In Mr. Fairfax's amended "letter of interpretation," he first reiterates the FHWA final rule on high visibility (Worker Visibility, 23 CFR Part 634) that explains that all workers on a Federally-funded roadway who are exposed to traffic or to construction equipment are required to wear high-visibility safety apparel that complies with ANSI 107-204.
The Director than adds, "Accordingly, high-visibility apparel is required under the General Duty Clause to protect employees exposed to the danger of being struck by public and construction traffic while working in highway/road construction work zones. Typically, workers in a highway/road work zone are exposed to that hazard most of the time."1
The full clarification letter can be found on OSHA's site here.