This book explores the various economic and institutional factors that explain why huge investments are made in unworthy transportation mega-projects in the US and other countries. It is based on research, the general literature, economic analyses, and results from a specifically collected database showing that a significant proportion of implemented mega-projects have been found to be inferior ex-ante or incapable of delivering the returns they promised ex- post. Transportation infrastructure and other public investments of a similar scope (¿mega-projects¿) reflect public sector priorities and objectives, non- pecuniary as well as financial constraints, and a range of decision-making processes. This book describes how decisions made in the