Induced demand

Induced demand – related to latent demand and generated demand – is the phenomenon that after supply increases, more of a good is consumed. This is entirely consistent with the economic theory of supply and demand; however, this idea has become important in the debate over the expansion of transportation systems, and is often used as an argument against increasing roadway traffic capacity as a cure for congestion. This phenomenon, called induced traffic, is 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 everyone thoughtful seems to acknowledge, yet almost no one is willing to act upon.'Induced demand is often used as a catch-all term for a variety of interconnected effects that cause new roads to quickly fill up to capacity. In rapidly growing areas where roads were not designed for the current population, there may be a great deal of 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 clogging them up again.During the last two or three years before , a few planners had...begun to understand that, without a balanced system , 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 this force the building of more highways – which would generate more traffic and become congested in their turn in an ever-widening spiral that contained the most awesome implications for the future of New York and of all urban areas....every time a new parkway was built, it quickly became jammed with traffic, but the load on the old parkways was not significantly relieved.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....on average, a 10 percent increase in lane miles induces an immediate 4 percent increase in vehicle miles traveled, which climbs to 10 percent – the entire new capacity – in a few years....the findings reinforce the overall conclusion of the original study—namely, that well-designed and well-implemented schemes to reallocate roadspace away from general traffic can help to improve conditions for pedestrians, cyclists or public transport users, without significantly increasing congestion or other related problems. Induced demand – related to latent demand and generated demand – is the phenomenon that after supply increases, more of a good is consumed. This is entirely consistent with the economic theory of supply and demand; however, this idea has become important in the debate over the expansion of transportation systems, and is often used as an argument against increasing roadway traffic capacity as a cure for congestion. This phenomenon, called induced traffic, is 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 everyone thoughtful seems to acknowledge, yet almost no one is willing to act upon.' The inverse effect, or reduced demand, is also observed (see below). According to CityLab: The technical distinction between the two terms, which are often used interchageably, is that latent demand is travel that cannot be realized because of constraints. It is thus 'pent-up'. Induced demand is demand that has been realized, or 'generated', by improvements made to transportation infrastructure. Thus, induced demand generates the traffic that had been 'pent-up' as latent demand. 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 materializes as actual usage. The effect was recognized as early as 1930, when an executive of a St. Louis, Missouri electric railway company told a 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' biographer, Robert Caro, in The Power Broker: The same effect had been seen earlier with the new parkways that Moses had built on Long Island in the 1930s and 40s, where 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 newly create 200 miles 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?: 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. 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: