Yes, you can get a DUI if you are caught driving under the influence of Washington D.C., just like any other controlled substance. If the officer believes you are under the influence of marijuana they will administer a field sobriety test which will check and monitor your behaviors. If you fail this test you will be arrested and charged with a DUI or some equivalent. Marijuana is treated roughly the same as alcohol.
Driving while under the influence of marijuana is illegal in Washington D.C. First offense is a mandatory 6 month suspension of driving privileges and possibly up to 90 days in jail. They would tell you are intoxicated by observing you and then confirm it through blood tests. There is no set threshold for marijuana intoxication and so the officer’s observations would be the determining factor.
Yes you can get a DUI after smoking and driving. Technically if you are tested for drugs for any reason at a police stop, and have the drug in your system, you CAN be arrested for drugged driving. Marijuana, even as prescribed, is still considered a drug you can not drive on. Such as, you could still get arrested for driving on pills you are prescribed to. It would depend on the officer and if they want to arrest you or believe you are impaired and guilty of drugged driving.