A judge says evidence shows Mary Richardson Kennedy was unaware that medications she had taken would impair her ability to drive when she was arrested by state police on Aug. 21, 2010.
She was initially pulled over for speeding on the Taconic State Parkway, but police said they determined she was under the influence of a prescription medication.
Pleasant Valley Town Justice Paul Caltagirone dismissed the charge Friday.
He noted in his decision that prosecutors opposed the motion.
Kennedy was arrested one month after a judge suspended her license following her guilty plea of driving while impaired near her Bedford home on May 15.