A medical marijuana card in Rhode Island does not authorize you to drive while impaired. You can still be charged with marijuana DUI even with a valid medical card if the prosecution can prove actual impairment at the time of driving. The medical card provides legal protection for possession and use, not for impaired operation of a motor vehicle. The same defense angles apply (lack of per se THC limit, weak field sobriety reliability for marijuana, chain of custody) but the card itself is not a defense to DUI. If anything, prosecutors sometimes view medical patients as more knowledgeable about their dosing, which can cut against you at sentencing.
