Sources of error
When forecasting for the medium to long term we need to be clear in acknowledging that we are dealing with an uncertain future. There are a number of factors which may contribute to errors and uncertainty in forecasts. Sources of uncertainty when forecasting travel demand include:
Demographic and socioeconomic input data: employment and population market segments; economic activity: impacts on drivers for freight and passenger demand.
Assumptions about physical networks (base year and future projects): observed counts may affect model calibration and validation; road and public transport network assumptions.
Values of travel time (VTT): strategic transport models use an imputed VTT for each main trip purpose. VTT are usually estimated from the coefficients of mode choice models.
Time of day effects: factors are usually used for different time periods, (peak, daily, weekly, monthly, and annually), based on historical data. Such factors will not deal with peak spreading or mixes in the traffic composition. Methods to model time-of-day choice explicitly are beginning be implemented in practice.
Optimism bias: Use should be made of strategic transport models to investigate the impact of proponent optimism bias on overall results. Patronage optimism bias should be examined using sensitivity testing.