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158 I N N O VAT I O N S I N T R AV E L D E M A N D M O D E L I N G , V O L U M E 2 There are no easy answers. If MPOs are motivated to Changes in Land Use, Socioeconomic, and transition to the new wave of model systems only if Demographic Characteristics activity-based models perform better than four-step travel models, then the more important question is: Travel demand models should be responsive to changes What constitutes a better model? If a better model is in land use, socioeconomic, and demographic character- defined in terms of meeting a higher standard of valida- istics (i.e., the inputs that play a key role in driving travel tion with the same number of or fewer adjustments to forecasts). Activity-based model systems should be sub- model components and parameters, then it is likely that jected to sensitivity tests in which population and the answer to both questions is yes. Clearly, this is open employment characteristics are altered, both across the to debate. region and in selected zones, land use subdivisions, or The debate also speaks to the merit of performing market areas. Characteristics that might be subjected to comparisons with four-step models. There is no doubt change include population and employment totals; that any model can be adjusted, refined, tweaked, and-- household distributions by zone, income, car ownership, if all else fails--hammered to replicate base-year condi- size, dwelling unit type, and number of children; employ- tions. Thus, simply comparing models is not enough. ment distributions by zone and occupation, industry, and This, the authors believe, is important because the state type; and person distributions by age, employment sta- of the practice appears to be focused on using replication tus, and gender. These variables should be subjected to a of base-year travel patterns as the sole or primary yard- range of changes. stick to assess models' performance. On the other hand, the primary objective of travel model development is forecasting future travel patterns when conditions may Changes in Multimodal Transport be quite different from base-year conditions or assessing Network Characteristics travel pattern shifts after the implementation of a major change in transportation services or policies, not repli- Travel demand models should be responsive to changes cating base-year patterns. Thus, the emphasis needs to in transport network characteristics, which directly be on capturing travel behavior patterns adequately from impact modal level of service attributes such as distance, base-year data, so that these behavioral patterns are time, and cost. There are a variety of ways in which these transferable. changes can be introduced. First, attributes associated The above discussion raises the issue of assessing the with existing modal facilities may be changed. Attributes performance, usefulness, and robustness of alternative such as highway network speeds and transit route fre- travel demand modeling, without focusing on replicating quencies may be altered. Second, new facilities may be base-year travel patterns. This issue is discussed next. introduced. New highway links, new transit routes, new transit stops, new bicycle and pedestrian facilities, and so on may be introduced into the system. A consideration ASSESSMENT OF ACTIVITY-BASED in determining the efficacy of a model is to examine the TRAVEL DEMAND MODELS model's ability to quantify induced or suppressed travel demand that may occur because of the modal change. The question of what constitutes a better model is open to debate. There is a belief that the superiority of a model is best judged in terms of the validation to base-year traf- Implementation of Transportation Policies fic conditions. However, given that any model can be adjusted to replicate a given set of base-year traffic con- Travel demand models should be responsive to a range of ditions, such measures are not always useful. contemporary and emerging transportation policies and The quality of a travel demand model system is better issues. These include, but are not necessarily limited to, judged on its ability to respond to a range of scenarios and policies of interest. It is in this context that a true assess- · Pricing policies such as value pricing, variable (time ment can be performed and comparisons between existing of day) pricing, area-based congestion pricing, parking four-step travel models and newer activity-based model pricing, tolls, public transit fare policies (free fare zones, systems become meaningful. Thus, assuming that there free intermodal transfers, and so forth), cash subsidies, are two models--an existing four-step travel model and a fuel prices and taxes, and employer reimbursement newer activity-based travel model--that have been vali- schemes; dated to a set of base-year traffic measures, here is how the · Policies aimed at encouraging alternate mode use performance, usefulness, applicability, and robustness of including HOVHOT lanes, rideshare programs, mixed the model systems can be assessed and compared. land use development, transit- and pedestrian-oriented