A warning device used for road safety includes a warning sign and a driving mechanism. The driving mechanism includes a base, a driving unit, at least one wheel, a plurality of supporting units, and a plurality of driving members. The driving unit is accommodated in the base. The at least one wheel is connected to and driven by the driving unit. The plurality of supporting units are movably connected to the base. The plurality of driving members can manipulate the supporting units. Each of the supporting units is movable between a receiving or retracted position and a supporting or extended position. For stability in placement, the device can be lifted a small distance above the road by the supporting units when these are deployed. The driving mechanism is also disclosed.