A first aspect relates to an apparatus, system and method for calculating a driving behaviour risk indicator for a driver of a vehicle. Said aspect involves obtaining a count of events occurring in each of a plurality of predetermined categories based on inputs from an inertial unit mounted on the vehicle, the inertial unit including a 3D inertial sensor with 3D gyroscope functionality, each event being indicative of at least one of dangerous and aggressive driving; and calculating a driving behaviour risk indicator based on the number of events in each category. According to a second aspect, an apparatus and method for reconstructing a vehicle trajectory is provided. Said aspect includes updating a sensor error model.