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180 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 ment. In addition, the market segmentation is limited to analysis zones, which will be used as the basis for high- a single variable (usually income) in conventional mod- way travel time and some other level of service network els whereas all characteristics of the simulated individu- skims. This means that aggregation error will still exist als can be retained in the activity-based approach. in the model (although to a lesser extent than in a con- Another way in which transportation project analysis ventional model). However, current model design antici- is improved compared with the use of conventional mod- pates storing each household and job at the point level, els is that the effects of new projects on travel demand mitigating some aggregation errors by allowing detailed (i.e., induced travel) can be modeled directly. Conven- calculation of walk skims. tional trip-generation models consider only demographic variables and do not consider transportation level of ser- vice. The magnitude of the effects of improved trans- IMPROVING THE MODELING PROCESS portation level of service stemming from new projects on the amount of travel demand can be estimated through In conclusion, it is clear that existing modeling tools come the incorporation of level-of-service variables in all steps up short in their ability to address the planning analysis of the demand modeling process. The use of logsum vari- needs of the Denver region. While the proposed activity- ables from subsequent model steps provides a way to do based approach is not a panacea for all of the shortcom- this while maintaining consistency among the level-of- ings, it does provide many improvements to the modeling service data for all model components. process that specifically address some of the issues. These It is worth briefly discussing some of the ways in include the ability to introduce distributions for the value which the proposed activity-based modeling approach of travel time in road pricing analyses, the use of a more fails to address some of the planning analysis needs. One accurate tour-based time-of day-modeling procedure in of the most significant is that a conventional static traf- road pricing and other analyses, the use of additional seg- fic assignment process will be used. Although it would mentation variables in such analyses as the development be desirable to consider traffic microsimulation or of urban centers and transit-oriented development, the dynamic traffic assignment (DTA) procedures, the ability ability to directly model trip chaining, and the use of to implement and validate such procedures when they transportation level-of-service variables in all steps of the are applied at a regional level (at least in a region as large model to estimate the effects of induced travel demand. as the Denver metropolitan area) has not yet been These advantages led DRCOG to begin development of proven. Lack of a fully disaggregate or at least DTA pro- an activity-based model as the main travel demand esti- cedure will limit the model's ability to analyze the effects mation tool for future planning analyses. of queuing of traffic and to examine variations in traffic flow within peak periods. This inhibits the full explo- ration of the effects of tolling options and other highway REFERENCE operations analyses. Another issue is that, despite its use of microsimula- 1. Cambridge Systematics, Inc., M. Bradley, and J. Bowman. tion of individuals, the model will still have some aggre- Model Design Plan for the DRCOG Activity-Based Model. gate elements. The region will still be divided into January 2006.