Environmental Engineering Reference
In-Depth Information
Table 1 Partial views of the freight system
Freight generation
Shipper/
Producers
Carriers
Distribution
centers/
Warehouse
Consumers of
cargos(receivers)
Transportation
agencies
Yes
a
Yes
a
Yes
a
Yes
b
Amount of cargo
No
Yes
a
Yes
a
Yes
a
Number of loaded
vehicle-trips
Not always
At Key links(no
distinction
between
loaded and
empty)
Yes
a
Number of empty
vehicle-trips
No
No
Yes
a
Yes
a
Yes
a
Yes
b
Number,frequency,
of deliveries
No
Yes
a
Yes
a
Yes
b
Commodity type
Not
always
only at some Ports
of entry
Yes
a
Yes
a
Yes
a
Yes
b
Shipment size
No
Yes
a
Yes
b
Cargo value
Not
always
Not always
only at some Ports
of entry
Yes
a
Yes
a
Yes
a
Yes
a
Land use patterns
All
a
b
Notes
Only of the cargo that they handle.
For all the cargo they receive
happens in the entire system. This has important implications for data collection
efforts, as it implies that the surveys used to collect data from the participants in
the freight activity would only be able to capture the information the agent sur-
veyed could provide. The fundamental challenge is how to put together a com-
prehensive
picture
of
the
system
that
is
relatively
accurate,
practical,
and
conceptually correct.
Another important aspect when characterizing the freight system is to under-
stand that freight—and by extension its generation—can be measured by many
metrics. These include the value of the cargo, the amount of cargo transported, the
vehicle trips produced, and the number of stops and deliveries made. Following
Holguín-Veras et al. (
2011
), it is important to make a distinction between freight
and freight trip generation. Freight generation (FG) is related to the generation of
demand, e.g., tons, while freight trip generation (FTG) is the generation of traffic,
e.g., truck-trips. In Holguín-Veras et al. (
2011
), the authors discuss this difference
based on logistical concepts, explaining how FG and FTG are related depending
on logistical decisions of shipment sizes, frequencies and types of modes used.
Furthermore, freight trip generation is also affected by logistic decisions about the
locations of the facilities and agents involved. These differences are of great
importance, since they have many implications for data collection. For instance
while understanding the connection between the commodity flow between an
origin and a destination may seem straightforward, the truck traffic may not follow
the same pattern (because the distribution of these goods may follow a delivery
tour). In addition, a large proportion of the truck traffic may be of empty trips, as
has been found in previous studies (U.S. Census Bureau
2004
), and not including
these trips can have negative implication in freight modeling (Holguín-Veras and
Thorson
2003
).
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