A raised road marker has a flat bottom surface with an outer periphery. At least one cavity opens onto the bottom surface laterally inwardly of the periphery and extends upwardly into the marker body. A venting groove extends along the bottom surface from the cavity to the periphery to provide a vent for air trapped in the cavity when the marker is urged into a pool of adhesive to secure the marker to a roadway. The venting of air provides a stronger bond between the marker and adhesive. For markers having a rounded top surface, there is preferably an indexing projection on the top surface and a complementary recess on the bottom surface. This enables interlocking of a plurality of markers in a vertical stack to prevent tilting of the markers.