8 referring now to fig5 the curves a 1 and a 2 representing respectively the acceleration of the vehicle in a high speed crash and a relatively low speed crash have been plotted so that the curves a 1 and a 2 first intersect at a time t 01 or t 02 when each of the curves a 1 or a 2 first exceeds the predetermined acceleration value a 0 and the initiation criterion is thus fulfilled the u201c sliding window u201d of 8 ms is indicated as being a period of 8 ms before the times t 01 and t 02 when the curves a 1 and a 2 first exceed the predetermined acceleration value a 0 the values of the acceleration a of the vehicle during this 8 ms u201c sliding window u201d are the values of acceleration a which are processed by the second processing arrangement 8 it is to be appreciated that as the second processing arrangement 8 carries out an integrating calculation over the classification time period to calculate the value k the value k will correspond to the area beneath each of the curves a 1 and a 2 during the classification time period it can be seen that as the acceleration a of the vehicle rises more rapidly during the initial stages of a high speed crash situation represented by curve a 1 as opposed to a low speed crash situation represented by curve a 2 the area beneath the high speed crash curve a 1 is less than the area beneath the low speed crash curve a 2 over the classification time period thus the value k calculated by the second processing arrangement 8 is less in the case of a high speed crash than the value k calculated during a low.