The tractor unit (1) of an articulated road vehicle includes an angle sensor unit (8) arranged to sense the angular position of the trailer (2) relative to the tractor and to cause the rear view mirror (4) for the driver to move so that he can always see the tail end of the trailer. According to this invention the sensor unit (8) is deflectable mounted (12, 12A) on the tractor unit (1) so that it be moved out of harms way when hit, e.g. by a trailer.
