This project continues ongoing research activity that will provide guidance to State agencies and industry partners on the appropriate use of phosphoric acid as an asphalt modifier. Issues still not clearly resolved are the reaction of phosphoric acid with asphalt from different crude sources, reaction with lime and limestone aggregates, and the effect on rutting and fatigue. The comparison of phosphoric acid modification with polymer modification needs to be augmented. There are also unresolved forensic issues in New York State concerning phosphoric acid and premature pavement failures. Until this is resolved, other State agencies will likely reject the use of this potentially cost-saving additive.