Late one February night in 2013, Massachusetts state trooper Eric French pulled over a blue SUV with its rear lights out. When he approached the car, he saw smoke and smelled pot. The driver, Thomas Gerhardt, could count backwards from 75 to 62 and recite the alphabet from D to Q. But he couldn’t stand on one leg or walk nine steps and turn, standard measures on a field sobriety test. The trooper determined that Gerhardt was impaired, and he was arrested and charged with driving under the influence of marijuana.