More Roads, More Traffic?

Stuck in Traffic, Coping with Peak-Hour Traffic Congestion by traffic expert Anthony Downs, published in 1992, contains ideas that underlie current criticisms of road-building projects - that they will only create more of what they are trying to reduce or eliminate. Mr. Downs spoke at Simon Fraser University in early March. Media reports of his talk indicate that his current thinking remains consistent with his original analysis:

Traffic congestion worsens with population growth and ever-increasing levels of vehicle ownership and usage;
Peak-hour traffic congestion “is a socially suboptimal condition that annually wastes billions of dollars worth of time and fuel and…adds to pollution.”
Peak-hour congestion is rooted in behaviour patterns that are fundamental cultural norms in North America: a standardized work day and freedom to choose where to live and work;
The strategies for reducing peak-hour congestion are two-fold: increasing the transportation system’s carrying capacity, supply-side strategies, and reducing system usage during peak hours, the demand-side strategies;
Supply-side tactics include building more roads and increasing public transit capacity;
Demand-side tactics include peak-hour road pricing, higher gasoline taxes, and clustering high-density housing around transit stops.
Such tactics involve market, regulatory-based approaches or both.

In determining which tactics will be most effective, Mr. Downs emphasizes, what he calls the “four principles of traffic analysis” which influences traffic flow, must be borne in mind.

The first “principle”, triple convergence describes the offsetting of any initial reduction of peak-hour travel times on, for example, an expanded expressway by drivers resuming use of that expressway who formerly used alternative routes (spatial convergence), traveled at other times (time convergence), or used public transit (modal convergence). The traffic volume of the expanded expressway keeps rising until “vehicles are once again moving at a crawl during the peak period.” In short, triple convergence ensures that even a great increase in road capacity will not long reduce peak-hour congestion.

The second “principle”, dual swamping by growth, describes the phenomena whereby small reductions in traffic congestion (by whatever tactics) are fully offset within a few years by growth.

The third “principle”, imperviousness of growth to local public policies, refers to no one locality being able to substantially affect, “the overall population or job growth of its metropolitan area as a whole.” Mr. Downs explains that, concerted action to limit growth “is extremely difficult to arrange in America’s fragmented system of local government.”

Because of these three principles, reducing peak-hour congestion permanently is extremely difficult if not impossible by adopting one tactic alone, even if applied on a very large scale. This leads to the fourth “principle” which Mr. Downs calls “the principle of one-hundred small cuts,” according to which, a metropolitan area can reduce its peak-hour traffic congestion only by applying “many different remedies simultaneously.”

The combined effect of these principles is that regardless of the tactics used, in a fast growing metropolitan centre, controlling traffic congestion is more a matter of slowing down its rate of increase than eliminating it.

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