  Stress impacts driving-related cognitive functions like attention and
decision-making, and may arise in automated vehicles due to non-driving tasks.
Unobtrusive relaxation techniques are needed to regulate stress without
distracting from driving. Tactile wearables have shown efficacy in stress
regulation through respiratory guidance, but individual variations may affect
their efficacy. This study assessed slow-breathing tactile guidance under
different stress levels on 85 participants. Physiological, behavioral and
subjective data were collected. The influence of individual variations (e.g.,
driving habits and behavior, personality) using logistic regression analysis
was explored. Participants could follow the guidance and adjust breathing while
driving, but subjective efficacy depended on individual variations linked to
different efficiency in using the technique, in relation with its attentional
cost. An influence of factors linked to the evaluation of context criticality
was also found. The results suggest that considering individual and contextual
variations is crucial in designing and using such techniques in demanding
driving contexts. In this line some design recommendations and insights for
further studies are provided.
