This report presents a review of experience with highway air quality assessments in four case study areas: New York State; Connecticut; Puget Sound (Seattle/Tacoma), Washington; and Oregon. Four case studies document the procedures used by federal, state and local governments: (1) To identify the need for an assessment of the air quality effects of a proposed highway project; (2) To determine the appropriate level of analysis when an assessment is required; (3) To provide for review of the air quality assessment by agencies and the public; (4) To resolve differences over technical and policy issues and arrive at a decision. The studies show a wide range of approaches to highway air quality review, with significant differences in aspects such as: (1) Relationships among indirect source review and other state and federal air quality reviews; (2) Number and size of highway facilities to which indirect source review applies; (3) Threshold levels for applicability of indirect source review to new or modified highways; (4) Pollutants covered.