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CHAPTER 4
Conceptual Model
This chapter includes the development of a Conceptual · Identifies the major sources of potential errors in emission
Model for freight transportation activity as it relates to emis- estimation parameters and the steps in emissions calcula-
sions calculations. The Conceptual Model offers a structure tions that warrant improvement; and
for comprehensive representation of freight activity in the · Tracks trends in freight emissions over time, and identifies
United States, covering all modes and relationships between which parameters were responsible for changes in emission
modes. In order for this model to be effective in improving outputs.
emissions estimates, the Conceptual Model captures factors
in freight movement and freight equipment that most influ-
4.1 Model Overview and Uses
ence emissions.
The Conceptual Model is a functional model, which The Conceptual Model provides the link between economic
includes the specifications of an information system in the activity, freight transportation activity, freight-related emis-
form of functional areas, business processes, and information sions, and associated health effects. The Conceptual Model
flows between them. The Conceptual Model does not include uses commodity flows derived from economic activity fore-
a formal data model, so it does not contain the description of casts to determine freight activity (Exhibit 4-1). Even though
all data elements that are necessary for all business processes. the Conceptual Model does not model dispersion of emissions
It does, however, identify the information needs from busi- or health effects, it provides the spatial and temporal allocation
ness processes. of emissions, which are necessary inputs for dispersion models
The Conceptual Model serves several purposes, as follows: and health risk assessments.
The Conceptual Model is based on a "link and node"
· Estimates multimodal emissions associated with specific transportation network. The link and node framework is the
supply chains, transportation corridors, freight facilities, basis for representing roadway networks in travel demand
and geographic regions; models, and it is also regularly applied to other modes (e.g.,
· Assists shippers, carriers, and logistics providers in incor- rail). The link and node concept provides an effective way to
porating emissions in the planning and operations of their link different modes in one supply chain and to represent the
logistics activities; intermodal connections and freight transloading that are
· Assists public agencies in incorporating emissions in the common in urban freight systems but poorly represented in
planning of transportation infrastructure, transportation current modal-specific analyses.
investment decisions, and development of transportation The Conceptual Model includes the definition of all
regulations and/or voluntary programs; functional areas and business processes necessary for the cal-
· Identifies elements of freight activity that are not well culation, allocation, and evaluation of freight transportation-
represented by available data and methods; related emissions. Based on input parameters, the Conceptual
· Identifies how new and emerging freight data and methods Model includes a set of information flows (between business
relate to existing data and methods, and how they can pres- processes) that are needed to calculate freight transportation
ent a comprehensive picture of freight movement; emissions. Some basic equations for emissions calculations are
· Identifies opportunities to link modal-specific freight activ- provided, but the Conceptual Model is not designed to replace
ity data and tools in a unified framework that spans multiple existing emission models. Instead, it is designed to calculate
modes and possibly geographic and temporal dimensions; and characterize transportation activity in such a way that it
