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vehicles, with ton-per-vehicle rates determined separately for the introduction of new facilities. As part of its TRANSEARCH
each commodity group. These rates are based on values (by commodity flow database, Reebie Associates provides the
commodity group) from the Surface Transportation Board option to map truck freight flows on a highway network. This
Rail Waybill sample and the assumption that each truckload routing is accomplished through the use of special files that
carries 40% of the load carried by a railcar. contain:
· A highway network with unique highway identifiers for
4.5 Traffic Assignment
each highway segment;
As shown in Figure 4.5, the assignment model component · A set of paths between origin and destination zones con-
uses the table or matrix of freight flows by mode between all sisting of the highway links used to travel from origins to
zones produced by the mode split model component to fore- destinations; and
cast freight volumes on individual links of the modal net- · An O-D table of truck flows by commodity with the iden-
works. The assignment model component customarily tifier of the path used by those flows.
processes each mode separately using a network for that
mode with attributes important to freight in order to find the The TRANSEARCH Highway Network is available as a
optimum path or sequence of links between all geographic Microsoft Access table and an ArcView shapefile. By initiat-
zones. For truck freight flows, the travel times on the highway ing queries within Access and exporting those results to
network may account for the congestion caused by passenger ArcView, it is possible to develop maps of the flows of some
autos and other vehicles. In that case the freight truck trip or all commodities on the highway system.
tables will be assigned together with those auto tables to find In freight truck only assignments, the freight truck trip
the total link travel times and volumes. For the economic table is assigned to the highway network using an all-or-
activity model class, the link volumes are used to adjust the nothing assignment process. Since a straight all-or-nothing
original economic forecast in an iterative process until an assignment typically loads too many trips onto the interstate
equilibrium is reached. highways, a procedure to adjust the link speeds for noninter-
Network assignment models, as used in statewide freight state highway segments is often applied. This serves to draw
forecasting, apply the modal freight trips to paths identified more trips from the interstate roads to the competing U.S.
from the modal network. Essentially three types of assign- and state highways that run parallel to them. The unfortunate
ment models are used: rules-based assignment, freight truck part of the assignment step is the failure to address the possi-
only network assignment, and multiclass network assign- bility of congestion due to the presence of a large number of
ment. Rail networks are typically rules-based assignment passenger vehicles sharing the road.
models, given the difficulties of including rail business prac- The Freight Analysis Framework (FAF) uses a methodol-
tices in an assignment model. Freight truck only mode and ogy to estimate trade flows on the nation's highway infra-
multiclass assignments typically apply only to trucks on high- structure, seeking to understand the geographic relationships
ways. between local flows and overall transportation. Truck assign-
Rules-based assignment techniques may be developed by ment in the FAF is accomplished using TransCAD's Stochas-
the analysts or purchased as part of the existing O-D survey. tic User Equilibrium and with other vehicles, such as auto-
The distinguishing feature of a rules-based assignment is that mobiles, preloaded on the network. FAF is an improvement
the analyst does not have the ability to change the paths to be over the all-or-nothing assignment because it accounts for
used in response to changes in performance on the system or congestion.
4 4 4 4
Truck Rail Water Air
Matrix Matrix Matrix Matrix
Highway Rail Water Air
Assignment Assignment Assignment Assignment
Truck Rail Water Air
Volumes Volumes Volumes Volumes
5 5 5 5
Figure 4.5. Traffic assignment.
