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If 5 p.m. is rush hour, September feels a lot like rush month, the annual moment of peak gridlock.

I think part of it probably is related to everybody back from vacation, everybody’s on routine schedules

Some of this spike is likely in the eye of the beholder. Every traffic jam seems like the worst ever until the next, especially if holiday memories remain fresh.

But there is a science beneath the anecdotal complaints, and surveys of traffic patterns in places as diverse as Holland, China, America and Canada show that if September is not the worst, it is at least tied with April.

“My guess is that the surge of traffic that you see in September in part is observation bias,” said Morris Flynn, associate professor in mechanical engineering at the University of Alberta, who has studied the dynamics of traffic flow using the principles of fluid dynamics.

“I think part of it probably is related to everybody back from vacation, everybody’s on routine schedules, people aren’t taking sick days, everybody thinks that they have to get to work at 7:55, they don’t realize that they can blow in at 8:05 and nobody really notices,” he said.

He has done research on “phantom traffic jams,” when a uniform stream of traffic mysteriously clogs up in a spontaneous spike of density, as opposed to a real traffic jam caused by a fixed obstacle like a pile-up or construction site. He learned that when roads exceed their capacity, traffic becomes “a self-reinforcing system” in which “everything just gets progressively more and more bogged down.”

“If you have this over-capacity of people on the roadways, and everybody on September the first feels locked into this rigid schedule, you’re going to get all kinds of manifestations of an over-capacity of people on the roadway, and that includes things like phantom jams, but as soon as phantom jams happen, then the likelihood of an accident goes up, and then you have a real traffic jam.”