It was discussed in the preamble to the final highway rule to allow case-by-case looking at a variety of factors (see pages 5073 and 5077, foot note). Unfortunately the provisions never made it into the regulations. We will be doing minor rule changes on this to mirror the preamble language.
Question and Answer was originally posted at Questions and Answers on the Clean Diesel Fuel Rules (PDF)(135 pp, 888 K, EPA420-B-06-010, July 2006, About PDF)