Conditions and Performance Chapter Listing

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Introduction

Highway Sensitivity Analysis

Transit Sensitivity Analysis

Introduction

This chapter explores the effects of varying some of the assumptions that were used to develop the investment requirement projections in Chapter 7. In any modeling effort, evaluating the validity of the underlying assumptions is critical. The results produced of Highway Economic Requirements System (HERS) and Transit Economic Requirements Model (TERM) are strongly affected by the values they are supplied for certain key variables. This chapter was added to the report to open up more of the modeling process, and to make the report more useful for supplementary analysis efforts.

There is a great deal of uncertainty about the appropriate values for the 20-year travel growth rates on which HERS and TERM rely. The highway and transit sections both show the impact that changing these assumptions would have on the investment requirement projections. The highway section of this chapter also explores a number of other variables, in part to show the impacts of some of the assumptions that were modified for this version of the report. The changes in the highway investment requirement methodology are discussed more fully in Appendix G.

One of the key parameters used in projecting investment requirements is the forecast rate of transit travel growth. The sensitivity of the estimated investment requirements to the growth rate forecast is analyzed by allowing three alternative growth rate inputs: 50 percent higher than the forecast, 50 percent below the forecast, and 100 percent below the forecast (i.e., zero transit passenger mile growth).