A sensing device 26 is attached to a tensioned strap 20 holding a load on a vehicle. A probe 44 engages the strap 20 and causes a signal lamp 28 to light up if the tension in the strap 20 is below a predetermined threshold value. The sensing device 26 helps the vehicle operator to set the tension correctly before a journey and also provides a visible warning if a strap becomes loose during the journey. The warning is visible beyond the vehicle, to alert other road users to a possible hazard.