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Formal program guidance is issued by FHWA during each transportation authorization; this usually takes the form of an update of the existing document that reflects changes provided in new legislation. In addition to the formal guidance development, the agency may need to issue intervening guidance memoranda between the authorizations, reacting to issues that arise and that are not covered by any existing guidelines. Examples would be guidance memoranda on CMAQ and transit station rehabilitation, freight and intermodal efforts, and others. Outside the CMAQ program office, there are other policy and guidance aids, including materials from the EPA on a number of issues, such as diesel retrofit technologies.