Why do traffic jams occur?

By Nathan Dyck

You’re cruising on your way to school, making good time, when suddenly *bam,* you’re in the middle of a traffic jam.

Transportation problems are as old as mankind, and we see more and more problems as our cities become increasingly crowded. But as long as we drive cars, we’ll have traffic.

"People should be prepared for longer and longer delays, whether or not we have a working transit system," said University of Calgary Civic Engineering professor Dr. J.F. Morrall. "If we all lived in 20-storey apartments, we could probably make transportation more efficient."

But he doesn’t think this will happen, because of lifestyle preferences and willingness.

"This is a very spread-out city… you can’t replace the automobile very easily," he said.

So, do we need to build bigger roads to handle more traffic?

"This city designs its road systems right at capacity… this doesn’t give us any cushion," said Morrall. But road capacity isn’t everything, and looking at it straight-on isn’t best: building a road from A to B, and making it big enough to fit the right number of cars may not be that effective. Despite this, a study on traffic congestion funded by nine State departments in the U.S. concluded tactics used include "add road space" and "lower the number of vehicles."

According to Dan Bolger, who is responsible for Coordinated Systems Planning of the City of Calgary, the city takes demographic and land-use patterns and tries to make a model of what roads are needed and where, and tries to encourage people to use other forms of transportation. He admitted, however, that as a part of their stated policy, "[there] may be situations where we can’t handle the demand." He suggested commuters "just try and live with it."

New ways of looking at traffic behaviour and road use may explain what causes many problems. The visualization of traffic as a flowing gas has proved to be useful. It explains many situations and events in real-world traffic; phenomena not accounted for in the strictly linear classical techniques.

According to Morrall, "the saturation flow of a traffic lane is about 1,600-1,800 vehicles per hour of green." When more vehicles are added to this mix, they develop queues, and these queues form a shockwave and ripple-back effect. When a flowing gas enters a bottleneck, it becomes compressed as the molecules begin to crowd together. Each molecule can be viewed as a car, except molecules are never late for an appointment. The shock wave travels upstream.

These bottlenecks aren’t necessarily traffic lights or burning wrecks. Systems engineers describe "ghosts," which can cause breakdowns in the flow for hours after the initial problem is gone.

Imagine driving down the highway and Bozo the clown steps in front of you. You swerve and manage to only nick his big red shoe. This is termed an "incident." However, when you swerve, you cut off someone beside you and they come to a stop. The person behind them also comes to a stop, and the one behind them jams on his brakes. The flurry of cars stopping and starting travels back up the highway. The effects of this can linger in the right traffic conditions, and hours later, cars are still slowing down, even though the cars at the front are free to speed up again once they have cleared the "ghost" of Bozo.

Other times, there is no clown to blame. In any system that contains many parts, each part affects the others. Tiny fluctuations can grow in huge and unpredictable ways.

One way to see this is to imagine some dogs on a log in the middle of the river. If one dog moves, it sets up a disturbance, forcing the other dogs to move to keep their balance. Pretty soon the log is rocking violently and many of the dogs may get wet. The expanding network of roads and lights designed to try and keep up with demand may not always be helpful, since drivers aren’t making rational decisions and add to problems. Increased capacity in a limited area often works only to pile more dogs onto the log.

Strangely, it is often when the traffic is densest that it flows smoothest. There are often fewer starts and stops backlogging the road, and less jostling for position, which adds less chaos to the flow.



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