A process for determining lateral overturning of a vehicle involves
establishing a threshold value line S separating a lateral overturning
region farther from an origin and a lateral non-overturning region closer
to the origin on a two-dimensional map made using a rolling angle .theta.
and a rolling angular speed .omega. of the vehicle. When a hysteresis line
for actual rolling angles and actual rolling angular speeds of the vehicle
traverses the threshold value line from the lateral non-overturning region
to the lateral overturning region, it is determined that there is a
possibility of lateral overturning of the vehicle, and correspondingly an
occupant protecting system may be deployed. Also, to assure accuracy, a
lateral acceleration sensor for detecting a lateral acceleration for
establishing the threshold value line S is disposed on a center plane of a
vehicle body.