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[ Negative feedback | Positive feedback | Counter-intuitive ]

Stable systems dominated by negative feedback

The Watt steam governor (click image for larger version)

The purpose of this device is to regulate the speed of a steam-powered engine. The governor is a pair of metal balls, each affixed to the top of a vertical rod by a hinged arm. As the steam engine turns this rod, centrifugal force causes the balls to rise towards a horizontal position. The hinged arms are connected to the steam valve that feeds the engine, and as they approach the horizontal position the steam is shut off. This causes the engine to slow, and as it does so gravity plays its part in counteracting the centrifugal force on the arms, with the result that they drop. With the arms dropping, the engine is free to increase its speed again. This mechanism thus causes the speed of the engine to oscillate between two points.

(Source: The Selfish Gene, Richard Dawkins, Oxford University Press, 1989, pp50-51.)

Restrictions on traffic growth (click image for larger version)

The increase in the amount of traffic on our roads, along with the increase in the number of roads themselves, is a classic positive feedback loop. As the amount of congestion worsens, more roads are built to accommodate the growing number of cars. These new roads swiftly fill up with cars however, encouraging the construction of more. While this has been continuing for decades, it is only now that we're beginning to see balancing loop which may have a dampening effect on traffic and road growth.

It was inevitable that eventually something would cause growth to slow before the ultimate limit was reached: no land remaining for road construction. The severity of congestion could have caused people to rethink the benefits, but this wasn't deterrent enough, and what seemed like an easy solution (more roads) was more attractive. It seems that increasing air pollution may be the factor which brings about balancing feedback.

The decreasing quality of air pollution is causing governments, local and national, to begin measures curbing certain kinds of growth. The Environmental Protection Agency has put restrictions on federal money for highway construction in areas near Atlanta due to the pollution. This has spurred businesses in the area to look at what they can do to improve matters, such as consolidating distributed offices into centralised facilities. In addition, the Georgia Regional Transportation Authority (Greta) has the power to construct (or not) roads and public transport, and only intends to approve new development in densely populated areas (cutting down on travel distances) or near mass transit. Whether Greta achieves this or not, it is one of many indicators that governments are slowly taking action to restrict traffic growth before businesses and consumers would have done so. There are of course reactions against this control, such as the desire of landowners to sell their land to developers, but this movement may at least slow the rate of traffic growth.

(Source: 'Suburban Comforts Thwart Atlanta's Plans to Limit Sprawl', by David Firestone, New York Times, 1999-11-21.)

[ Negative feedback | Positive feedback | Counter-intuitive ]