Effects of Changes in Bus Supply Level on Urban Rail Demand Forecasting

Abstract

The problem of misleading forecasts on the road is less severe and less one-sided than for rail. As the cause of this severe and nonrandom error of rail demand forecasting, this study focuses on the unexpected increase of bus supply level and the modal share prediction errors caused by the unexpected bus supply level change in the process of mode split. If a new rail line is planned for an area where large land developments are expected, the level of bus supply at the time of development planning and level of bus supply at the time of development completion would be different, and this may cause overestimation of rail demand. Therefore, this study developed a model to forecast the bus supply level and suggested a method to apply the model in the urban rail demand forecasting process. To consider the effect of change of future bus supply level, post-processing analysis which re-estimates urban rail demand by using cross-elasticity and the differential rate between bus supply level of existing model and the proposed model. By using the proposed model, four cases of previous urban rail demand forecasting studies were re-reviewed. In Shinbundang-line, Yongin light rail, Gimpo metro line and Byollae-line case studies, the impacts of bus supply level change on the rail demand forecasts ranged from 16% to 41%. In this proposed model, the error was less than 5% and showed high predictability except for the case of Byollae-line. In Byollae-line case, the results of both of the existing model and proposed model showed large error rate to the observed demand. The source of this relatively large error of Byeollae-line case is supposed that several land development plans are neighboring, and buses running through the districts may be overlapped. It is assumed that the overlapping buses cause errors in several districts, as a result, causing relatively large errors. This study suggests the unexpected change of bus supply level as a major source of rail demand forecasting error, which has been failed to be considered in previous studies and guidelines. This study is distinct from previous studies regarding the impact of change of bus supply level on urban rail demand forecast was quantified, and a model to forecast the bus supply level was developed and applied to improve the reliability of demand forecasting.