Scenario/sensitivity testing in project-level traffic forecasting

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A “scenario” is comprised of the set of factors that can influence the forecast but are not defined by a project alternative. A scenario might involve the economic, demographic or land-development environment of the project. A “sensitivity” is the amount of change in a single output of a forecast, given a change in a single input to a forecast; formally, a sensitivity is equivalent to an elasticity from the field of economics. Sensitivity testing is useful during model development, but scenario testing is more relevant to the decision process. Scenario testing has the advantage of placing bounds on the range of a forecast and of alerting decision makers about how the forecast might be affected by extraneous factors and assumptions about the future. Scenario testing has the advantage of removing the burden of needing to know future conditions very precisely.