Abstract: A collision diagnosis (12) is generated for a traffic event involving a vehicle, by receiving an output from a self-capacitance sensor (6) having a single electrode comprising at least part of the bodywork of the vehicle (3) and comparing the received output with at least one threshold (8) to generate provisional collision detection data (9). The provisional collision detection data (9) is correlated with further data (11) relating to the traffic event to produce correlation data and the collision diagnosis (12) is generated on the basis of the correlation data.