Since World War II, per capita ownership of automobiles in the United States has more than doubled, partly because of the increase in real income. Increases in automobile ownership have gone hand in hand with the process of suburbanization, each reinforcing the other. In terms of direct costs, private transportation more or less pays for itself, whereas public transportation requires heavy subsidization. The decline in public transportation since World War II was attributed partly to increased automobile ownership and partly to a more spread-out pattern of land use.
The present pattern of land use shapes the present demand for transportation, and transportation investment decisions shape the future pattern of land use. In the best of circumstances, land-use and transportation planning proceed as coordinated rather than isolated processes.
Large-scale transportation planning is a four-step process:
- 1. Estimating trip generation. Without considering destinations, the planners, using such variables as household size, household income, and number of vehicles per household, estimate the total number of trips that will originate from each zone.
- 2. Estimating trip distribution. Using a gravity model or other mathematical model, the planners will then estimate the number of trips from each zone to each zone.
- 3. Estimating modal split. If more than one mode of transportation is available, say, automobile and bus, the planners will use mathematical models to estimate the number of travelers using each mode from each origin to each destination. The main variables in such models are cost and travel time.
- 4. Predicting trip assignment. Where there is more than one route from one zone to predicting another zone, the final item to be modeled is the number of trips that will be made via each route.
After the model is built, it is calibrated, that is, adjusted, so that its output duplicates the actual travel behavior of the region. Then the model can be used to examine different transportation alternatives. Benefit-cost analysis is frequently used to evaluate and rank different investment possibilities. Ultimately, though, deciding on major investments is a political matter because transportation system investments can have large consequences for land values, neighborhood quality, and the entire pattern of development.
We noted the interest in the development of intercity rail expressed in the last several years as well as the very formidable financial problems it poses. This chapter also discussed TSM and TDM as a means of improving the performance of the highway system, the use of sophisticated toll systems, and the privatization of some highways. The chapter closes with a discussion of changes in automotive technology, in particular the potential for the driverless automobile and some of its possible implications.