Vehicles gain more sensors every year. Outward-facing sensors get the most attention: active braking, proximity sensing, rain-adapting. But what about the vehicle interior? We teamed up with experts on human-machine interaction to collect information on how sensors in vehicles might measure occupants’ stress levels and other safety-relevant behaviors. This literature review covers two types of soft sensors, wearable and in-vehicle, and discusses their potential to improve the human-factors side of transportation safety.