On the Trail of Phantom Traffic Jams

Jan Hofmann is a regular contributor to Orange County Life

Is there anything more frustrating than the traffic snarls caused by the usual overturned trucks, stalled station wagons and fender-bender chain reactions?

Only one: the tie-ups that don’t seem to have a cause. You know, when--for a change--you’re zipping along in high gear and suddenly you’re stopped by a wall of brake lights. You slog along for a mile or two, stop, inch forward, stop again, go again, looking around for whatever the problem is. Then almost as suddenly, the road opens up again. Relieved but confused, you press the pedal back down and wonder, “What was that all about?”

If it’s any consolation, traffic engineering experts are also trying to understand those phantom traffic jams--how they work, what causes them, and most important, what to do about them.

Transportation engineers have put together mathematical models of how phantom traffic jams work, says Will Recker, director of UC Irvine’s Institute of Transportation Studies. But you and I don’t have to be good with numbers to understand how the model works. All we need to do is go to the beach.


See that wave breaking in front of us? Far out beyond the horizon, some minor disturbance, such as a gust of wind, blew it into being. From there it developed a life of its own, swelling and dropping, up and down until finally it surged ashore with a crash.

Phantom traffic jams work exactly the same way, Recker says, at least according to the model. “We can sort of predict it mathematically,” he says. “But whether that’s in fact what’s going on, we don’t really know.

“You take steadily moving traffic, add some oscillation, and under the right conditions, that wave can be amplified based on the car-following theory.”

The car-following theory, Recker explains, is something most Orange County drivers practice whenever they get onto a crowded freeway. “That’s where you base your response--accelerating, braking--on what the vehicle in front of you is doing. You’ve heard the adage, ‘Leave one car length in front of you for every 10 miles an hour of speed you’re traveling.’ It comes from that.”


So something, however slight, interferes with the flow up ahead. “If you have the right combination of circumstances, this oscillation can produce a wave that has that oscillation amplified so everything just comes to a halt,” Recker says. The wave travels in both time and space, in the opposite direction of the traffic flow, just as an ocean wave travels against the flow of the water.

“The point where it stops may be far away from where it began,” says Recker, who is also a professor of civil engineering at UCI. Many miles? “No, more like many feet,” he says.

OK, so that would make something like the Orange Crush interchange (where the Garden Grove Freeway meets the Santa Ana and Orange freeways) the traffic equivalent of the famous Wedge in Balboa, where the waves bounce off the Newport Harbor jetty and are amplified into moving mountains. (If there was a way to surf these freeway swells, we might make it home for supper a lot more often.)

So what gets these waves going in the first place? The considered, informed opinion of Recker as well as experts at the California Department of Transportation and the California Highway Patrol is: You name it.


“It could be the lingering effect of an incident that’s been previously cleared,” says Marian Hofstad, government affairs coordinator for the local Caltrans district. “It could be the CHP doing enforcement driving up ahead. Construction projects, people picking up trash, anything for people to look at along the side: a near-miss, a rapid lane change that causes everybody in the whole area around it to slow. Sometimes a car leaves one turn signal on, and everybody stays away from them. It could be an unusual car. Sometimes it’s just a plain old overcautious driver, an older person, or someone who’s not familiar with the area.”

Caltrans engineers say even the sun can sometimes be the culprit, according to Hofstad. “At sunset and sunrise, especially at sunset, if the sun is coming toward traffic, there will just about always be a slowing,” she says.

Ken Daily of the CHP’s San Juan Capistrano office confesses that sometimes he or his colleagues set off traffic waves. “If we’re running a traffic break, slowing down traffic to create a gap so we can remove a road hazard or an accident ahead, the traffic right behind us may slow down to about 35. Then the traffic coming up behind them will slow even faster, and eventually it will come almost to a complete stop,” he says.

Sometimes the CHP unintentionally interferes with traffic flow, he says. “I had a woman come up to me in traffic school and say, ‘The other day there was a Highway Patrol car going 65, and he held us all up.’ ”


But seriously, Daily says, “I remember one time when I had a report of an accident over the side, and I was going 40 or 45 looking for it. I looked back and everybody was lined up behind me, afraid to go around.”

If a CHP officer is traveling at or near the speed limit, Daily says, it makes sense not to pass. But if he’s going slower, drivers shouldn’t be so shy.

Stopped Highway Patrol cars can also set off waves, Daily says. “When an officer just makes an enforcement stop, people slow down just to look.” For that reason, he says, officers are trained to be as inconspicuous as possible, turning off their flashing lights as soon as possible.

Now the hard part: What can we do about phantom traffic jams?


On this point, the experts are in utter agreement. Not much, they say.

“We can offer a plausible explanation,” Recker says. “What we do about it is another question.” But that is the goal in setting up those mathematical models and studying the phenomenon. “The hope is that by understanding the process better, we may be able to come up with something,” he says.

One possibility, he says, are pacing lights, “almost like a strobe, something you can use to adjust your speed.” If all drivers traveled at about the same pace, the waves would flatten out. But such solutions, if indeed that’s what they are, are years away.

Meanwhile, it’s up to individual drivers. “If you see brake lights way ahead of you, prepare to stop, but don’t just slam on your own brakes,” Daily says. “Just back off the gas. If every driver would do that, we wouldn’t have that accordion effect. But people just don’t do that.”


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