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Assessing Uncertainty

Scenario testing places bounds on the range of possible forecast values, given variations in future conditions. Within a single scenario, there is still the possibility of uncertainty in a forecast. The range of uncertainty in the forecasts can be established through validation statistics or, in the case of time series models, goodness-of-fit statistics such as the standard error of the estimate.

The range of uncertainty should be reported consistently with the parameters of the extended half-lane rule. The procedures for calculating the range of uncertainty will differ between techniques. For most projects, a 50% confidence interval should be used, which corresponds to the “probable error” of the forecast.

Measures of Effectiveness (MOEs)

Aggregated results have comparatively less uncertainty than disaggregated results, due to cancelling of random errors. Therefore, the use of measures of effectiveness (MOEs) are encouraged to the extent that they provide useful information to the decision processes. MOEs include such items as vehicle-miles-traveled (VMT), vehicle-hours-traveled (VHT), average speed, percent delay and total fuel consumption.