Psychophysiological evidence of mental workload and subjective anxiety from in-vehicle voice message complexity

Purpose This study aims to investigate how the complexity of in-vehicle voice messages affects drivers. The authors examine its multifaceted impact on cognition, physiology and psychology to guide the design of safer, more sustainable interfaces. Design/methodology/approach A within-subjects experimental design was used with 75 participants. Behavioral performance (reaction time, accuracy), physiological measures (heart rate variability [HRV], blink rate) and subjective responses (NASA-Task Load Index, Dundee Stress State Questionnaire, STAI-State) were recorded during naturalistic driving while participants were exposed to messages systematically varying in length, number of items and information type. Findings Complex messages (long, containing 2–3 items, mixed-type) significantly increased reaction times by nearly 40% and decreased recall accuracy by over 20 percentage points. Physiologically, they induced a pronounced stress response, marked by suppressed HRV and elevated blink rate, which persisted into recovery phases. Subjectively, participants reported substantially higher mental demand, effort, distress and situational anxiety. A critical metacognitive gap was observed, wherein drivers underestimated their significant performance and physiological impairment. Practical implications The naturalistic setting introduced environmental variability, and the study captured acute, not long-term, effects. The implications are profound for interface designers and policymakers. Findings necessitate design principles prioritizing brevity, information chunking and adaptive delivery based on real-time physiological monitoring. Regulatory standards must expand beyond visual-manual distraction to include cognitive and mental health impacts. Originality/value This study provides original, multimodal evidence establishing auditory message complexity as a potent hidden stressor that extends beyond cognitive load to impact driver mental well-being. Its major contribution is a holistic, psychophysiological framework that validates the need for “psychologically sustainable design” and offers evidence-based guidelines to mitigate technostress in the automotive context and beyond.