A deformation guiding system for a road safety device and a road safety device group, wherein the device has a frame and a deformable longitudinal body with a first end fixed to the frame and a second end. A head is fixable to the second end of the body. A guide rail is reciprocally arranged with respect to the deformable body for identifying a deforming direction thereof which is parallel to the guide rail. Coupling means fixed to the head couple the head to the guide rail for sliding motion along the guide rail. Stabilizing means fixed to the head prevent oscillations of the head about the guide rail. The coupling and stabilizing means are reciprocally arranged and conformed so that, in an impact of a vehicle against the head, the head can slide along the guide rail without oscillating about the guide rail so as to guide the deformation of the deformable body in the deforming direction.