Since the unfortunate events of September 11th, the United States must focus additional attention on determining vulnerabilities to terrorist attack. One such vulnerability that has received consideration is the threat of terrorist action against a transportation target. This report summarizes results of research to investigate cost effective measures to improve bridge security against a terrorist threat. It discusses previous research performed on risk management and threat assessment, and discusses the dynamics of extreme loadings on structures. It also discusses the analysis methods and results of parameter studies used to determine cost effective bridge retrofit or design change options for improved security. This research provides a guideline for a bridge engineer to create a bridge design to protect against terrorist blast loads.