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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
