Status of the Nation's Highways, Bridges, and Transit:
2002 Conditions and Performance Report
|Chapter 10: Sensitivity Analysis|
Part I: Description of Current System
Part II: Investment Performance Analyses
Part III: Bridges
Part IV: Special Topics
Part V: Supplemental Analyses of System Components
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 by the Highway Economic Requirements System (HERS) and the Transit Economic Requirements Model (TERM) are strongly affected by the values they are supplied for certain key variables. This chapter was first added to the 1999 C&P 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. Alternative estimates of highway investment requirements are shown for scenarios in which future highway travel growth rates match those observed over the last 20 years and for scenarios in which travel growth is substantially lower than that projected in the Highway Performance Monitoring System (HPMS) sample data. The sensitivity of the estimated transit 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).
The chapter also includes other sensitivity analyses showing the impact of using alternative values for certain key model parameters (whose estimated values may be subject to some uncertainty). Both the highway and transit sections analyze the impact of increasing the unit improvement costs in HERS and TERM by 25 percent and the effects of variations in the value of time. The highway section also considers alternative values for additional parameters, including the value of a statistical life, truck shares, and travel demand elasticity.