Wednesday, March 7, 2012

MODELLING TRANSPORT: A Synthesis of Transport Modelling Methodologies

by Aruna Sivakumar

The transport sector contributes to a significant proportion of the energy consumed in an urban area (Figure 1, for instance, depicts energy consumption in London, by sector, for the year 2001). In order to quantify the energy consumed by the transport sector, it is necessary to develop travel demand models that can predict the travel needs of the population by mode, time of day, duration and location. Such travel demand models must consider the travel needs not only of individuals but also of businesses and other organizations.

The need for travel demand models was recognized by urban and transport planners and researchers as far back as the mid-nineteenth century with the macroeconomic modelling of the spatial flows of people and commodities (see, for example, Carey, 1859). For nearly a century transport planners relied on various aggregate approaches of estimating spatial movements and flows, such as entropy- and gravity-based models. The mid-twentieth century saw the development of a sequential process of estimating travel demand, based on aggregate approaches, that was known as the four-step model. A relatively disaggregate version of the 4-step model is used to this day by several metropolitan planning organisations worldwide.
Parallel to the development of the four-step model of travel demand, urban planners also recognised the intricate interactions between the transport network and the rest of the urban system. Figure 2 presents a conceptual representation of the interactions between the various players in an urban system (Southworth, 1995). At the core of this figure is the transport system, which is influenced by the land-use configuration and the travel needs of people and businesses, and regulated by government plans and controls. Changes in the transport supply, in turn, influence the residential and work location choices of the population as well as business location decisions, thus influencing the land-use configuration. Further, there are demographic and firmographic processes independent of the transport system that influence the land-use configuration and thus indirectly influence the demand for transport. The final piece of this puzzle is the environment, in the form of emissions and energy-consumption resulting from transport and other activities undertaken by people and businesses. The environmental link has long been considered as being external to the land use-transport system, and it is only in recent times that the importance of internalising environmental impacts has been acknowledged.
In recognition of the complex dynamics of an urban system, urban planners in the 1950s and ’60s initiated the development of integrated land use-transport (LU-T) models. The first of these models to gain popular notice was Lowry’s model of Metropolis in 1964. Since then there have been several integrated LU-T models developed worldwide such as ITLUP (Putman, 1983), MEPLAN (Echenique, 1985), MUSSA (Martínez, 1992), and UrbanSim (Waddell, 2002). While the land-use components of these LU-T models have been rapidly evolving from simple aggregate representations to complex economic and econometric models of the market processes, the 4-step model continues to represent the transport modelling component.

Evolution of land use modeling, Waddell, P. (2005). Building an Integrated Model: Some Guidance, presented at TRB Workshop 162 on Integrated Land Use-Transport Models, Washington D.C.

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