A safety system for a vehicle, the safety system comprising: a first sensor arrangement to determine a current velocity of the vehicle; a second sensor arrangement to detect a mobile object exterior to the vehicle, and determine a current position and velocity of the mobile object relative to the vehicle; a safety device which may be activated to protect an occupant of the vehicle and/or the mobile object; and a control unit operable to: receive and analyse signals from the first and second sensor arrangements (4, 2) determine whether the vehicle will collide with the object if the current velocities of the vehicle and of the object are maintained, and if so to determine the time and position of the collision; and generate an activation signal to activate the safety device based on the analysis of the signals from the first and second sensor arrangements (4, 2), characterized in that the analysis of the signals from the first and second sensor arrangements (4, 2) comprises: defining a vehicle front line extending across all or part of the front of the vehicle, and calculating likely future movement of the front line through space to determine a vehicle plane, in both space and time coordinates, comprising the future path of the vehicle front line through space based on the calculated likely future movement of the front line; defining an expected future trajectory for the mobile object; and calculating the position and time of an intersection between the vehicle plane and the expected future trajectory of the mobile object, wherein the calculated position includes a position of the intersection along the vehicle front line. a first sensor arrangement to determine a current velocity of the vehicle; a second sensor arrangement to detect a mobile object exterior to the vehicle, and determine a current position and velocity of the mobile object relative to the vehicle; a safety device which may be activated to protect an occupant of the vehicle and/or the mobile object; and a control unit operable to: receive and analyse signals from the first and second sensor arrangements (4, 2) determine whether the vehicle will collide with the object if the current velocities of the vehicle and of the object are maintained, and if so to determine the time and position of the collision; and generate an activation signal to activate the safety device based on the analysis of the signals from the first and second sensor arrangements (4, 2), characterized in that the analysis of the signals from the first and second sensor arrangements (4, 2) comprises: defining a vehicle front line extending across all or part of the front of the vehicle, and calculating likely future movement of the front line through space to determine a vehicle plane, in both space and time coordinates, comprising the future path of the vehicle front line through space based on the calculated likely future movement of the front line; defining an expected future trajectory for the mobile object; and calculating the position and time of an intersection between the vehicle plane and the expected future trajectory of the mobile object, wherein the calculated position includes a position of the intersection along the vehicle front line. receive and analyse signals from the first and second sensor arrangements (4, 2) determine whether the vehicle will collide with the object if the current velocities of the vehicle and of the object are maintained, and if so to determine the time and position of the collision; and generate an activation signal to activate the safety device based on the analysis of the signals from the first and second sensor arrangements (4, 2), characterized in that the analysis of the signals from the first and second sensor arrangements (4, 2) comprises: defining a vehicle front line extending across all or part of the front of the vehicle, and calculating likely future movement of the front line through space to determine a vehicle plane, in both space and time coordinates, comprising the future path of the vehicle front line through space based on the calculated likely future movement of the front line; defining an expected future trajectory for the mobile object; and calculating the position and time of an intersection between the vehicle plane and the expected future trajectory of the mobile object, wherein the calculated position includes a position of the intersection along the vehicle front line. defining a vehicle front line extending across all or part of the front of the vehicle, and calculating likely future movement of the front line through space to determine a vehicle plane, in both space and time coordinates, comprising the future path of the vehicle front line through space based on the calculated likely future movement of the front line; defining an expected future trajectory for the mobile object; and calculating the position and time of an intersection between the vehicle plane and the expected future trajectory of the mobile object, wherein the calculated position includes a position of the intersection along the vehicle front line.