Overall, the DOT vulnerability assessment is excellent, but it should not be a one-time effort. Attention to the following points will help improve future assessments:

• The basic assumptions and predicted consequences of each scenario should be given a further reality check, in cooperation with appropriate industry representatives.
• Further assessment of threat likelihood is highly desirable, although very difficult.
• A more complete, balanced, and clearly defined analysis of possible means of attack will be needed for future assessment efforts.
• Cyber and other attacks on the C³ systems of surface transportation are given insufficient attention in the assessment. The discussion that does appear is focused too narrowly on the introduction of computer viruses.
• Chemical and biological vulnerabilities must be considered carefully and seriously—not left to others to worry about. Future assessment efforts should distinguish carefully between chemical attacks and biological attacks and between attacks involving agents with different properties.
• Strategic, systemic vulnerabilities deserve close attention. These may result from interdependencies within the surface transportation system, interdependencies with other infrastructure sectors, or special circumstances involving nationally important functions. To improve its understanding of these issues, DOT should undertake (1) a strategic assessment of the surface transportation system's redundancies and interdependencies, and (2) an analysis of lessons learned about impact and mitigation from past accidents and natural disasters.