Definitions According to
CityLab: 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.)
History Latent demand has been recognised by road traffic professionals for many decades, and was initially referred to as "
traffic generation". In the simplest terms, latent demand is demand that exists, but, for any number of reasons, most having to do with human psychology, is suppressed by the inability of the system to handle it. Once additional capacity is added to the network, the demand that had been latent materialises as actual usage. 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. 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 entrance of the United States into World War II], a few planners had ... begun to understand that, without a balanced system [of transportation], 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
Verrazzano–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." 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. An 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. and the
opportunity cost of the time spent travelling, which is usually calculated as the product of travel time and the
value of travellers' time. These cost determinants change often, and all have variable effects on demand for transport, which tends to be dependent on the reason(s) as well as method of travel. For roads or highways, the supply relates to capacity and the quantity consumed refers to
vehicle miles traveled. The size of the increase in quantity consumed depends on the
elasticity of demand. 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 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.
Sources of induced traffic In the short term, increased travel on new road space can come from one of two sources: diverted travel and induced traffic. Diverted travel occurs when people divert their trip from another road (change in route) or reschedule their travel to avoid peak period congestion – but if road capacity is expanded, peak congestion is lower and they can travel at the time they prefer. 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. 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”.
Policy implications When evaluating induced demand traffic demand theoretically, consideration is mainly given to the actual amount of traffic that will arise from a certain scenario. In real world applications, policymakers must consider the benefits of new infrastructure with the potential negative impacts on the environment, public health, and social equity. 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.
Criticism Critics of induced demand arguments generally accept their premise, but argue against their interpretation. Steven Polzin, former director of the Center for Urban Transportation Research and former Senior Advisor at the US Department of Transportation, argues that most forms of induced demand are actually good things and that, due to changing transportation trends, past data cannot be applied to present circumstances. Specifically, he argues: • One type of induced demand is simply keeping up with population growth. This is a good thing. • Another is traffic moving out of neighborhoods and onto newly expanded freeways. This is a very good thing. • Another is people adjusting timing of trips to their desired timing, thus improving business efficiency and quality of life - both good things. • Another is shifting transportation from non-auto transport to auto transport. Polzin does not argue that this is good, but rather that it's irrelevant (at least in a US context) as non-auto transport is such a small fraction of the total, and thus cannot meaningfully induce demand anymore (unlike in the past). By contrast, going in reverse would require unprecedented growth rates in public transport systems even just to keep up with population growth. • Another is people taking trips to places that they wouldn't have gone before, such as shopping in new places or living further from work. Beyond arguments that this implies improved quality of life, while this appears to have been a major driver in induced demand in the past, it ignores trends. From 1980 to 2015 increases in road capacity in the US didn't even keep up with population growth, yet vehicle miles per capita doubled - a detachment between capacity growth and demand. But since the late 2000s, vehicle miles per capita have stagnated - and growing trends of
telecommuting and
e-commerce are likely to apply further downward pressure. I.e.: people don't drive further to shop or work if they're shopping or working from home either way. • As personal road travel declines, commercial and service travel increases. This travel is not sensitive to road capacity and is not readily shifted to alternate modes of transportation. Rather than limiting demand by reducing road capacity, Polzin argues for limiting demand via highway pricing, such as
managed lanes,
toll highways,
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 Urbanist director Scott Beyer, Professor of City and Regional planning Robert Cervero, studies such as from WSP and Rand Europe, and numerous others. ==Film-induced demand==