Product Code Database
In economics, induced demand – related to latent demand and generated demandSchneider, Benjamin (September 6, 2018) "CityLab University: Induced Demand" CityLab – is the phenomenon whereby an increase in supply results in a decline in price and an increase in consumption. In other words, as a good or service becomes more readily available and mass produced, its price goes down and consumers are more likely to buy it, meaning that the quantity demanded subsequently increases. This is consistent with the economic model of supply and demand.
In transportation planning, induced demand, also called " induced traffic" or consumption of road capacity, has become important in the debate over the expansion of systems, and is often used as an argument against increasing roadway traffic capacity as a cure for congestion. Induced traffic may be a contributing factor to urban sprawl. City planner Jeff Speck has called induced demand "the great intellectual black hole in city planning, the one professional certainty that every thoughtful person seems to acknowledge, yet almost no one is willing to act upon."
The inverse effect, known as reduced demand, is also observed.
Induced demand is a catch-all term used for a variety of interconnected effects that cause new roads to quickly fill to capacity. In rapidly growing areas where roads were not designed for the current population, there may be significant latent demand for new road capacity, which causes a flood of new drivers to immediately take to the freeway once the new lanes are open, quickly congesting them again. But these individuals were presumably already living nearby; how did they get around before the expansion? They may have taken alternative modes of transport, travelled at off-peak hours, or not made those trips at all. That’s why latent demand can be difficult to disentangle from generated demand—the new traffic that is a direct result of the new capacity. (Some researchers try to isolate generated demand as the sole effect of induced demand.)
The technical distinction between the two terms, which are often used interchangeably, is that latent demand is travel that cannot be realised because of constraints. It is thus "pent-up". Induced demand is demand that has been realised, or "generated", by improvements made to transportation infrastructure. Thus, induced demand generates the traffic that had been "pent-up" as latent demand.Mokhtarian, Patricia L. (ndg) "Understanding the Concept of Latent Demand in Traffic" State of California Department of TransportationRodrigue, Jean-Paul (2016) "Transportation as a Derived Demand" The Geography of Transport Systems
The effect was recognised as early as 1930, when an executive of a St. Louis, Missouri, electric railway company told the Transportation Survey Commission that widening streets simply produces more traffic, and heavier congestion. Report of the Transportation Survey Commission of the City of St. Louis (1930), p.109, cited in Fogelson, Robert M. (2001) Downtown: Its Rise and Fall, 1880–1950 New Haven, Connecticut: Yale University Press. p.66. In New York, it was clearly seen in the highway-building program of Robert Moses, the "master builder" of the New York City area. As described by Moses's biographer, Robert Caro, in The Power Broker:
During the last two or three years before the, a few planners had ... begun to understand that, without a balanced system of, roads would not only not alleviate transportation congestion but would aggravate it. Watching Moses open the Triborough Bridge to ease congestion on the Queensborough Bridge, open the Bronx-Whitestone Bridge to ease congestion on the Triborough Bridge and then watching traffic counts on all three bridges mount until all three were as congested as one had been before, planners could hardly avoid the conclusion that "traffic generation" was no longer a theory but a proven fact: the more highways were built to alleviate congestion, the more automobiles would pour into them and congest them and thus force the building of more highways – which would generate more traffic and become congested in their turn in an ever-widening spiral that contained far-reaching implications for the future of New York and of all urban areas.The same effect had been seen earlier with the new parkways that Moses had built on Long Island in the 1930s and 40s, where
... every time a new parkway was built, it quickly became jammed with traffic, but the load on the old parkways was not significantly relieved.Similarly, the building of the Brooklyn–Battery Tunnel failed to ease congestion on the Queens-Midtown Tunnel and the three East River bridges, as Moses had expected it to. By 1942, Moses could no longer ignore the reality that his roads were not alleviating congestion in the way he expected them to, but his answer to the problem was not to invest in mass transit, it was to build even more roads, in a vast program which would expand or create of roads, including additional bridges, such as the Throgs Neck Bridge and the Verrazano Narrows Bridge.
J. J. Leeming, a British road-traffic engineer and county surveyor between 1924 and 1964, described the phenomenon in his 1969 book, Road Accidents: Prevent or Punish?:
Motorways and bypasses generate traffic, that is, produce extra traffic, partly by inducing people to travel who would not otherwise have done so by making the new route more convenient than the old, partly by people who go out of their direct route to enjoy the greater convenience of the new road, and partly by people who use the towns bypassed because they are more convenient for shopping and visits when through traffic has been removed.Leeming went on to give an example of the observed effect following the opening of the Doncaster Bypass section of the A1(M) in 1961. By 1998, Donald Chen quoted the British Transport Minister as saying "The fact of the matter is that we cannot tackle our traffic problem by building more roads."Chen, Donald D. T. (March 1998) "If You Build It, They Will Come ... Why We Can't Build Ourselves Out of Congestion" Surface Transportation Policy Project Progress; quoted in
In Southern California, a study by the Southern California Association of Governments in 1989 concluded that steps taken to alleviate traffic congestion, such as adding lanes or turning freeways into double-decked roads, would have nothing but a cosmetic effect on the problem. Also, the University of California at Berkeley published a study of traffic in 30 California counties between 1973 and 1990 which showed that every 10 percent increase in roadway capacity, traffic increased by 9 percent within four years time. A 2004 meta-analysis, which took in dozens of previously published studies, confirmed this. It found that:
... on average, a 10 percent increase in lane miles induces an immediate 4 percent increase in vehicle miles travelled, which climbs to 10 percent – the entire new capacity – in a few years.Salzman, Randy (December 19, 2010) "Build More Highways, Get More Traffic" The Daily Progress, quoted inAn aphorism among some traffic engineers is "Trying to cure traffic congestion by adding more capacity is like trying to cure obesity by loosening your belt."
