A vehicle safety system comprising: a pre-crash sensor, to detect a pedestrian with which the vehicle will collide, or is likely to collide, before a collision between the vehicle and the pedestrian takes place; a crash sensor, to detect directly a collision between the vehicle and the pedestrian; a control unit; and an air-bag to protect a pedestrian in the event of a collision, wherein the safety system is adapted: in response to a determination from the pre-crash sensor that a collision is inevitable or likely, but before a determination is made that a crash is occurring, to inflate the air-bag; and in response to a determination from the crash sensor that a collision is occurring, to vent the air-bag after inflation in such a way as to protect a pedestrian who is struck by the vehicle, wherein the venting of the air-bag is initiated a set length of time after the determination from the crash sensor, and wherein the control unit is adapted to calculate the set length of time based upon measured parameters relating to the size of the pedestrian, the relative speed between the vehicle and the pedestrian, and/or the speed of the vehicle, and wherein the calculation of the set time takes into account: an air-bag pressure profile, representing the pressure within the air-bag over time following inflation of the air-bag in the absence of a significant impact or other significant external forces acting on the air-bag; a drop in pressure within the air-bag over time following venting of the air-bag; a desired pressure within the air-bag at the moment of impact between the air-bag and a pedestrian; and an estimated length of time between the determination by the crash sensor and the moment of impact between the air-bag and a pedestrian. a pre-crash sensor, to detect a pedestrian with which the vehicle will collide, or is likely to collide, before a collision between the vehicle and the pedestrian takes place; a crash sensor, to detect directly a collision between the vehicle and the pedestrian; a control unit; and an air-bag to protect a pedestrian in the event of a collision, wherein the safety system is adapted: in response to a determination from the pre-crash sensor that a collision is inevitable or likely, but before a determination is made that a crash is occurring, to inflate the air-bag; and in response to a determination from the crash sensor that a collision is occurring, to vent the air-bag after inflation in such a way as to protect a pedestrian who is struck by the vehicle, wherein the venting of the air-bag is initiated a set length of time after the determination from the crash sensor, and wherein the control unit is adapted to calculate the set length of time based upon measured parameters relating to the size of the pedestrian, the relative speed between the vehicle and the pedestrian, and/or the speed of the vehicle, and wherein the calculation of the set time takes into account: an air-bag pressure profile, representing the pressure within the air-bag over time following inflation of the air-bag in the absence of a significant impact or other significant external forces acting on the air-bag; a drop in pressure within the air-bag over time following venting of the air-bag; a desired pressure within the air-bag at the moment of impact between the air-bag and a pedestrian; and an estimated length of time between the determination by the crash sensor and the moment of impact between the air-bag and a pedestrian. in response to a determination from the pre-crash sensor that a collision is inevitable or likely, but before a determination is made that a crash is occurring, to inflate the air-bag; and in response to a determination from the crash sensor that a collision is occurring, to vent the air-bag after inflation in such a way as to protect a pedestrian who is struck by the vehicle, wherein the venting of the air-bag is initiated a set length of time after the determination from the crash sensor, and wherein the control unit is adapted to calculate the set length of time based upon measured parameters relating to the size of the pedestrian, the relative speed between the vehicle and the pedestrian, and/or the speed of the vehicle, and wherein the calculation of the set time takes into account: an air-bag pressure profile, representing the pressure within the air-bag over time following inflation of the air-bag in the absence of a significant impact or other significant external forces acting on the air-bag; a drop in pressure within the air-bag over time following venting of the air-bag; a desired pressure within the air-bag at the moment of impact between the air-bag and a pedestrian; and an estimated length of time between the determination by the crash sensor and the moment of impact between the air-bag and a pedestrian. an air-bag pressure profile, representing the pressure within the air-bag over time following inflation of the air-bag in the absence of a significant impact or other significant external forces acting on the air-bag; a drop in pressure within the air-bag over time following venting of the air-bag; a desired pressure within the air-bag at the moment of impact between the air-bag and a pedestrian; and an estimated length of time between the determination by the crash sensor and the moment of impact between the air-bag and a pedestrian.