Abstract: A mobile device for forcing a highway post into the ground in the vertical position, especially where the highway post is flexible and made of fiberglass, which involves a vertical column spaced from and supported by the mobile device, guide means mounted on the vertical column to hold a reflective highway post in a vertical position while the post is forced into the ground by repetitive blows from a hydraulically driven hammer and means for moving the vertical column and the guide means upwards in a vertical direction when the post has been forced a sufficient distance into the ground.