According to city planner Jeff Speck, the "seminal" text on induced demand is the 1993 book The Elephant in the Bedroom: Automobile Dependence and Denial, written by Stanley I. Hart and Alvin L. Spivak.
When road capacity is increased, initially there is more road space per vehicle travelling than there was before, so congestion is reduced, and therefore the time spent travelling is reduced – reducing the generalised cost of every journey (by affecting the second "cost" mentioned in the previous paragraph). In fact, this is one of the key justifications for construction of new road capacity (the reduction in journey times).
A change in the cost (or price) of travel results in a change in the quantity consumed. Factors such as petrol prices, as well as fuel costs, are the most common variables that influence the quantity demanded for transport. This can be explained using the simple supply and demand theory, illustrated in this figure.
The elasticity of demand for transport differs significantly depending on the reason people are choosing to travel initially. The clearest example of inelastic demand in this area is commuting, as studies indicate that most people are going to commute to work, regardless of fluctuations in variables such as petrol prices, as it is a required activity to generate income. This exemplifies the fact that activities that yield a high economic benefit, in this case, financial gain in the form of income, tend to be inelastic. In contrast, travel for recreational or social reasons has a low tolerance for price increases, and as such the demand for recreational travel when prices spike sees a sharp decline.
A review of transport research suggests that the elasticity of traffic demand with respect to travel time is around −0.5 in the short term and −1.0 in the long term. This indicates that a 1.0% saving in travel time will generate an additional 0.5% increase in traffic within the first year. In the longer term, a 1.0% saving in travel time will result in a 1.0% increase in traffic volume.
Induced traffic occurs when new automobile trips are generated. This can occur when people choose to travel by car instead of public transport, or decide to travel when they otherwise would not have.
Shortening travel times can also encourage longer trips as reduced travel costs encourage people to choose farther destinations. Although this may not increase the number of trips, it increases vehicle miles travelled. In the long term, this effect alters land use patterns as people choose homes and workplace locations farther away than they would have without the expanded road capacity. These development patterns encourage automobile dependency which contributes to the high long-term demand elasticities of road expansion.
In the UK, the idea of induced traffic was used as grounds for protests against government policy of road construction in the 1970s, 1980s and early 1990s, until it became accepted as a given by the government as a result of their own Standing Advisory Committee on Trunk Road Assessment (SACTRA) study of 1994. However, despite the concept of induced traffic now being accepted, it is not always taken into consideration in planning.
On the other hand, a comparison of congestion data from 1982 to 2011 by the Texas A&M Transportation Institute suggested that additional roadways reduced the rate of congestion increase. When increases in road capacity were matched to the increase demand, growth in congestion was found to be lower.
A study by Robert Cervero, a professor of City and Regional Planning at the University of California, Berkeley, found that "over a six-to eight-year period following freeway expansion, around twenty percent of added capacity is 'preserved,' and around eighty percent gets absorbed or depleted. Half of this absorption is due to external factors, like growing population and income. The other half is due to induced-demand effects, mostly higher speeds but also increased building activities. These represent California experiences from 1980 to 1994. Whether they hold true elsewhere is of course unknown."
And Mokhtarian et al. (2002) paired eighteen California state highway segments whose capacities had been improved in the early 1970s with control segments that matched the improved segments with regard to facility type, region, approximate size, and initial volumes & congestion levels. Taking annual data for average daily traffic (ADT) and design-hour-traffic-to-capacity (DTC) ratios during the 21 years 1976–1996, they found the growth rates between the two types of segments to be “statistically and practically indistinguishable, suggesting that the capacity expansions, in and of themselves, had a negligible effect on traffic growth”.
Carbon emissions have become a primary concern for policymakers in recent times and continues to be a consideration for infrastructure planning. An example of this is the Expansion of Heathrow Airport, where hopes of additional runways would spur economic growth within the UK: increasing both the amount and frequency of direct flights. These expansion proposals posed climate concerns and prompted studies into its environmental viability. It was estimated by the government that such expansion plans would create 210.8 million tons of CO2 annually. In addition, approximately 700 homes, a church, and eight listed buildings would have to be destroyed to make way for the project. In 2020, the court of appeal ruled the expansion plans illegal due to the ministers’ lack of consideration towards the government’s commitments to climate change.
