A rear underrun protection device (1; 100; 200; 300) for a towing vehicle, the rear underrun protection device (1; 100; 200; 300) comprising: an impact bar intended to be arranged such that a longitudinal extension of the impact bar extends laterally with respect to the towing vehicle; two support arms (3, 3'), each arm (3, 3') having a first end portion (3a, 3a') mounted to the impact bar on different sides of a longitudinal center of the impact bar and a second end portion (3b, 3b') intended to be rotatably connected to the towing vehicle so that the support arms (3, 3') are rotatable around an axis of rotation; and at least one locking member (12; 104; 208; 302; 402) configured to move between an engaged position, in which the locking member (12; 104; 208; 302; 402) is intended to engage a structure of the towing vehicle, and a disengaged position, in which the locking member (12; 104; 208; 302; 402) is intended to be disengaged from the towing vehicle, the locking member (12; 104; 208; 302; 402), when in use and in the engaged position, being adapted to lock one of the support arms (3, 3') in a first position, in which the impact bar is positioned to prevent another vehicle from running under the towing vehicle during a collision, and in a second position, in which the impact bar is positioned to allow a trailer to be connected to the towing vehicle, characterized by an operating assembly having a first point of engagement operatively connected to the locking member (12; 104; 208; 302; 402) and a second point of engagement with said one of the support arms (3, 3'), the operating assembly comprising a driver (9; 201; 301; 401) movable in a uniform direction and a transmission arrangement configured to, upon a uniform movement of the driver (9; 201; 301; 401): in a first phase, move the first point of engagement substantially parallel to said axis of rotation, to move the locking member (12; 104; 208; 302; 402) to its disengaged position; in a second phase, move the second point of engagement around said axis of rotation, to bring said one of the support arms (3, 3') to rotate between said first and second positions; and in a third phase, move the first point of engagement substantially parallel to said axis of rotation, to return the locking member (12; 104; 208; 302; 402) to its engaged position. an impact bar intended to be arranged such that a longitudinal extension of the impact bar extends laterally with respect to the towing vehicle; two support arms (3, 3'), each arm (3, 3') having a first end portion (3a, 3a') mounted to the impact bar on different sides of a longitudinal center of the impact bar and a second end portion (3b, 3b') intended to be rotatably connected to the towing vehicle so that the support arms (3, 3') are rotatable around an axis of rotation; and at least one locking member (12; 104; 208; 302; 402) configured to move between an engaged position, in which the locking member (12; 104; 208; 302; 402) is intended to engage a structure of the towing vehicle, and a disengaged position, in which the locking member (12; 104; 208; 302; 402) is intended to be disengaged from the towing vehicle, the locking member (12; 104; 208; 302; 402), when in use and in the engaged position, being adapted to lock one of the support arms (3, 3') in a first position, in which the impact bar is positioned to prevent another vehicle from running under the towing vehicle during a collision, and in a second position, in which the impact bar is positioned to allow a trailer to be connected to the towing vehicle, characterized by an operating assembly having a first point of engagement operatively connected to the locking member (12; 104; 208; 302; 402) and a second point of engagement with said one of the support arms (3, 3'), the operating assembly comprising a driver (9; 201; 301; 401) movable in a uniform direction and a transmission arrangement configured to, upon a uniform movement of the driver (9; 201; 301; 401): in a first phase, move the first point of engagement substantially parallel to said axis of rotation, to move the locking member (12; 104; 208; 302; 402) to its disengaged position; in a second phase, move the second point of engagement around said axis of rotation, to bring said one of the support arms (3, 3') to rotate between said first and second positions; and in a third phase, move the first point of engagement substantially parallel to said axis of rotation, to return the locking member (12; 104; 208; 302; 402) to its engaged position. in a first phase, move the first point of engagement substantially parallel to said axis of rotation, to move the locking member (12; 104; 208; 302; 402) to its disengaged position; in a second phase, move the second point of engagement around said axis of rotation, to bring said one of the support arms (3, 3') to rotate between said first and second positions; and in a third phase, move the first point of engagement substantially parallel to said axis of rotation, to return the locking member (12; 104; 208; 302; 402) to its engaged position.