The campaign against drowsy driving is moving to the courtroom, with law enforcement increasingly pushing to hold sleepy drivers criminally accountable when they cause fatal crashes, the New York Times reports. Jurors in the Bronx have been wrestling with the question of how tired is too tired to drive? Ophadell Williams was driving a bus on what prosecutors said was just a few hours of sleep when it crashed in the early morning hours, killing 15 passengers. He faces up to 15 years in prison if convicted of manslaughter and criminally negligent homicide.

Drowsiness has been cited in criminal cases against drivers in more than half-a-dozen states, including Florida, New Jersey, and Texas. In Virginia last month, a bus driver was convicted of involuntary manslaughter; authorities said he fell asleep before a crash that killed four passengers and injured dozens of others. The shift to prosecution follows successful efforts to criminalize other dangerous driving habits, like speeding, drinking alcohol, and using cellphones. Drowsy driving, which the AAA Foundation for Traffic Safety calls “one of the most significant, unrecognized traffic safety problems,” faces tougher legal hurdles. A blood-alcohol test can show whether a driver was drunk. Skid marks may betray a speeder. Cellphone records can show whether someone was texting right before a crash. Drowsiness is a personal and often fleeting state of mind that leaves no record.