Explain the four-step travel demand forecasting model.

Four-step model predicts future travel patterns: (1) Trip Generation - determines number of trips produced and attracted by each zone based on land use and demographics, (2) Trip Distribution - allocates productions to attractions using gravity model or growth factors, (3) Mode Choice - splits trips among travel modes (auto, transit, walk) based on utility functions, (4) Traffic Assignment - loads trips onto network using shortest path or equilibrium methods. Models require socioeconomic projections, network data, and calibration to existing travel patterns.