The “accordian effect” and “shockwave traffic jams”

The in basket: Dustin Butler of Port Orchard writes, “I’ve noticed over the years that the merge at the Bremerton treatment plant on Highway 3 doesn’t really cause the huge backups. The same is true coming onto the freeway from Navy Yard City too, now that the HOV lane is there.  The problem actually seems to start with people not speeding up again after the merge or slowing down well after the merge, possibly nervous when meeting with the other lane.
Recently, Dustin said, “I went through there during the normal (about 4 p.m.) rush hour. Since it was holiday, there was no back-up of cars merging, but well after we were in one lane, but before meeting up with the second lane, all the cars came to a complete stop then started creeping slowly not gaining speed until after meeting the other lane. That is just one example of seeing this hundreds of times in this area.
“In other areas (I can’t recall an exact area, but California I believe),” he said, “I have seen signs that say maintain speed or similar wording.  Is there any evidence these signs work and has the state considered trying something like this in this area?  The small cost of the experiment would save millions in waste even if it just worked a little.” he concluded.

I doubt that very many of those who pass through that area at weekday rush hour would agree that the merge doesn’t cause the backups, but I often see what Dustin describes slightly ahead at about Windy Point, when all the merging of Highway 3 and 304 traffic is complete.

I asked state officials if the highway pros have an explanation for the phenomenon there and other places where traffic regularly comes to a standstill for no reason that is apparent when one finally gets to the point where traffic starts moving freely again. I also ask about the practical impact of “Maintain Speed” signs.

The out basket: Steve Bennett, traffic operations engineer for the Olympic Region of state highways, replies, “I’m not sure if this applies to your question exactly, as I generally think of this happening on longer corridors, but there is something called the accordion effect that may explain this phenomenon.

“Basically, it occurs went traffic is heavy and for some reason a driver slows. This then sends a ripple of braking down the corridor, each driver, in turn, slowing slightly more than the driver ahead of him.  If the line is long enough or speeds slow enough, this can eventually get the trailing traffic to zero mph.”

Steve referred us to a New York Times article about “shockwave traffic jams” that can be seen at

It includes a video of a Japanese experiment that created  a shockwave jam in which a vehicle slowing creates a shockwave behind it that grows until traffic is barely moving  at all. The test in the video involved a whole bunch of cars traveling in a circle at 30 miles per hour until something caused the cars to bunch up and slow. Check it out.

As for “Maintain Speed” signs, Steve says, ‘No, there is no evidence that these types of signs have any effect on traffic.  Drivers tend to drive the speed they feel comfortable going and will not modify their behavior because a sign tells them to do so.”

One thought on “The “accordian effect” and “shockwave traffic jams”

  1. I think the main reason people slow in that merge area is probably because of past experience with drivers who will suddenly “jump” to get in the other lane, even before the end of the thick, white, “don’t cross here” line that’s supposed to keep people in their own lane(s). I’ve had it happen to me, going along fine and suddenly, there’s some driver who just HAS to be in my lane NOW. If you look at the people as those two lanes come together, thre’s lots of head turning going on, as people look to change lanes OR look for someone else doing so. That’ll make people put their foot on the brake and so it begins…

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