49 C.F.R. § 553.19
A petition for extension of the time to submit comments must be received not later than 15 days before expiration of the time stated in the notice. The petitions must be submitted to: Administrator, National Highway Traffic Safety Administration, U.S. Department of Transportation, 400 Seventh Street SW, Washington, DC, 20590. It is requested, but not required, that 10 copies be submitted. The filing of the petition does not automatically extend the time for petitioner's comments. Such a petition is granted only if the petitioner shows good cause for the extension, and if the extension is consistent with the public interest. If an extension is granted, it is granted to all persons, and it is published in the FEDERAL REGISTER.
49 C.F.R. §553.19