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Urban sprawl and traffic congestion: Why does every new road bring more traffic?
With the rapid expansion of cities, traffic congestion has become a more serious problem, especially after the construction of new roads, the original plan to improve traffic often backfires. This phenomenon is called "induced demand" in economics, which means that an increase in supply often leads to an increase in demand, and even in some cases leads to increased traffic congestion. When capacity on a public transportation system or road increases, the number of riders seems to rise immediately, often confusingly.
Induced demand is seen as the “great intellectual black hole” of urban planning, something that almost every thoughtful person acknowledges exists but few are willing to take action to address.
The concept of induced demand
Induced demand is an important concept in economics, which includes latent demand and generated demand, and aims to explain the changes in consumer behavior when supply increases. In transportation planning, induced traffic usually refers to new traffic that is caused by building new roads or expanding highways. This new traffic flow is formed partly because people who used to commute change their travel patterns and flow to the newly opened roads, causing congestion. This "latent demand" may have been lurking on the fringes of the transportation system, waiting to be activated.
Historical Evidence
As early as the 1930s, experts pointed out that expanding roads would not only fail to alleviate traffic congestion, but might even aggravate it. After building many bridges and highways in New York City, famous urban planner Robert Moses found that the new traffic routes were quickly filled up, resulting in the original relief effect not being achieved. The book The Power Broker points out that adding roads only further creates a vicious cycle of traffic.
More highways only allow more cars to flow in, eventually creating new congestion and forcing people to build more roads.
Factors that influence demand
When public transport capacity is expanded, more roads will initially be available for travel, thus reducing travel time. The result is that the cost of travel is greatly reduced, prompting more people to choose to drive. This phenomenon is not limited to new traffic, but also includes traffic diverted from other sections of the road. For commuters in a hurry, arriving on time remains a major consideration, even with new construction on the road.
Price and Demand Elasticity
The elasticity of transportation demand lies in the impact of price changes. Generally speaking, commuting shows relatively rigid demand due to its necessity. The demand for leisure or social travel is relatively elastic, and when oil prices rise, this part of the demand may fall rapidly. This reflects the differences in demand sensitivity for different travel purposes.
Consideration of induced demand in transportation planning
While planners consider future traffic growth when planning new roads, these growth figures are usually based on increases in car ownership and economic activity rather than the induced traffic that would result from the new roads. This results in possible flaws in the calculation of traffic forecasts, which fail to accurately reflect changes in traffic flows following various new constructions.
New model for sustainable transportation development
In contrast, Bogotá is a successful example of managing induced demand by investing in bike lanes and green transport infrastructure. The city began building bicycle lanes in 1974 and later invested heavily to build more than 300 kilometers of dedicated bicycle lanes, successfully attracting citizens to choose bicycles for travel, greatly alleviating traffic congestion problems and reducing carbon emissions.
Faced with worsening traffic and environmental problems, what choices should we make in urban planning? How to find a balance between demand and sustainability so that urban transportation can operate in a healthier way?
