49 CFR § 397.13 - Smoking.
No person may smoke or carry a lighted cigarette, cigar, or pipe on or within 25 feet of -
(a) A motor vehicle which contains Class 1 materials, Class 5 materials, or flammable materials classified as Division 2.1, Class 3, Divisions 4.1 and 4.2; or
(b) An empty tank motor vehicle which has been used to transport Class 3, flammable materials, or Division 2.1 flammable gases, which when so used, was required to be marked or placarded in accordance with the rules in § 177.823 of this title.
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