  It is known that car drivers tilt their head toward the center of a curve. In
addition, drivers are generally less susceptible to carsickness than are the
passengers. This paper uses a mathematical model to investigate the effect of
the head-tilt strategy on motion sickness. It is shown that tilting the head in
the centripetal direction reduces the estimated motion sickness incidence
(MSI), defined as the percentage of subjects who vomited. In addition, the head
movements of both drivers and passengers were measured in a real car. It is
also shown that the estimated MSI of the drivers is smaller than that of the
passengers. Experimental results presented in previous studies demonstrated
that the severity of motion sickness was reduced when passengers imitated the
head tilt of the driver. These results strongly suggest that the driver's head
tilt reduces motion sickness, and this can be understood as a subjective
vertical conflict.
