Roadmarkers are mounted on the surface of a roadway, such as along its center line or shoulders, to delineate paths or lanes for fast-moving traffic, or at intersections to define left-turn lanes, stopping lines or cross-lanes for traffic, both vehicular and pedestrian. Markers of this type are mounted in spaced apart relationship and serve to guide traffic in following or traversing a roadway, or in following a curve or grade in the roadway. Particularly, to assist a driver of a vehicle at night, these markers have light reflectors which catch and return rays of light from vehicle headlights back toward the driver. Road markers further contribute to traffic safety when roads are wet from rain, when fog tends to obscure the center line or shoulder boundaries, or when the glare of oncoming headlights makes it difficult to discern center lines or shoulder boundary markers. Indeed, under the conditions described above, road markers can frequently be the only means of orienting a driver to a change in the direction of a highway.
Many forms of light reflectors have been suggested and patented, but they all suffer from one or more limitations, such as reflecting too small a proportion of incident light back toward an approaching vehicle, or in reflecting back insufficient light such as on an inside or outside curve, when the vehicle's headlights' beams are not at precisely 90.degree. with respect to a flat reflective roadmarker face. Furthermore, for any given set of vehicle headlights, there will be an optimum distance ahead of the vehicle when maximum reflectivity is realized from conventional roadmarkers, with such roadmarkers more distant, and those closer to the vehicle, reflecting less light. The driver of the vehicle therefore, has the benefit of only a relatively short segment of continuing reflectivity during forward progress.
In order to avoid interference with traffic, roadmarkers are usually of a relatively low-profile configuration, so that wheeled traffic is free to roll over them without appreciably interfering with the forward progress of the vehicle. Exemplary of such roadmarkers which have proved moderately successful in the past are those covered by the U.S. Pat. No. 3,332,327, to Heenan, issued in July, 1967. That roadmarker was characterized by having a pair of substantially flat, planar retroreflective faces reflecting in opposite directions, to thereby serve as a centerline marker or lane divider, for traffic traveling in opposite directions. Preferably, the planar face of the Heenan roadmarker formed an angle of 30.degree. with the supporting pavement.
U.S. Pat. No. 4,076,383 to Heasley, issued in February, 1978, exemplifies an attempt to increase the efficiency of a retroreflective roadmarker by coacting three, retroreflective, substantially planar faces adapted to intercept light that was to be retroreflected.
One of the principal drawbacks of both the Heenan and Heasley retroreflectors is that, by design, they return most retroreflected light in a path substantially parallel to that of the incident light. In other words, as the incident light strikes the planar faces of Heenan and Heasley at an increasing angle past 90.degree., less and less retroreflected light is observed by the driver of the vehicle.
In other words, unless the incident light from an oncoming vehicle is substantially normal to the leading edge of a planar retroreflector, there is a reduction in reflected light back toward the driver, which reduction increases proportionately to the reduced angle at which the incident light strikes the planar retroreflector, vis-a-vis a line describing its leading, straight edge.