- Case Studies
- Impact Methodologies
- Site Map
- Search
|
|
| Planning |
|
Case Study:Sacramento, CaliforniaMethodologyLand Use-Transportation ModelMEPLAN The MEPLAN framework draws on over 25 years of spatial economic modeling experience and has been used around the world in cities such as Naples, Helsinki, and Bilbao, but the Sacramento model is the first application in the U.S. Moreover, this is the first study in which an integrated land use and transportation model uses separate a.m., p.m., and off-peak assignment models (as opposed to an average daily assignment model) for more accurate emissions analysis. Abraham and Hunt (1999) provide a description of the MEPLAN model, from which the following description is drawn. The basis of the MEPLAN modeling framework is the interaction between two parallel markets, the land market and the transportation market. This interaction is illustrated in Figure 3. Behavior in these two markets is a response to price signals that arise from market mechanisms. In the land markets, price and generalized cost (disutility) affect production, consumption, and location decisions by activities. In the transportation markets, money and time costs of travel affect both mode and route selection decisions. The cornerstone of the land market model is a social accounting matrix (SAM). The SAM is an input-output table that is expanded to include relationships among four factors: industries, households, building floor space, and land. The SAM will tell, for example, that one unit of manufacturing industry requires X units of floor space, Y units of industrial land, and Z workers, as well as specified amounts of inputs from other economic sectors. (Input-output tables can be obtained from the Bureau of Economic Analysis' Regional Input-Output Modeling System or from IMPLAN.) The SAM is spatially disaggregated at the zonal level. Logit models of location choice are used to allocate volumes of activities in the different sectors of the SAM to geographic zones. The attractiveness or utility of zones is based on the costs of inputs to the producing activity (which include transportation and rents), location-specific effects, and the costs of transporting production to consumption activities. The resulting patterns of economic interactions among activities in different zones are used to generate origin-destination matrices of different types of trips. These matrices are loaded to a multimodal network representation that includes nested logit forms for the mode choice models and stochastic user equilibrium for the traffic assignment model (with capacity restraint). The resulting network times and costs affect transportation costs, which then affect the attractiveness of zones and the location of activities, and thus the feedback from transportation to land use is accomplished. Figure 3.
Source: Johnston, Rodier, Choy, and Abraham (2000). The framework is moved through time in steps from one time period to the next, making it "quasi-dynamic." In a given time period, the land market model is run first, followed by the transportation market model, and then an incremental model simulates changes in the next time period. The transportation costs arising in one period are fed into the land market model in the next time period, thereby introducing lags in the location response to transport conditions. See Hunt (1994), Hunt and Echenique (1993) or Abraham and Hunt (1999) for descriptions of the mathematical forms used in MEPLAN. Sacramento MEPLAN Model As described in Johnston, Rodier, Choy, and Abraham (2000), the Sacramento MEPLAN model uses 11 industry and service categories that are based on the SAM and aggregated to match available employment and location data. Households are divided into three income categories (high, medium, and low). There are seven land use categories in the model. Constraints are placed on the amount of manufacturing land use to represent zoning regulations that restrict the location of heavy industry. Each of these land uses (except agricultural) locates on developed land represented by the factor "urban land." Two factors are used to keep track of the amount of vacant land available for different purposes in future time periods, and the development process converts these two factors to "urban land." A calibration parameter allows differential rents to be paid by different users of the same category of land. In most applications, including Sacramento, MEPLAN is a "sketch-planning" tool in the sense that the zone system and transportation network are quite coarse. In the Sacramento application, there are 32 analysis zones. The level of detail is primarily a result of the difficulty both in obtaining the required data and calibrating the model for a more detailed zone structure. An automated calibration procedure recently developed for the MEPLAN model (Abraham and Hunt, 2000) may help overcome this limitation to some extent. Another option would be to use MEPLAN and the regional travel demand model in conjunction, by taking land use results from MEPLAN and adjusting land use inputs into the travel demand model accordingly. This approach is being taken by the Baltimore Metropolitan Council, which is in the process of calibrating TRANUS - a land use model similar to MEPLAN - for the Baltimore region (see Box 1). Data Requirements MEPLAN can be developed for an area using locally available data sources. The following data were used in developing the Sacramento model:
According to the study authors, the most important data limitation was a lack of information on building stock, specifically, on the amount of floorspace by building type in each zone. As a result, floorspace was not included directly in the model. Instead "space" variables were developed that approximately represented the amount of developed land. A related problem was a lack of data to calibrate models of developer behavior. Data on floorspace through time would have facilitated the development of such a model. Without these data, theoretical constructs and rules-of-thumb were used to predict the location and extent of development. Data on freight flows in the Sacramento region and data on the relationship between economic links and trip rates for freight volumes, were also desired in order to model benefits to goods movement. Unfortunately, these data were lacking, so a "goods movement" flow was omitted from the Sacramento version of MEPLAN. For an example of data collection on commodity flows that would assist in freight model development, see the Portland case study. The study authors report that the Sacramento MEPLAN model was designed around the available data, and unanticipated trends and inconsistencies in the data led to changes in the model design. Some of these changes were improvements, others might be considered compromises. The project demonstrates that a useful model of urban land use and transport interaction can be developed within this framework in the United States using existing data sources. Calibration The parameters in the MEPLAN land use and transport interaction model of the Sacramento, California region were estimated using the sequential approach. (Calibration of the Sacramento model is discussed in Abraham and Hunt, 1998; the sequential approach and alternative calibration approaches are discussed in detail in Abraham and Hunt, 2000. The authors have developed an automated calibration approach that should expedite model application in the future.) Time-series data would have been preferable, but the available data were sufficient to obtain a working model. The following sets of measured variables were used to formulate the objective function for estimating the overall parameters:
[TOP] |
