Uncertainty Propagation in an Integrated Land Use-Transportation Modeling Framework: Output Variation via UrbanSim

The effect of uncertainty in the land use component of a partially integrated land use–transportation modeling system called UrbanSim is examined. Outputs from the land use model (LUM) act as inputs for a traditional four-step travel demand model (TDM), and travel times from the traffic-assignment stage of the TDM are fed forward into the subsequent years’ LUM. This work examines the propagation of uncertainty across model stages as well as at each model stage over time. A factorized design approach is used to model uncertainty in demographic inputs (which include aggregate growth rates and mobility rates) to the land use model, as well as uncertainty in various model parameters. Results suggest that although several model inputs may affect model outputs in the short run, only those inputs that have a cumulative effect are likely to have a significant impact on outputs in the long run. Results also suggest that uncertainty in model outputs may increase for the first few years for which the model is run, as modified inputs send shocks through the urban system. However, the level of uncertainty appears to come down in later years, as households, jobs, and developers respond to changed input conditions.