This investigation addressed the identification of potential risks from highway transportation of hazardous materials that would result in severe permanent, irreparable or catastrophic consequences, and the identification of practical and implementable physical protective systems to reduce accident incidents and/or mitigate consequences. The primary concern was to reduce or prevent contamination of surface or ground water resources from flows or other movements of materials from accidental spills of hazardous materials. The hazardous spill substances are likely to be directly toxic or indirectly result in reduced quality of receiving waters. This report developed a methodology using a State's panel to identify 11 generalized ranked extreme risk scenarios and identified protective systems for each.