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4 Baseline Baseline Traffic-Flow OD Tables Network Improvement Growth Growth Redistribution Redistribution Module Module Assign HCM Traffic Assignment Assign Traffic Assign Revised Traffic HCM Assignment Revised HCM Using HCM Curves Module Using Module HCM Curves Using HCM Curves Assignment Module HCM Traveler Assignment Traveler Traveler Behavior Response Behavior Response Behavior Response Module Module Module Module USER'S GUIDE Assign Revised Revised HCM Traffic Assign Revised Revised HCM Traffic VMT VHT Using HCM Curves Using HCM Curves Assignment Module Assignment Module VMT VHT VMT VHT Base Short Term Long Term VehicleModal Vehicle ModalActivity Activity Module Module VehicleEmissions Vehicle Emissions Module Module Results Figure 1. The NCHRP 25-21 methodology. The third round of analysis (long term) feeds the short- separate methodology with separate data requirements. Trucks term results into the Growth Redistribution Module, which are presumed to be a fixed percent of current and future traffic computes the impacts of the traffic-flow improvements on demands in this methodology. the relative growth rates of zones within the region. The The methodology does not forecast socioeconomic changes, revised growth rates are used to redistribute the origins and traffic condition changes, or emission changes that are due to destinations of the trips in the short-term OD tables. The factors other than traffic-flow improvements. The proposed revised OD tables are then fed back through the Traveler methodology, therefore, must be used in conjunction with Behavior Response Module one more time to obtain mean some other model for predicting future baseline conditions, speed and flow for each highway link. The information is usually a conventional travel demand model. then fed to the Vehicle Modal Activity and Vehicle Emis- sions Modules to obtain emissions for the long term. The methodology generally follows the recommendations 2.3 HCM ASSIGNMENT MODULE of NCHRP Project 8-33. The methodology is designed to pre- dict the changes due to the traffic-flow improvement projects. On the opening day, drivers will experience the maximum It does not predict baseline conditions. Baseline conditions travel time savings provided by an improvement project. The (the baseline OD tables) must be input to the methodology. improved road section will have higher operating speeds The methodology does not separately model the demand and fewer and milder acceleration/deceleration events. If the response of heavy-duty vehicles to traffic-flow improvements. improvement also increases peak capacity, then more vehicles Modeling truck demand response would require a completely will be able to pass through the improved segment during the