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D I R E C T I O N S F O R C O O R D I N AT E D I M P R O V E M E N T O F T R AV E L S U RV E Y S A N D M O D E L S 87 The variables listed above have already been exam- corrections and adjustments until the final decision was ined in various research and modeling frameworks and made and the corresponding activity was implemented. contexts. These are measures that can be quantified and The three approaches described here are not actually added to a survey instrument. What is needed is to move alternatives: they are sequentially inclusive. All factors, these research achievements into practice for travel sur- variables, and observed statistics pertinent to the con- veys and models. In particular, widening the range of ventional outcome-based approach are still relevant for explanatory variables should eventually allow for the the cause-based approach, and causality is still a part of removal of flat mode-choice constants and distribution the decision-making screening. However, in addition to K-factors that dominate the current models and what happens as a result of the combination of explana- "explain" most of the observed variability. tory variables, the cause-based approach offers insights An important but underresearched area is the exami- into the why sequence of decisions and events that led to nation of long-term trends in travel behavior. Travel the modeled what. The decision processbased approach behavior obviously undergoes a significant evolution takes an additional step in mapping the whole how that is not captured by static travel demand models. chronology of the decision making that built up around There have been only several attempts to capture long- the modeled event. The modeling complexity and term trends in VOT estimates with the corresponding amount of information needed for these approaches consequence for the choice model coefficients. grows exponentially from what to why and then to how. Chronological peculiarities of individual decision making are less important for large-scale models and fre- CAUSAL LINKAGES quently lead to complicated multistage procedures with numerous feedbacks that are difficult to convert into In the authors' view, focusing on causality represents a operational models. Understanding of casual linkages is constructive intermediate stage between a fairly stan- a simpler task although it is a limited view of travel dard outcome-based approach and the new process- behavior. It may significantly improve the structure of based approach. The difference between outcome-based, the travel model system and sequencing of the modeled cause-based, and process-based approaches can be illus- choices and associated decision-making steps. trated by the following example of location choice for The cause-based approach to surveys pragmatically shopping. serves the existing static structure of choice models and The conventional outcome-based approach would try helps in improving it. It is not a substitute for a full- to explain the chosen location by means of the location fledged process-based approach; it is a simplification characteristics (size, distance from home, accessibility by that is practically helpful in the short term. It may also be different modes) and personhousehold characteristics helpful in the longer term as well, however, because the (person type, gender, age, car ownership, presence of knowledge and understanding acquired in causal analy- children, etc.) in a single-choice framework in which all sis may be of great value for the subsequent process- location, person, and household attributes would be based analysis. blended in the utility function and all other locations Introducing causality and proper sequencing in a stat- (zones) would be considered as available alternatives. ic framework requires adding to the household surveys The cause-based approach would be focused on for- specific questions that would refer to the order and con- mation of the available choice set under the given condi- ditionality of decisions as well as to the formation of the tions of the person that are considered as earlier in the choice set. In particular, for each visited activity location causal chain and prove that these conditions indeed were and the corresponding choice of destination, mode, and fixed in the decision making at the time of making the time of day (TOD), the following set of questions can be modeled decision (available time window, car availabil- added to either RP or SP surveys: ity, usual spatial "domain" of the person) and then for- mulation of a choice model that would take maximum Was this activity-preliminary scheduled or under- advantage of the causalconditional variables and the taken as a result of occasionally saved time in the course conventional variables. The cause-based approach is ori- of the day? ented to proper sequencing and conditioning of decision- Were the destination, mode, and TOD choices making steps in an overall static environment. made simultaneously or was there a certain order to con- The decision processbased approach would be ditional choices? Which of these choices are usual and focused on both causal and chronological aspects of the stable over time and which are subject to change? decision making associated with the modeled event. Ide- If the actually chosen alternative was not available, ally, this would include a historical sequence of prelimi- what would be the second-best choice? nary decisions about the time and location for the Is there any predetermined area from which the modeled shopping activity, probably including numerous locations choice was made (like shopping on the same