36 CFR 4.23 - Operating under the influence of alcohol or drugs.
(a) Operating or being in actual physical control of a motor vehicle is prohibited while:
(1) Under the influence of alcohol, or a drug, or drugs, or any combination thereof, to a degree that renders the operator incapable of safe operation; or
(2) The alcohol concentration in the operator's blood or breath is 0.08 grams or more of alcohol per 100 milliliters of blood or 0.08 grams or more of alcohol per 210 liters of breath. Provided however, that if State law that applies to operating a motor vehicle while under the influence of alcohol establishes more restrictive limits of alcohol concentration in the operator's blood or breath, those limits supersede the limits specified in this paragraph.
(1) At the request or direction of an authorized person who has probable cause to believe that an operator of a motor vehicle within a park area has violated a provision of paragraph (a) of this section, the operator shall submit to one or more tests of the blood, breath, saliva or urine for the purpose of determining blood alcohol and drug content.
(2) Refusal by an operator to submit to a test is prohibited and proof of refusal may be admissible in any related judicial proceeding.
(3) Any test or tests for the presence of alcohol and drugs shall be determined by and administered at the direction of an authorized person.
(4) Any test shall be conducted by using accepted scientific methods and equipment of proven accuracy and reliability operated by personnel certified in its use.
(d) Presumptive levels.
(1) The results of chemical or other quantitative tests are intended to supplement the elements of probable cause used as the basis for the arrest of an operator charged with a violation of paragraph (a)(1) of this section. If the alcohol concentration in the operator's blood or breath at the time of testing is less than alcohol concentrations specified in paragraph (a)(2) of this section, this fact does not give rise to any presumption that the operator is or is not under the influence of alcohol.
(2) The provisions of paragraph (d)(1) of this section are not intended to limit the introduction of any other competent evidence bearing upon the question of whether the operator, at the time of the alleged violation, was under the influence of alcohol, or a drug, or drugs, or any combination thereof.