Generated traffic has three different impacts to consider in transportation planning and project appraisal: 1. It reduces the congestion-reduction benefit that can result from increased road capacity. 2. It often increases external costs, such as downstream congestion, parking demand, uncompensated crash damages and environmental damages. 3. Since it consists of marginal value trips (vehicle travel consumers are most willing to shift or forego if their costs increase), consumer benefits from generated traffic tend to be modest. This article describes how generated traffic occurs, defines different types of generated traffic, discusses their impacts and describes ways to incorporate generated traffic into transport planning and modeling.