Pavement-marking sheet material made from unvulcanized elastomer precursors provide traffic control markings of superior durability because of their deformability and reduced elasticity. Such sheet material deforms readily into intimate contact with the irregular pavement surface; it absorbs the energy of wheel impacts without fracture; and its low elasticity avoids the stretch-return action that has been found to loosen sheet material from a roadway.
A deficiency of such deformable marking materials is that they have been unavailable in satisfactory retroreflective forms, apparently because their deformability prevents traditional ways of providing retroreflectivity. Conventional pavement markings include a firm supporting structure, such as a metal foil or a dried polymeric paint matrix, on which retroreflective microspheres may be supported. The deformable pavement-marking sheet materials or tapes do not provide such a support, with the result that microspheres applied to the top surface of such markings become embedded into the tapes under the pressure of road traffic.
One prior-art teaching (Eigenmann, U.S. Pat. No. 3,587,415) seeks to avoid this deficiency in deformable pavement-marking tapes by making such a tape in two levels, one level comprising a continuous base strip adhered to a roadway, and the second level comprising cross-strips adhered to the top of the base strip and filled with microspheres, desirably in an amount of about 80 weight-percent. Microspheres contained in the cross-strips are said to be exposed at the vertical edges of the strips to provide reflection of light from the headlamps of vehicles traveling on the roadway. Whether or not useful retroreflectivity would be provided by the apparently minimally exposed microspheres, the construction is clearly not a fully effective answer to the need for a retroreflective deformable pavement-marking tape: such a two-level tape is expensive to manufacture; the base strip remains deformable, such that the cross-strips can be pressed into it; and vertical edges as described typically become covered by collected dirt.
It has also been contemplated that deformable pavement-marking sheet materials be reflectorized by use of very large retroreflective elements having diameters larger than the thickness of the pavement-marking sheet material. However, serious consideration of such an approach has been prevented by the practical unavailability of retroreflective elements having the needed strength, size, and optical properties for such a use.
Others have sought to reflectorize deformable sheet materials by use of stiffer, less deformable tape formulations, but these constructions sacrifice the superior durability provided by reduced elasticity and deformability.
In short, none of the prior-art suggestions has resulted in a deformable pavement-marking sheet material that exhibits desired durability, reflectivity, and moderate cost. Until there is such a sheet material, the full potential of deformable pavement-marking sheet materials for traffic control purposes will not be realized.