I don't think a first DUI without fatalities is a felony under the
relevant D.C. law. It DOES apply to driving under the influence of drugs, though:
No individual shall, when . . . under the influence of . . . any drug . . . operate or be in physical control of any vehicle in the District. . . . Any individual violating any provision of this paragraph, upon conviction for the first offense, unless the individual has previously been convicted for a violation of paragraph (2) of this subsection, shall be fined $300 and may be imprisoned for not more than 90 days. In addition, if the individual's blood contains at least .20%, but not more than .25%, by weight, of alcohol, the individual shall be imprisoned for an additional mandatory minimum period of 5 days, or if the level is more than .25%, by weight, of alcohol, for an additional mandatory minimum period of 10 days. The additional mandatory minimum period shall not be suspended by the court.
The problem is, without detaining or questioning the driver, the police cannot determine whether or not a felony has taken place. And they cannot detain or question a Congressman who is on his way to a debate or a vote in Congress.