Freight and Distribution

State-of-the-art metropolitan freight truck discounts models are hybrids that blend commodity flow modeling techniques with freight truck modeling techniques. Commodity flow databases tend to be relatively accurate for inter-county flows, but undercount intra-county flows because commodity flow databases rely partly on economic input-output data that ultimately are based on financial transactions between producers and consumers of goods. However, in an urban area, many freight shipping companies truck moves are not easily traced to such transactions. Moves from warehouses and distribution centers, repositioning of fleets, drayage moves, parcel delivery, and the like are generally short-distance trips in which there may not be an economic exchange of the goods from one party to another. To compensate for the lack of inclusion of the shorter distance trips in commodity flow data, and to account for types of trucks that do not carry freight, local truck trips are generated based on local employment and economic factors using trip generation rates. These trips are usually generated at the zone level and trip distribution uses methods such as gravity models. The trip rates are calibrated so that the truck traffic volumes that are generated from the combined commodity flow and locally generated truck trips match those from available truck counts. Several terms are used to refer to these two trip types, including commodity-flow trips versus locally generated trips, external versus internal truck trips and long-haul versus local truck trips. Taking advantage of the relative strength of the commodity long-haul approach and the truck short-haul approach within the same model has been called a “hybrid approach.” The two modeling frameworks – freight truck models and commodity-flow models – are described briefly in the following sections. These two models form the basis for the freight/truck hybrid forecasting procedures.

Freight shipping companies

For statewide freight models, data are needed to develop and specify the equation used in the various steps, and forecast adapt is needed in the same format to create freight flow forecasts. These tables tend to have limitations that must be overcome in using them to survey as freight surveys for model development. The CFS is publicly available only for 114 zones nationally, but the number of zones increases the purchase price. The challenge in the use of both models is to develop zone structures that are detailed within the model study area, the state, and increasing less detailed at distances from the state model area. The commodity table typically has what is referred to as two-digit level of detail. Employment data are needed at an industry detail matching this freight commodity structure. Even the 40-50 commodities available provide data management and computational challenges and commodities carried forward are typically those that are the largest and most important to the study area. The associated employment must be available for those important commodities but may be aggregated to less detail matching the aggregated commodities. For example, printing may be included with all non-durable manufactured goods while food products would be retained as a separate category.

Freight carrier

These commodity-flow surveys also provide information needed to calibrate the trip distribution and mode split steps. Commodity flows will typically need to be converted into units of daily vehicles because this more easily integrates with passenger forecasts and other transportation design, and operations tasks are typically based on daily flows. Data are needed to develop factors that can be used to convert from annual tons to daily trucks. The model needs to be validated to observed counts. This validation data, on highways, is observational, such as truck classification counts and typically will have no information on the commodities being carried. Since observational counts also include no information on truck purpose, those counts probably include trucks carrying local delivery of local freight or trucks used in construction, service, and utility trucks, none of which are included in the freight commodity model. Conversion from annual flows to daily modal vehicle flows is needed only for those modes that will be used in assignment.

The total number of axles on the trucks are normally categorized into four axle categories – two axles with four tires, two axles with six tires, three axles, and four or more axles. This information on vehicles can be obtained by visual identification or manual counts, or the use of axle sensor-based counters that are often used to collect accurate truck counts. The number and spacing of axles is used to classify trucks into FHWA’s 13-category classification scheme. Most of the vehicle classification count studies across the country classify freight carrier trucks into these 13 categories, as listed below:

• Class 1:  Motorcycles (Optional) – All 2 or 3-wheeled motorized vehicles. Typical vehicles in this category have saddle type seats and are steered by handlebars rather than steering wheels. This category includes motorcycles, motor scooters, mopeds, motor-powered bicycles, and three-wheel motorcycles. This vehicle type may be reported at the option of the state.
• Class 2:  Passenger Cars – All sedans, coupes, and station wagons manufactured primarily for the purpose of carrying passengers and including those passenger cars pulling recreational or other light trailers.
• Class 3:  Other Two-Axle, Four-Tire Single Unit Vehicles – All two-axle, four-tire vehicles, excluding passenger cars. Included in this classification are pickups, panels, vans, and other vehicles such as campers, motor homes, ambulances, hearses, carryalls, and minibuses. Other two-axle, four-tire single-unit vehicles pulling recreational or other light trailers are included in this classification. Because automatic vehicle classifiers have difficulty distinguishing Class 3 from Class 2, these two classes may be combined into Class 2.
• Class 4:  Buses – All vehicles manufactured as traditional passenger-carrying buses with two axles and six tires or three or more axles. This category includes only traditional buses (including school buses) functioning as passenger-carrying vehicles. Modified buses should be considered to be a truck and should be appropriately classified.
• Class 5:  Two-Axle, Six-Tire, Single-Unit Trucks – All vehicles on a single frame, including trucks, camping and recreational vehicles, motor homes, etc., with two axles and dual rear wheels.
• Class 6:  Three-Axle Single-Unit Trucks – All vehicles on a single frame, including trucks, camping and recreational vehicles, motor homes, etc., with three axles.
• Class 7:  Four-or-More-Axle Single-Unit Trucks – All trucks on a single frame with four or more axles.
• Class 8:  Four-or-Fewer-Axle Single-Trailer Trucks – All vehicles with four or fewer axles consisting of two units, one of which is a tractor or straight truck power unit, used by freight shipping companies.
• Class 9:  Five-Axle Single-Trailer Trucks – All five-axle vehicles consisting of two units, one of which is a tractor or straight truck power unit.
• Class 10:  Six-or-More-Axle Single-Trailer Trucks – All vehicles with six or more axles consisting of two units, one of which is a tractor or straight truck power unit.
• Class 11:  Five-or-Fewer-Axle Multitrailer Trucks – All vehicles with five or fewer axles consisting of three or more units, one of which is a tractor or straight truck power unit.
• Class 12:  Six-Axle Multitrailer Trucks – All six-axle vehicles consisting of three or more units, one of which is a tractor or straight truck power unit.
• Class 13:  Seven-or-More-Axle Multitrailer Trucks – All vehicles with seven or more axles consisting of three or more units, one of which is a tractor or straight truck power unit.