In fact, the Germans' analysis suggested, such spontaneous breakdowns in traffic flow probably occur quite frequently on highways. Under the right conditions a small, brief, and local fluctuation in the speed or spacing of cars - the sort of fluctuation that happens all the time just by chance on a busy highway - is all it takes to trigger a system-wide breakdown that persists for hours after the blip that triggered it is gone. Traffic can be flowing freely along, at a density still well below what the road can handle, and then suddenly gel into a slow-moving ooze. No bottlenecks or other external causes are necessary. The eeriest thing that came out of these equations, however, was the implication that traffic congestion can arise completely spontaneously under certain circumstances. That is precisely analogous to the well-known slowing and queuing of cars behind a traffic bottleneck: as cars slow at the obstruction, cars behind them slow too, which causes a wave of stop-and-go movement to be transmitted "upstream" along the highway. When a flowing gas encounters a bottleneck, for example, it becomes compressed as the molecules suddenly crowd together - and that compression travels back through the stream of oncoming gas as a shock wave. The moving-gas model of traffic reproduces many phenomena seen in real-world traffic. But the physicists added some terms to the equations to take the differences into account, and the overall description of traffic as a flowing gas has proved to be a very good one.
GRIDLOCK COMPONENT CROSSWORD DRIVERS
Cars do not behave exactly like gas molecules, to be sure: for example, drivers try to avoid collisions by slowing down when they get too near another car, whereas gas molecules have no such aversion. The Germans had noticed that if one simulated the movement of cars and trucks on a highway using the well-established equations that describe how the molecules of a gas move, some distinctly eerie results emerged.
SUCH was the happy, practical, and deterministic state of affairs up until a few years ago, when several German theoretical physicists began publishing papers on traffic flow in Physical Review Letters, Journal of Physics, Nature, and other publications not normally read by civil engineers. Find a way to eliminate the bottlenecks and flow will be restored. The mathematical and computer models indicate that when traffic jams occur, they are the result of bottlenecks (merging lanes, bad curves, accidents), which constrict flow. These included "add road space" and "lower the number of vehicles." This would not, as the saying goes, appear to be rocket science.Įven when American traffic engineers have ventured closer to rocket science, with computer simulations of traffic flow on multi-lane highways, the results have tended to reinforce the American reputation for practicality and level-headedness.
A recent study funded by nine state departments of transportation to examine the doubling in congestion on urban highways and primary roads that has occurred over the past two decades listed in its final report various ways that traffic engineers have tried to alleviate the problem. As an American academic discipline, traffic engineering is centered in civil-engineering departments, and civil engineers tend to believe in solving problems by going at them head on. Bigger and better highways are as American as fast-food restaurants and sport utility vehicles, and when it comes to making the crooked straight and the rough places plain, the practicality of American traffic engineers is hard to argue with. The latest field to host this conflict of national styles is one that seems at first glance to offer little prospect of a sporting contest.
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