In most cities in the developed and developing world, key decision makers often tend to think that the obvious way to reduce congestion and improve traffic flow is to build more roads and add new lanes to their highways. Conventional engineering and traffic planning approaches have been too focused on improving the efficiency, flow and number of vehicles they move through the road network.
When stakeholders think of ways to reduce traffic congestion, they think in terms of creating more road connections, and expanding the size of existing ones. Making roads smaller and ripping out sections of a highway would have never even been considered back in the 20th century, when the automobile ruled the roads. But these ideas are starting to change.
In recent years, evidence has proven that the conventional approach doesn’t always work. It has been found that through a phenomenon called Induced Demand, whereby adding new road capacity only sustains the current level of driving and can even encourage a growing number of cars on the road, as new drivers take to the roads even if others switch to other modes of transport.
Research on US cities, in particular, by economists Matthew Turner and Gilles Duranton in 2009 found that as the road capacity was increased by a certain percentage between 1980 and 2000, the total vehicle miles travelled in that same period increased by the same amount. They note that while correlation doesn’t equal causation, in their study period the other factors to explain the increase in driving were not strong enough.
Why don’t bigger roads reduce traffic congestion?
The concept of Induced Demand suggests that as you supply more of something, in this case – roads, the demand to use it will rise. With road traffic it creates a vicious cycle that begins when congestion consistently develops on a road, which leads to drivers and residents in the vicinity demanding for the widening of the roads with increased lane capacity. Under these social and political pressures, roads are widened, and congestion is relieved for the time being.
Due to the reduction in congestion, the road now becomes more attractive for both existing and new drivers, as well as for economic growth and property development in the area the road serves, bringing more residents and businesses to the surrounding area. Consequently this leads to more people and goods travelling on the new road, which over time causes congestion to develop once again.
Where do all these new drivers come from?
These new drivers are either new drivers altogether, drivers who travelled at off-peak hours, or previous drivers who took an alternative mode of transport. The concept of Triple Convergence can explain Induced Demand in a 3-pronged approach.
- Spatial Convergence: Occurs when those who previously took a different route now decide to take the route with the new lane capacity, as they believe it will get them to work faster due to the relieved congestion.
- Time Convergence: Occurs when those who previously travelled just before or after the peak rush hour period to avoid congestion, decide to travel during rush hour now that they believe the road will be less congested during this time.
- Mode Convergence: Occurs when those who previously took public transit or another mode of transport other than driving to their destination, now decide to drive because of the new and increased road capacity.
Todd Litman of the Victoria Transport Policy Institute summarised this phenomenon by saying, “Traffic congestion tends to maintain equilibrium. Congestion reaches a point at which it constrains further growth in peak-trip periods. If road capacity increase, the number of peak-period trips also increases until congestion again limits further growth.” This essentially says, no matter how much we invest into our road infrastructure, as long as the roads are there and new roads constructed, congestion will eventually always develop, given no other changes in social behaviour or policy.
How can congestion effectively be alleviated?
A similar article by Adam Mann outlines a few policies that affect the demand side of driving rather than supply side, such as congestion charging, increasing the cost of parking, and improving the quality of public transportation services.
Congestion charging has been successfully implemented in cities such as London, Singapore and Stockholm, and works by raising the price of driving on certain roads or areas at peak times when demand is high, influencing some drivers to either drive at off-peak hours to avoid the charge, or instead use an alternative mode of transport.
By increasing the cost of parking in busy downtown areas, and improving the quality, affordability, timeliness and comfort of public transportation, pedestrian and cycling infrastructure, drivers could be discouraged from driving to busy downtown areas, and make use of alternative modes of public and active transport as their means of mobility.
What about inducing a reduction in demand by reducing supply?
While these are demand side interventions to reduce the amount of vehicles and congestion on the roads, the same effect of induced demand also works in reverse of increasing road capacity. In this sense, reducing road capacity, or even removing an entire highway, doesn’t always lead to more congestion, but rather to a redistribution of the traffic to other streets or onto alternative modes of transportation, given the appropriate policies and counter-measures.
One notable example is the Cheonggyecheon Restoration Project, in which an elevated highway built in the 1970s through Seoul over an inner city river was demolished and replaced by a 3.6-mile long urban park alongside the uncovered river. The effects were modelled, including adjustments to other streets and an improvement in public transit system, and, to much surprise, the results found that it would actually reduce travel times in downtown Seoul. The project was seen as a major success in terms of urban renewal and city beautification, with surrounding property values rising, a reduction in the “urban heat island” effect, and a reintroduction and increase in species of bird, fish, and insects in and around the river.
