Abstract:U.S. transportation control measure (TCM) policies have evolved over the past several years, especially with respect to the identification of reasonably available control measures (RACM). U.S. TCM experiences are used to examine tramportation conformity in light of two important emerging policy issues: identification of TCMs that are RACM and the substitution of TCMs in existing state implementation plans (SIPs). The issues associated with forming TCMs into SIPs while buffering against conformity difficulties are outlined against the background of recent U.S. Environmental Protection Agency (EPA) rulemaking actions, court decisions, research findings, and various state SIP adoption efforts. Factors that are related to TCM substitution and take into consideration EPA approval actions and recent considerations raised by pubUc interest groups and planning agencies are identified. A process for identifying candidate TCMs for RACM analysis, explicit criteria for determining RACM TCMs, insights concerning TCMs that will be most effective, and case study-derived recommendations on how to structure SIP TCM commitments are provided.