49 C.F.R. § 10.29 – Social Security numbers.

(a) No individual is denied any right, benefit, or privilege provided by law because of such individual's refusal to disclose his Social Security account number.
(b) The provisions of paragraph (a) of this section do not apply to:
1. (1) Any disclosure which is required by Federal statute; or
2. (2) The disclosure of a Social Security number when such disclosure was required under statute or regulation adopted prior to January 1, 1975, to verify the identity of an individual.
(c) When an individual is requested to disclose his or her Social Security account number, that individual is informed whether that disclosure is mandatory or voluntary, by what statutory or other authority such number is solicited, what uses are made of it, and what detriments, including delay in the location of records, are incurred if the number is not provided.