A first DUI is not a small thing. It is a criminal conviction, a license suspension, an insurance catastrophe, and — for anyone with a professional license, a security clearance, or a commercial driver's license — a threat to their livelihood. A DUI with injury is a felony. A fourth is a felony. A DUI with a death can be charged as murder. Breath and blood evidence is not infallible and it is not unchallengeable. Instruments are calibrated by people. Blood is drawn, stored, and tested by people. Field sobriety tests are administered by officers, according to a protocol, and that protocol is frequently ignored. And every DUI case begins with a traffic stop that has to have been lawful in the first place.