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16 CHAPTER 5 THE GROWTH REDISTRIBUTION MODULE The Growth Redistribution Module predicts the very long- Ainew = predicted AM peak home-based work accessibil- term impacts of localized travel time changes (caused by ity of zone i after traffic-flow improvement, traffic-flow improvements) on the geographic distribution Aiold = AM peak home-based work accessibility of zone USER'S GUIDE of growth in a metropolitan area. There are already several i before traffic-flow improvement, sophisticated land-use models available (such as UrbanSim) CP = calibration parameter for model determined from that could be used for the purpose of this module. However, linear regression (CP is the slope of the least- these models require a great deal of specialized economic data squared error line constrained to go through zero), and effort (which are beyond the resources of many MPOs) to G = ratio of the total predicted number of households set up for a region. When a sophisticated land-use model exists in the region after the traffic-flow improvement in a region, it can be used to predict the long-term growth divided by the number of households in the region effects. When such a model is not available, the simplified before the improvement, and model described here is proposed for use to approximate the R = ratio of the total predicted accessibility in the long-term land-use effects of traffic-flow improvements. region after the traffic-flow improvement divided by the total accessibility in the region before the improvement. 5.1 MODULE DESCRIPTION The module presumes that total regional growth will be The Growth Redistribution Module requires that a baseline unaffected by traffic-flow improvements (in other words, the 20- to 25-year forecast of land-use growth (i.e., households model will not be sensitive to the potential effects of differ- and employment changes) be available for the metropolitan ing levels of regional traffic-flow improvements on the com- area. This baseline forecast should have been prepared either petitiveness of regions for attracting new households or jobs). manually or with a model, taking into account accessibility The module predicts only how regional growth might be real- changes as well as all of the other factors that commonly located from marginally less accessible zones to more acces- affect the distribution of growth within a region. sible zones within the region. The marginal change in zonal The Growth Redistribution Module consists of a simple accessibility is obtained by subtracting the average change in linear regression model that is fitted to the baseline forecast. regional accessibility from the zone-specific change in acces- The regression model predicts the change in the growth rate sibility (this is accomplished in Equation 12 by subtracting in households and employment in each zone of the region as the ratio R from the ratio of new to old accessibility for each a function of the relative change of accessibility for each zone i). For similar reasons, the amount of household growth zone. Although not sophisticated enough to predict actual that would have normally occurred in a zone (if the zone had growth, the module should be sufficient to predict how small grown at the regional average growth rate) is added to the changes in travel time accessibility can affect the predicted model-predicted growth rate that is due exclusively to mar- baseline growth rate in specific zones of the region. The mod- ginal changes in the zonal accessibility (this is accomplished ule is as follows: in Equation 12 by adding the ratio G). The effect of the above normalization is that if the ratio of the new accessibility to the old accessibility for a zone is less LUinew = LUiold G + CP iold - R A new Equation 12 than the average ratio for the entire region, then the zone's A i growth will be less than the regional average. If the zonal accessibility ratio is greater than the average regional acces- Where: sibility ratio, then the zone's growth will be greater than the LUinew = predicted sum of the number of households and regional average. jobs in zone i after traffic-flow improvement, The value of G will normally be 1.00, unless there is a LUiold = sum of households and jobs in zone i before traffic- significant period of time between the "before" and "after" flow improvement, traffic-flow improvement dates. The ratio G allows the ana-