Provisions for a carbon monoxide (CO) categorical hot-spot finding were included in the January 24, 2008 final conformity rule amendments at 40 CFR 93.123(a)(3) and explained in the preamble at 73 FR 4432-4434. 40 CFR 93.123(a)(3) states, "DOT, in consultation with EPA, may also choose to make a categorical hot-spot finding that §93.116(a) is met without further hot-spot analysis for any project described in paragraphs (a)(1) and (a)(2) of this section based on appropriate modeling. DOT, in consultation with EPA, may also consider the current air quality circumstances of a given CO nonattainment or maintenance area in categorical hot-spot findings for applicable FHWA or FTA projects".
FHWA's 2017 CO Categorical Hot-Spot Memo and Finding
Technical Documentation and Resources