A sheet-fed or web-fed printing machine comprises a main unit (1), at least one mobile carriage (2) adapted to be coupled to the main unit for cooperation therewith during printing operations and to be moved away from the main unit during maintenance operations, a control unit for controlling displacement of said mobile carriage toward and away from said main unit, and an access zone (9, 9') surrounding at least partly the mobile carriage for allowing a human operator to get access to the main unit and mobile carriage, which access zone lies outside of the displacement path (10) of the mobile carriage. The printing machine further comprises detection means (11) for monitoring the access zone and detecting presence of a human operator within the access zone. The control unit is operatively connected to the detection means so as to interrupt or prevent displacement of the mobile carriage in case presence of a human operator is detected within the access zone by the detection means and before the human operator penetrates in the displacement path of the mobile carriage.