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The anatomy of a traffic jam

by Abe Tekippe
Feb 16, 2011

Stop. Go. Stop. Go. Stop.

It’s a pattern that’s become part of many drivers’ everyday routines, especially in cities like Chicago, which ranks No. 1 in the nation for traffic congestion, according to a recent report by the Texas Transportation Institute. Chicago drivers lose an average of 70 hours a year because of this congestion, the study said.

But what causes traffic jams? How does traffic suddenly go from 55 mph to zero, only to speed up a few minutes later with no sign of an accident? And is there anything drivers can do to help stop congestion before congestion stops traffic?

Local transportation experts agree that the majority of traffic jams, both in Chicago and elsewhere, are caused by at least one of three things: overcrowded roads, adverse weather conditions, and accidents and breakdowns. These factors, experts say, trigger a chain reaction — similar to the push and pull of an accordion or Slinky — that eventually causes traffic to slow down or, in some cases, come to a standstill.

“If the person in front of you slows down a little bit and you don’t notice right away, then you have to slow down a lot — otherwise you’ll overtake them — and that just sort of builds back through the traffic queue,” said Roy Lucke, a research manager at Northwestern University’s Center for Public Safety. “Each person has to slow down more than the person ahead of them.”

While accidents and breakdowns are the obvious causes of delays, Joseph Schofer, a professor of civil and environmental engineering at Northwestern, said something as simple as a piece of paper blowing across the road could cause drivers to brake suddenly, sending a shockwave up the traffic stream.

“I think the hardest thing for people to understand is that at some point you go through and get to where traffic is flowing smoothly again and say, ‘But I didn’t see an accident,’” Schofer said. “There’s a lot of cases where you don’t see anything because whatever it was happened really fast.”

This shockwave phenomenon, caused by drivers’ small, sudden fluctuations in speed, was at the center of a 2008 Japanese study in which 22 cars drove around a circular track at a constant speed of 30 kph.

“The research found that tiny fluctuations in speed, always existing when drivers want to keep appropriate headway space, have a cumulative effect,” a 2008 news release about the study said. “Once traffic reaches a critical density, the cumulative effect of gentle braking rushes back over drivers like a wave and leads to a standstill.”

Although drivers may not be able to defy physics and stop traffic jams altogether, Lucke and Schofer said there are measures they can take to help keep traffic flowing. The most important: paying attention.

“If you see traffic slowing ahead … then start slowing down so you don’t have to do that abrupt slowdown later on,” Lucke said. “The more gradual the slowdown, and the more gradual the speedup, the more likely it is that the traffic in any given queue will be better able to absorb that slowdown and speedup.”

Beyond alertness, Schofer said drivers can also leave earlier or later to avoid the busiest travel times, carpool or use public transportation. Ultimately, the choice is theirs, he said.

“People spend a lot of time in their cars, but they’ve made choices to do this,” Schofer said. “They’ve made the choice to live farther way form their workplace in order to get a cheaper or better housing situation and the price you pay is more commuting time.”