output a signal which is indicative of the lateral acceleration of the sensor 5 the vehicle 1 also comprises one or more safety devices such as a side air bag 6 to protect a vehicle occupant in the event of a side impact a control unit 7 which comprises one or more processors interprets signals output by the sensors 2 3 4 5 and determines whether the safety devices should be triggered using the information derived from some or all of these sensors 2 3 4 5 it is possible to make a determination as to whether the vehicle 1 is in a situation where it is likely to be involved in a harmful side impact in particular such an impact is more likely to occur where control of the vehicle has been lost or appears likely to be lost such a situation may be indicated by understeer of the vehicle 1 by oversteer of the vehicle 1 or by lateral slip of the vehicle 1 i e where the vehicle 1 skids in a lateral direction loss of control may also be occurring or be likely to occur if the vehicle 1 is undergoing avoidance manoeuvring or emergency braking or if it appears that the vehicle 1 has left the road on which it is travelling if any of these conditions are met the likelihood of the vehicle 1 being involved in a harmful side impact is increased in addition the likelihood of a harmful side impact occurring is increased if the longitudinal speed of the vehicle 1 is relatively high for instance if the vehicle 39 s speed is in excess of 5 km h and particularly if the speed is in excess of 15 km h it may be determined.