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on the availability of a highway network model from other
sources. If no statewide highway network or other vehicle trip
tables are available from a statewide travel demand model or
other source, a rules-based assignment model is used. If a
highway network is available or can be developed, a freight
truck only assignment can be used. If a highway network and
other vehicle trip tables are available, a multiclass assignment
can be used.
Case Studies and References
Two case studies demonstrate the O-D factoring method:
the Ohio Freight Model Case Study and the Freight Analysis
Framework Case Study. These are described in Sections 8.4
and 8.5, respectively.
Oklahoma Model This model and forecast system were
developed in 2000 by TranSystems Corporation. It is a con-
ventional model based on Reebie TRANSEARCH data.
Kentucky Corridor Model This model was developed in
1997 by Wilbur Smith Associates.12 The network and base
data were updated in 2001 by Wilbur Smith, without chang-
ing the model methodology.13 Figure 6.3. The truck
model.
6.3 The Truck Model
forecasts both passenger and goods movement and conse-
Description
quently use a simultaneous assignment of truck trips with
As shown in Figure 6.3, truck models use the trip genera- automobile trips.
tion and distribution model components to produce a table Truck models follow a three-step process of trip genera-
of truck trips and uses assignment model components to tion, trip distribution, and traffic assignment. The truck types
assign that table of truck trips. As truck models address only often considered in a truck model are broadly classified into
the single mode of trucks, they do not require a mode split light, medium, and heavy trucks based on gross vehicle
component. weight (GVW) ratings. Although weight-based, these classi-
Truck models usually attempt to account for all shipments fications are loosely correlated to other defining characteris-
of goods, including local delivery. Freight truck volumes, as tics of trucks, which are described in the Quick Response
freight is defined in most data sources such as the CFS and Freight Manual.3
TRANSEARCH, dominate in rural areas between distant
cities. Truck models that include local delivery are more use- · Light trucks are defined as vehicles with four or more tires
ful for states with closely spaced or contiguous urban areas. and two axles, with a GVW of less than 16,000 pounds.
For this reason, the sole example of a state truck model · Medium trucks are defined as single-unit vehicles with six
identified for inclusion in this Toolkit was developed for or more tires and two to four axles, with a GVW of 16,000
New Jersey, the most densely populated state in the nation. to 52,000 pounds.
However the nation's largest metropolitan planning organi- · Heavy trucks are defined as double-unit, triple-unit, or
zation, the Southern California Association of Governments, combination vehicles with five or more axles, with a GVW
is included as an example of a statewide truck model because greater than 52,000 pounds.
it uses the same techniques and the region's geographic and
population size is greater than that of many states. Truck Using these definitions, medium trucks directly correlate
models are more commonly a component of urban travel to single-unit trucks collected in truck surveys and heavy
forecasting models. trucks directly correlate to double- and triple-unit trucks.
Truck models obviously cannot analyze shifts between The truck counts do not usually separate light trucks from
modes, since by definition they include only the truck freight passenger cars and are sometimes estimated as part of pas-
mode. They are usually part of a comprehensive model which senger vehicle travel.
