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VA L I D AT I O N A N D A S S E S S M E N T O F A C T I V I T Y- B A S E D S Y S T E M S 159 development, neo-traditional neighborhood develop- desirable to have a model system that is reasonably ment, and new transit, bicycle, and pedestrian facilities robust to changes in spatial and temporal resolution. It is and services; and possible that zone systems will be altered, zones will be Alternative work and school arrangements such as split, and zones will be added. In general, a travel satellite or home-based telecommuting, flexible work demand model should be aspatial and thus unaffected by hours, and distance-learning classes. the definition of the zonal system. If additional time-of day periods are desired, it should be easy to re-estimate Travel demand models should be able to provide and recalibrate the components of the model system quantifiable impact measures, by market segment, that affected by the re-definition of time periods. address issues such as market equity, social exclusion, environmental justice, quality of life, and environmental (emissions) impacts of policy measures. Accommodation of Emerging Behavioral Some of the policies identified here can be reflected by Paradigms and Concepts adjusting a modal level-of-service variable associated with one or more facilities. For example, a new toll on a There is literature documenting behavioral phenomena bridge can be reflected by imposing a cost on the specific inadequately captured by traditional travel demand highway links that represent the bridge. Other policies modeling paradigms. Despite concern about the lack of a may be subtler and may not be as easy to capture or in a sound behavioral theory driving or underlying innova- model. For example, how does one represent a flexible tive model development, there is a growing body of work work hour policy to reflect its impacts on travel behav- that is helping to identify behavioral paradigms and con- ior? Potentially, activity-based travel demand models cepts that ought to be incorporated into models of activ- that include consideration of work constraints, flexibil- ity and travel demand. While one may debate the need to ity and rigidity of different activities, and activity inter- accommodate these concepts, the profession must move dependencies would be able to accommodate the effects toward recognizing established behavioral relationships, of a flex work hour policy. if only to make the models more defensible and explica- ble. Some concepts include the following: Consideration of New Technologies Interdependencies and interactions: There are interdependencies and interactions that are key to activ- Technology is playing an increasingly bigger role in shap- itytravel demand modeling. These include modal, tem- ing human activity patterns, residential and work loca- poral, and spatial (location) interdependencies among tion choices, travel behavior, use of time, and freight trips in a chain and among chains in a daily activ- logistics. The interactions between technology and travel itytravel pattern, interdependencies in activity engage- behavior are closely intertwined with people's use of ment across days and weeks, interactions among time. On the one hand, technology may substitute for household members, and residenceworkschool loca- travel while, on the other hand, technology may comple- tion interdependency. ment or lead to more (spontaneous) travel. Similarly, Constraints and flexibility: There is much to be there are new transportation technologies including trav- learned about constraints and flexibility associated with eler information and guidance and navigation systems, various activities and their attributes; much has been dis- intelligent transportation systems, and alternative fuel covered as well. There are many constraints that play a vehicle technologies that impact travel behavior. Travel key role in shaping activitytravel patterns, including demand models used for forecasting should reflect the modal, situational, institutional, household (obligatory), telecommunicationstravel behavior interaction. and personal constraints. Positive utility of travel: There is some evidence that suggests that travel is not purely a disutility that is Changes to Spatial and Temporal Resolution minimized by individuals. A model system that could accommodate alternative utilitarian paradigms might be Many implementations of tour-based model systems are able to capture the situations in which travel, by itself, based on the traditional zone-based spatial representa- offers a positive utility. tion of a region and discrete time-of-day periods. Until Time use and activity patterns: Travel demand is models move toward a truly continuous representation inextricably tied to the demand for pursuing activities of the spacetime domain (which is happening at a rapid that are distributed in time and space and the time avail- pace in R&D), the rather discrete representation of space able to pursue them. Thus, time use and activity analysis and time is likely to continue. In such case, it would be play an important role in modeling travel demand. His-