Other similar projects around the world have seen similar effects, with projects in San Francisco, Madrid, and Portland, seeing new city life and more green space brought back to their downtowns in the place of the highways. Currently, Seattle is undertaking a similar project with its Alaskan Way Viaduct, by tearing down the elevated highway, reducing the size of the road and sinking it as an underground tunnel, reclaiming public space at the ground level and creating connections between local neighbourhoods and Seattle’s waterfront. Boston’s Big Dig project has shown that this is entirely possible and can be very successful, with its relocation of several elevated downtown highways underground and replacing it with a 1.5-mile urban green park.
A similar but temporary phenomenon occurred in Hong Kong during the 2014 Occupy Central Movement, where demonstrators blocked off a section of Connaught Road Central causing the temporary closing of one of Hong Kong Island’s busiest road traffic corridors. Contrary to what most people thought, this did not cause citywide gridlock, traffic was re-routed and the majority of businesses operated as usual. Understandably, it may have taken some people longer to get to where they needed to go due to the detours, but ultimately, people were still able to reach their destination through alternative means, and the closing of the road resulted in substantial roadside air quality improvements in the Central and Admiralty districts, and city space that was taken from vehicles and given back to the people.
While developed countries are gradually coming to terms with the concept and consequences of Induced Demand, developing countries are following in their footsteps and making the same mistakes. This is especially the case in cities in China and India, where the massive growth of urban populations only exacerbates the problem. Rising incomes and an expanding middle class, combined with the status symbol of private cars in Asia has caused private car ownership to rapidly outpace infrastructure development in roads and public transportation.
In India, the state government is pursuing the construction and widening of a variety of highways that connect major cities. One significant project is the widening of the four-lane National Highway 4 and the six-lane Mumbai-Pune Expressway to six and eight lanes respectfully, including the construction of a new bypass between Khopoli and Sinhagad. The goal is to reduce the travel time between Mumbai and Pune, relieve congestion and reduce the number of accidents that occur.
China’s two largest cities, Shanghai and Beijing, have seen much new expressway construction and expansion since the early 2000s. With many second and third tier cities growing at an increasing rate, connections between these cities and the first tier global cities of Shanghai and Beijing are being constructed, as well as preparing inner city expressways for the increased traffic. Additionally, expressways are being constructed in rural areas to facilitate connections between the rural and urban areas.
Although a key part to economic growth, these new and wider expressways will eventually attract more and more cars and trucks, funnelling increasing amounts of cars into populated urban centres. Eventually, this will lead to major congestion in the long run, which cities such as Beijing and Shanghai already suffer from immensely.
In attempts to curb the growing traffic congestion, both Beijing and Shanghai have adopted a license plate lottery system, whereby only a set number of licenses – required to register a car on the road – are issued every year. This leads to the auctioning of license plates, where a license can be valued at more than a car itself. Shanghai has taken extra measures by prohibiting non-locally registered cars from using the downtown expressways during peak rush hours of the morning and evening. However, while these can be successful in restricting car ownership, in the long run it does not solve the existing problem of congestion unless investments into public transportation are made to encourage people to choose public transportation over a private car.
Why should cities think twice about building a new road or expanding existing ones?
Over time, as our interests and values shift from the automobile dominated streets to an increased focus on the pedestrian, public space and quality of urban life, more people will start to see the benefits of cutting back on road infrastructure spending and discouraging driving. The mind-set is shifting towards more vibrant, mixed-use and socially stimulating city streets and public areas, which will come with the reduction in dependence on private cars as a means of transport, towards more of a focus on providing a balanced proportion of modes of transport including public mass transit, active transit and cars.
With space taken away from cars and given back to the citizens, it leads to positive externalities such as increased green space, space for public congregation, and a higher quality urban environment that is more conducive to forms of public transit, and active transport, which may become its own virtuous cycle of improved sustainability in itself.
Do you agree with the concept of Induced Demand? What can be done to convince policy makers and car-dependent individuals of the benefits of reducing road capacity?
Written by Tim Rodgers with Arun Govada on behalf of UDP International. UDP International is a 20-year-old global strategic planning and urban design boutique head-quartered in Hong Kong, with offices in Hong Kong, Hyderabad and San Francisco. Please email firstname.lastname@example.org or email@example.com to continue the conversation.
1. Durnaton & Turner. Fundamental Law of Road Congestion. September 2009.
2. Litman, T. Generated Traffic: Implications for Transport Planning. Victoria Transport Policy Institute. January 2015.
3. Mann, A. What’s Up With That: Building bigger roads actually makes traffic worse. June 2014. WIRED.
4. Rao, K. Seoul tears down and urban highway and the city can breathe again. Grist. April 2011.
5. Walker, A. 6 Freeway Removals That Changed Their Cities Forever. Gizmodo. March 2014.
9. Photo by Sachin Jadhav