In contrast to negative externalities, Bogotá, Colombia, has been recognized as a success story in managing induced demand for transportation by investing in new bike infrastructure. The city’s first bike path was established in 1974, with heavy investment in the late 1990s which eventuated in over 300 kilometers of bike lanes and dedicated bike paths. This infrastructure has been credited with reducing traffic congestion through encouraging more people to bike as transport. Less traffic then directly leads to lesser emissions, improved air quality and healthier lifestyles for residents. In addition, the city has implemented additional policies such as a bike-sharing program, bike-friendly streets and education campaigns to promote biking as a healthy and sustainable mode of transportation.
Rather than limiting demand by reducing road capacity, Polzin argues for limiting demand via highway pricing, such as , toll roads, congestion pricing or cordon pricing, as this provides a revenue stream which can (among other things) subsidize public transportation.
Similar arguments have also been made by libertarian transportation policy analyst Randal O'Toole, economist William L. Anderson, transportation journalist and market urbanism director Scott Beyer, Professor of City and Regional planning Robert Cervero, studies such as from WSP and Rand Europe, and numerous others.
The 1998 study referred to about 150 sources of evidence, of which the most important were about 60 case studies in the UK, Germany, Austria, Switzerland, Italy, The Netherlands, Sweden, Norway, the US, Canada, Tasmania and Japan. They included major town centre traffic schemes to make closed to traffic, bus priority measures (especially bus lanes), bridge and road closures for maintenance, and closures due to natural disasters, mostly earthquakes. The 2002 study added some extra case studies, including some involving cycle lanes. The Annex by Kitamura and his colleagues reported a detailed study of the effects of the Hanshin-Awaji earthquake in Japan.
Taking the results as a whole, there was an average reduction of 41% of the traffic flows on the roads whose capacity had been reduced, of which rather less than half could be detected as reappearing on alternative routes. Thus, on average, about 25% of the traffic disappeared. Analysis of surveys and traffic counts indicated that the disappearance was accounted for by between 15 and 20 different behavioural responses, including changing to other modes of transport, changing to other destinations, a reduction in the frequency of trips, and car-sharing. There was a large variation around these average results, with the biggest effects seen in large-scale pedestrianisation in German town centres, and the smallest seen in small-scale temporary closures with good alternative routes, and small reductions in capacity in uncongested streets. In a few cases, there was actually an increase in the volume of traffic, notably in towns which had closed some town centre roads at the same time as opening a new by-pass.
Cairns et al. concluded that:
The European Union have produced a manual titled "Reclaiming city streets for people" "Reclaiming city streets for people: Chaos or quality of life?" European Commission that presents case studies and methodologies for traffic evaporation in urban areas.
Two widely known examples of reduced demand occurred in San Francisco, California, and in Manhattan, New York City, where, respectively, the Embarcadero Freeway and the lower portion of the elevated West Side Highway were torn down after sections of them collapsed. Concerns were expressed that the traffic which had used these highways would overwhelm local streets, but, in fact, the traffic, instead of being displaced, for the most part disappeared entirely. A New York State Department of Transportation study showed that 93% of the traffic which had used the West Side Highway was not displaced, but simply vanished.
After these examples, other highways, including portions of Harbor Drive in Portland, Oregon, the Park East Freeway in Milwaukee, Wisconsin, the Central Freeway in San Francisco, and the Cheonggyecheon in Seoul, South Korea, were torn down, with the same effect observed.
The argument is also made to convert roads previously open to vehicular traffic into , with a positive impact on the environment and congestion, as in the example of the central area of Florence, Italy. In New York City, after Mayor Michael Bloomberg's plan for congestion pricing in Manhattan was rejected by the New York State Assembly, portions of Broadway at Times Square, Herald Square and Madison Square were converted into pedestrian plazas, and traffic lanes in other areas taken out of service in favor of protected bike lanes, reducing the convenience of using Broadway as a through-route. As a result, traffic on Broadway was reduced, and the speed of traffic in the area lessened. Another measure instituted was the replacement of through-lanes on some of Manhattan's north–south avenues with dedicated left-turn lanes and protected bike lanes, reducing the avenues' carrying capacity. The Bloomberg administration was able to put these changes into effect as they did not require approval from the state legislature.
Despite the success of the Broadway pedestrian plazas in Manhattan, some pedestrian malls in the US, in which all traffic is removed from shopping streets, have not been successful. Areas with sufficient population density or pedestrian traffic are more likely to successfully pursue this path. Of the approximately 200 pedestrian malls created in the US from the 1970s on, only about 30 remained as of 2012, and many of these became poorer areas of their cities, as lack of accessibility caused commercial property values to decline. The exceptions, including the Third Street Promenade in Santa Monica, California, and 16th Street in Denver, Colorado, are indicators that conversion of shopping streets to pedestrian malls can be successful. Some of the failed pedestrian malls have improved by allowing limited automobile traffic to return. are common across cities and towns in Europe.
|
|
