EPA’s Office of Transportation & Air Quality (OTAQ) has issued Transportation Conformity Guidance for the 2015 Ozone NAAQS Nonattainment Areas.
A link to EPA’s Policy and Technical Guidance for State and Local Transportation webpage, with the document:
https://www.epa.gov/state-and-local-transportation/policy-and-technical-guidance-state-and-local-transportation
And this is the direct link to the document itself: https://www.epa.gov/sites/production/files/2018-06/documents/420b18023.pdf