Everyone who drives knows there’s bound to be a traffic jam around the next curve or corner.
But did you ever wonder how it got there?
Jams form, say those who study traffic, when a mix of certain conditions and human reactions come together.
It’s part science and part emotion.
As Americans rely more than ever before on their cars as their primary mode of transportation, congestion becomes more ubiquitous. So, for the last decade, scientists have been studying the traffic jam’s “properties” in hopes of reducing its effects.
“Traffic jams are a waste of time and money,” says Don Schmidt, an engineer with the Illinois Department of Transportation’s expressway traffic operations. “They’re totally inefficient.”
Dissecting why traffic jams occur, however, is harder than identifying how they occur, researchers say. “It’s easy to pinpoint any number of causes of a traffic jam,” says Schmidt:
– Congestion is the obvious reason, he said. For example, to maintain a free flow at about 30 to 35 miles per hour of cars on the Kennedy Expressway, there can be no more than 2,100 cars per lane per hour, he said. And that’s in optimal weather conditions. More than that will tax the roadway’s capacity and translate to more traffic jams. (Though IDOT says it doesn’t track traffic by rush hour, it did say that in 1995, the most recent figures available, 125,000 cars used the inbound Kennedy daily between North Avenue and Division Street. That figure was 123,000 outbound.)
“When additional vehicles try to get onto an already congested roadway, that can create an interruption that affects cars up the stream of traffic,” said Lilly Elefteriadou, an assistant professor of engineering at Penn State University in University Park, Pa.
The same mechanics apply to a bottleneck, said Elefteriadou.
– Weather can be a factor. Obviously a snow or rain storm can slow down cars and cause congestion, but so can bright sunshine in someone’s eyes, says Schmidt. Neither IDOT nor any of the traffic reporting services can give hard numbers on how much longer you can expect the commute to become.
– Mechanical breakdowns, especially in rush hour, are another cause, said Schmidt. “When the road is at capacity, it takes five minutes for traffic to clear for every one minute a car has been disabled on the road,” he said.
– Accidents can cause the same type of delays. “The more severe the accident, the longer it will take to clear up,” said Schmidt.
– Gapers’ blocks, usually the result of breakdowns or accidents, add to those delays.
– Road construction or problems such as potholes will also cause drivers to slow down, again providing a reason for traffic to jam.
Despite the number of causes, most traffic researchers agree that it’s the human condition that creates a traffic jam.
“Everything is complicated by human reaction,” Bob Ervin, head of the Engineering Research Division for the University of Michigan’s Transportation Research Institute in Ann Arbor.
This explains why traffic can lock up without any tangible cause in sight. “If the conditions are right, one person hitting the brakes can cause a chain reaction down the flow of traffic,” said Ervin.
A major effort at dissecting the traffic jam is taking place at the Los Alamos National Laboratory in New Mexico. Owned by the Department of Energy and managed by the University of California, the national security laboratory is running a project called TRANSIMS (Transportation Analysis and Simulation System) for the U.S. Department of Transportation.
The program of travel-forecasting models was created to give transportation planners detailed data on traffic impacts, congestion and pollution. The program was partially spurred by the Clean Air Act.
“The department wants the next generation of tools for forecasting the impact of traffic for regional transportation planning,” said Chris Barrett, leader of the basic and applied simulation science group at Los Alamos.
Barrett likens the mechanics of a traffic jam to an increasing flow of water in a pipe with one difference–the human condition.
“The more cars you have, the more likely they are to start to interact with each other,” he said. “If they were water molecules, they would start bumping into each other. But in traffic they just start slowing down and speeding up to miss each other and try to get down the road as fast as they can.”
When traffic flow is dominated by these interactions, said Barrett, there will start to be “little blips of slowdown” in that flow.
“So first you slow down and then the guy behind you slows down,” he said. “Sometimes it doesn’t go any further. Other times it propagates down the traffic flow.”
That slowdown can quickly reach a critical point known as a phase transition, which can produce a shock wave that runs down the flow, said Barrett.
The slowdown is exacerbated by each driver’s reaction.
“If the guy in front of you slows down, then you slow down and then the guy behind you slows down,” said Barrett. “Now there are time delays between those because of our individual reaction times and because we’re not all exactly the same distance between one another and we’re not all going the same speed.
“So the way these things propagate or damp out is extremely complicated,” he added.
Ervin said that comparing traffic during rush hour and non-rush hour times helps researchers better understand the formation of a traffic jam.
“When you’re driving in the expressway in the middle of the night, you’re going to be 400 feet behind the guy in front of you,” said Ervin. “So if there’s a disturbance–say he hits the brakes to avoid a pothole–you have time to react and you may not even have to apply your brakes. That stems any more reaction behind you.”
In the rush hour, however, a driver may be moving at 100 feet per second (70 m.p.h.) and be 50 feet behind the driver in front of them.
“You’re literally one to 1 1/2 seconds apart from each other,” said Ervin. “So if the driver in front of you puts on his brakes, you can’t afford to wait around to see how hard he’s braking.”
The driver following is probably going to slam his brakes a little harder. “And the guy behind him does the same,” said Ervin. “So you have this chain reaction, or this cascade effect, and everyone is going to crunch down in speed.”
Key here, said the researchers, is how quickly drivers can react and return the flow of traffic to normal.
“Once there is a breakdown, there has to be lower demand,” said Elefteriadou. “It’s the only way the freeway can recover.”
“When you’re packed in tightly it’s going to take more time to clear up because of that human reaction,” said Ervin.
Does this mean that when cars are driven by computers versus humans, traffic jams will be a thing of the past?
Not necessarily, said Barrett.
“It depends on how we do things like create cities and zones, as well as provide mass transit,” he said.
“Then it also depends on how many people are on those roads and how intense the interaction is among the vehicles.
“Ultimately, you put enough cars on the road and no matter how many computers you’ve got driving your cars, the traffic is going to stop,” Barrett said.
