules are clear to all drivers, it often works quite well to have the drivers take up both lanes until the point where one lane ends, and then have them take turns entering the through lane.
But there’s no single system that accounts for all possibilities.
Bet you’re wondering what Stroud said. This was part of her letter:
“When I see vehicles entering the highway, as a courtesy I try to move out of the lane so they can enter, or I speed up or slow down to make room for them, if possible. When I was commuting, I was considerably less accommodating of drivers who knew their lane was ending but ran all the way to the merge point and then tried to cut in.”
Dear Dr. Gridlock:
I read with interest your recent discussion about zipper merges, and talked about it with my nice Midwestern neighbor. Predictably, he is of the camp that says one should join in at the back of the line. I do that, as well, unless I am late for a pressing appointment, when I act like an Easterner and cut in front from the adjacent lane.
As you pointed out, traffic engineers have shown that the zipper merge moves everyone faster. This is apparently the case for lane mergers, but it does not work as well for exit queues. In that situation, drivers in the adjacent lane who go to the front and cut in cause non-exiting cars behind them to slow down or stop, creating an inefficient, and potentially dangerous, situation.
Steve Fahey, Kensington
DG: I’ve been a driver or passenger in every state but two, and I’ve found it impossible to predict driver behavior based on the region I’m traveling in. (I look for other more immediate clues to anticipate what people are going to do.)
But I do agree that the zipper system doesn’t do its best when one set of drivers has to slow very quickly to merge, or when the rules are unclear.
Drivers in the through lane get very possessive about their space when there’s no sign or police officer telling them to back off and let the other car in.
I’m glad we’ve now cleared the air on this merging stuff. You now know the correct thing to do in any situation, right?
Source Washington post