Tag Archives: back-up

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.”

Ferry traffic in Kingston needs to use holding lane

The out basket: Cliff Durant wrote on Aug. 12 to say, “I was at the parts house in Kingston this evening. When I attempted to leave, the ferry traffic was backed up through town and as far as I could see out of town.

“The big problem was that no one would use the ferry holding lane,” Cliff said, “therefore stopping all traffic into town. I waited at least 20 minutes when I finally persuaded a driver to back up enough so I could get into the two-way left-turn lane and get where I wanted to go. There were times when cars were trying to go both ways in the two-way left-turn lane.

“Since the use of tally slips, people quit using the holding lane in town,” he said, “and when it backs up out of town they don’t use it there either. I called (9-1-1) and reported it and was told that is just the way it is when the ferry traffic backs up. I said someone needs to direct the traffic into the holding lane and was told there wasn’t anyone available.

“This happens quite often,” he said, “and doesn’t need to if the ferry (traffic) would just use the holding lane. Maybe signs advising the use of the holding lane would help.”

The out basket: The State Patrol manages traffic around the ferry terminals, so I asked its local spokeswoman, Trooper Krista Hedstrom about this.

“This does occur from time to time when the boat off-loads and the traffic lights stop traffic in both directions,” she said. “That will cause some gridlock. During the evening rush hour (during the summer only) there can be a 5-10 minute back-up, which is self-correcting when the boat loads.

“The problem is there is a mixture of commuters and leisure travelers that have absolutely no clue what to do regardless of our direction,” she said. “WSP does routinely direct traffic when it causes a back-up, however this issue typically corrects itself once the boat loads.  It usually takes more time directing everyone onto the shoulder than just allowing the traffic jam to self-correct itself.”