What happened?
"It's a phenomenon," said Lois Morasco, spokeswoman for the Pennsylvania Department of Transportation. "I've encountered it on expressways so many times. It's frustrating to any motorist, and it's a hard one to explain."
Well, Ms. Morasco, we are going to try.
From his perch in a helicopter high atop the city and its environs recently, Pete Roscoe of the Shadow Traffic Network mused about the factors leading to traffic jams.
"All of them kind of have a reason," he concluded, "even if it doesn't look that way."
From the Schuylkill Expressway to Interstate 95, from U.S. Route 202 near King of Prussia to Route 70 in New Jersey, that reason can be as mundane as a slow-moving truck or sun glare in the morning.
The key is high volume.
Where there are too many cars on roads not built to handle the capacity, high volume makes traffic flow extremely vulnerable to the slightest change. A tap on the brakes coming around a curve, a frosting of snow, a motorist with a flat tire by the side of the road can cause traffic to sputter and stall.
As Jeff Greene of the Philadelphia traffic consulting firm Orth-Rodgers & Associates explained in reference to the Schuylkill Expressway:
"When traffic gets very dense, just about any change in the roadway, traffic composition or the environment can trigger a motorist to touch the brakes. Then the person in back slows down even more, and there's a ripple effect until someone has to stop."
Complex mathematical formulas predict and describe this behavior, but two words can explain the result: You're stuck.
Of course, traffic jams also result from easily discernible causes such as construction projects or traffic oddities, like four gallons of molasses spilling from a truck and turning the highway into a sticky mess (it actually happened, on the Schuylkill Expressway in 1963, causing an hour-long delay).
Obviously, accidents cause traffic jams, as well.
