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15 Socioeconomic Forecasting Methods Data 1 Figure 4.6. Economic/land use modeling. Multiclass network assignment of the truck trips can be depend on the relative accessibility between geographic zones based on an equilibrium highway assignment, and truck trips and the forecast is revised based on the resulting forecast of are usually assigned together with the passenger vehicle link volumes. These iterative adjustments may be made as part model because congestion has a significant impact on truck of a formal model process. travel times. Truck trips may also be assigned separately by Economic/land use modeling components in statewide vehicle size using the multiclass assignment technique. Many freight forecasting include modeling techniques known as a truck models are developed using a conversion of truck vol- spatial input-output (I-O) or econometric models. The land umes to passenger car equivalents (PCE) for assignment pur- use considerations in these models that consider state and poses. This factor provides a means of accounting for the fact national economic activity are generally far less developed that larger trucks take up more space on the roads than pas- than in metropolitan land use models and typically only fore- senger cars, and behave differently during acceleration and cast household and economic activity across county-level braking. This is important to determine the effects on capac- zones based on basic supply, demand, and cost relationships ity and congestion for assignment of both trucks and passen- for the state and national economy. These models may be ger cars. The Transportation Research Board's Highway used to develop the forecast socioeconomic variables that will Capacity Manual recommends PCE values of 1.5 and 2.0 for be used by the freight model. single unit trucks with six or more tires and combination Econometric models are seldom maintained and operated by units respectively. The truck model developed by the Balti- state departments of transportation. Most often they are oper- more Metropolitan Council indicated that the PCE value for ated by other state agencies, by state universities (such as, Uni- heavy truck varies from 2.0 to 4.0. This value depends on versity of Florida Bureau of Economic and Business Research, roadway grades, acceleration, and braking times. If observed University of Kansas Econometric Model) or by private firms data on passenger car equivalents are collected, then these (for example, Global Insights as WEFA for the FAF and Ohio assumptions by truck type should be modified. Interim Model and as DRI for the Nebraska Model, REMI, Woods & Poole for Indiana). When operated by others, the state departments of transportation may receive and use only 4.6 Economic/Land Use Modeling economic activity outputs, such as employment by industry and The economic/land use modeling component shown in population, to use directly in statewide freight forecasting. Figure 4.6 may be used to prepare the basic socioeconomic Alternatively, they may receive growth rates to apply to existing forecasts by geographic area used in freight forecasting. If the freight flows, or a complete forecast of future freight flows. economic forecasts are prepared independently of the freight When included as a component within the economic transportation forecasts they will serve as inputs to factor activity class of models, the economic/land use model com- facility flow or standalone commodity trip tables or as inputs ponents may be operated by the state DOT in cooperation to the trip generation model component. The forecasts with economic development agencies.