Abstract: A crash detector activates a safety device (3, 4) such as an air-bag in a vehicle. The crash detector incorporates an acceleration sensor (10), the output of which must pass a threshold (11) before the signal is processed by a signal processor (2). The signal is processed optionally in conventional ways (13, 14; 16, 17) to activate the safety device. Activation of the safety device is inhibited by a slam algorithm generator (18). In the slam algorithm generator a control signal is generated which converges towards an average of the acceleration signal during periods of time in which the acceleration signal indicates an inward acceleration in excess of a predetermined level, the control signal being reduced at a substantially constant rate with time during periods in which the acceleration is not in excess of said predetermined level. The control signal is passed to a threshold means (19) which enables (15) actuation of the safety device (3, 4).