Accuracy of Socioeconomic Forecasts for Transportation Planning by Medium-Sized Metropolitan Planning Organizations

Urban travel demand forecasting begins with 20-year forecasts of population, employment, vehicle ownership, and households for an entire region and for individual transportation analysis zones within the region. Yet even though these forecasts rely on socioeconomic forecasts, their long-term accuracy is generally not known, especially for medium-sized metropolitan planning organization regions with limited staff. This paper reports on a case study of the socioeconomic predictions made in 1981 for a horizon year of 2000, comparing predicted and actual results in Lynchburg, Virginia. The region percent error reflects the difference between forecast and observed values for the entire region. Although regional forecasts for the number of vehicles and employment showed errors of less than 10 percent, those for population and households showed errors of 48 and 14 percent, respectively. Two of the 68 zones accounted for approximately 80 percent of the population error and 90 percent of the household error. These large errors were the result of planned development in these zones that did not materialize. The zone percent error is the average of all individual zone percent errors. Population, households, and vehicles had similar zone percent errors of 61, 65, and 54 percent respectively, whereas the employment zone percent error was 154 percent. Although supporting the need to update socioeconomic forecasts more frequently than is the current practice, these results make a compelling case for executing the regional travel demand model twice: once with the given socioeconomic forecasts and once with those forecasts modified based on expected errors. In the absence of additional information, the expected errors from this paper may be used.