The provisions of §§ 391.21 (relating to applications for employment), 391.23 (relating to investigations and inquiries), and 391.31 (relating to road tests) do not apply to a driver who has been a single-employer driver (as defined in § 390.5 of this subchapter) of a motor carrier for a continuous period which began before January 1, 1971, as long as he/she continues to be a single-employer driver of that motor carrier.