GB2048508A - Retroreflective marking tape - Google Patents

Abstract

A retroreflective marking material comprises a sheet having at least some light-transmitting portions, a light- refracting surface on one side, and a plurality of trihedral light-reflecting units 35a, b of three mutually perpendicular facets on the other side adapted to be adhered to a working site 10 by adhesive 15. The light-reflecting units are oriented to reflect light having relatively high angles of incidence with respect to the sheet. The marking material may be used as a tape for marking roads. The sheet or tape may have light-receptors 34 which rise above the tape and are spaced apart along one direction of the tape, and the trihedral light-reflecting units are substantially opposite the light-receptors. Preferably, the light-reflecting units have the shape of a triangular-based pyramid, and the marking material retroreflects light in two opposite directions. <IMAGE>

Description

SPECIFICATION Retroreflective marking tape This invention reiates to a retroreflector which may be used wherever light reflection is desired, but which is particularly useful in the form of a marking tape for roads, pavements, airport runways, and the like, and will hereafter be described with respect to this use.

Retroreflective tapes or strips are mounted on the surface of a road, such as along its centre line or shoulders, to delineate paths or lanes for traffic, or at intersections to define stopping lines or cross-lanes for traffic, both vehicular and pedestrian. Markers of this type are mounted in spaced apart relation and serve to guide traffic in following or traversing a road, or in following a curve or grade in the roadway.

Particularly to assist a driver of a vehicle at night, these markets have light reflectors which catch incident beams of light from vehicular headlights and return them towards the source of the light.

Since automobiles of recent vintage have quite powerful headlights, the use of marking tapes has become more sidespread. Marking tapes contribute to traffic safety particularly when roads are set from rain and under conditions such as fog.

Several forms of retroreflective marking tapes have been suggested. They each suffer from one or more limitations, such as reflecting too small a proportion of incident light while an approaching vehicle is still at an appreciable distance; or being susceptible excessively to exposure to the weather, particularly rain, alone or with subsequent freezing.

The practice has been to place light-reflecting elements at or even above the upper level of the road marking tape, but at this location, the light-reflecting units are not only subject to wear and even breakage from vehicular and foot traffic, but also to erosion and the damaging effects due to entry of water and the like. Further, the accuracy and intensity of light-reflection of such marking tapes are often adversely affected when wet by rain.

We have now devised an improved retroreflector in sheet form, which is of relatively simple design, has efficient retroreflectivity and which is particularly well suited for use as a retroreflective marking tape. In the form of a marking tape, the lightreflecting units are shielded by the tape rather than exposed by it, so that the units are protected against wear and destruction by the elements such as rain water.

According to the invention, there is provided in a retroreflective marking material comprising a sheet having at least some light-transmitting portions, a light-refracting surface on one side and a plurality of trihedral light-reflecting units having three mutual perpendicularfacets on the other side, said units being oriented to reflect light striking said sheet at relatively high angles of incidence, and being adapted to be adhered to a working site.

The invention also provides a retroreflective marking tape comprising a light-transmitting strip having one side provided with light-receptors which rise above said one side and are spaced apart lengthwise of the strip, and light-reflecting units formed at the interface of said light-receptor and said one side of the strip, the other side of said strip being adapted to be adhered to a working site.

The invention further includes a bidirectional retroreflector comprising a light-transmitting body having a light-refracting surface and a plurality of light-reflecting units adapted to receive and retroref lect light refracted by said surface, said light reflecting units comprising juxtaposed rows of units disposed transversely to the direction of the light to be reflected, each row of units comprising threefaceted, triangular-based pyramids, said three facets being mutually perpendicular, and said pyramids being arranged in alternating order, such that the transversely disposed facets of two pyramids are separated by the point of an intervening third pyuramid, the bases of said pyramids merging with and forming a part of said body of the retroreflector.

The invention also provides an integral, retroreflective tape for marking roads and the like, comprising a light-transmitting strip having a light-refracting surface on one side and a plurality of trihedral light-reflecting units on the other side, said other side being adapted to be adhered to a working site, said trihedral light-reflecting units comprising juxtaposed rows of units disposed transversely to the direction of the light to be reflected, each row of units comprising three-faceted, triangular-based pyramids, said three facets being mutually perpendicular, and said pyramids being arranged in alternating order, such that the transversely disposed facets of two adjacent pyramids are separated by the point of an intervening third pyramid.

In one form, the retroreflective marking material of the invention is preferably fabricated from an organic polymeric resin in the form of a sheet having at least some light-transmitting portions, a lightreflecting surface on one side, and a plurality of trihedral light-reflecting units having three mutual perpendicular facets on the other side, such other side being adapted to being adhered to a working site. As a modification, light-receptors may rise above one side of the sheet and be spaced along one direction of the sheet, but in the present construction thetrihedral light-reflecting units are shielded by being spaced on the other side of the sheet or at the interface between the sheet and the light receptors.

The lightreflecting units are preferably coated with metal to aid in their reflecting function and can be arranged to be bi-directional with respect to retroreflecting light.

Although the light-reflecting units must be trihedral and therefore comprise three mutually perpendicular facets, the size of the facets and their optical orientation may vary. For example, such units may be cube corners or rectangular parallelepipeds. A preferred form is that a triangular-based pyramid which is well suited for use in bidirectional reflection.

In order that the invention may be more fully understood, reference is made to the accompanying drawings, in which: Figure 1 is a fragmentary, perspective view of a road having two forms (by way of illustration only) of retroreflective maring tape of the invention adhered thereto; Figure 2 is an enlarged cross-section of Figure 1 on the line 2-2; Figure 3 is an enlarged cross-section of Figure 1 on the line 3-3; Figure 4 is a cross-section like Figure 3 and illustrates light-reflecting units at the interface of light-receptors and the body of a marking tape; Figure 5 is a cross-section like Figure 3 and shows an alternate shape for a light-receptor as well as an alternate location of the light-reflecting units; Figure 6is a cross-section like Figure 3 and illustrates one arrangement for bidirectional lightreflection;; Figure 7 is a cross-section like Figure 3 and shows the use of an elastically deformable light-receptor; Figure 8 is a section of Figure 7 on the line 8-8; Figure 9 is a fragmentary, enlarged, longitudinal section of one form of marking tape of the invention, and a section of Figure 10 on the line 9-9, and shows the use of cube corners as light-reflecting units; Figure 10 is a view of Figure 9 on the plane of the line 10-10; Figure 11 is a fragmentary, enlarged longitudinal section of another form of marking tape of the invention, and illustrates the use of rectangular parallelepipeds as the light-reflecting units; Figure 12 is a view of Figure 11 on the plane of the line 12-12; Figure 13 is a fragmentary, enlarged, bottom plan view of another form of marking tape of the invention having triangular-based pyramids as lightreflecting units;; Figures 14 and 15 are sections of Figure 13 on the lines 14-15 and 15-15, respectively; and Figure 16 is a perspective view of triangular-based pyramidal light-reflecting units in which the outlines of the bases of the pyramids are isosceles triangles.

Referring to Figure 1, a road 10 which may be of any known construction has retroreflective marking tapes 11 and 12 adhered thereto which represent two different embodiments of the invention and which are shown in enlarged cross-section by Figures 2 and 3, respectively. Any embodiment of the tape may be in the form of a straight strip, continuous or discontinuous, or arranged in the form of circles, polygons, arrows, letters, symbols, and the like. Preferably, the tape is flexible to facilitate handling and application.

In Figure 2, tape 11 has a refracting surface 13 and a plurality oftrihedral light-reflecting units- 14, having three mutually perpendicular facets, on the other side. An adhesive 14 secures rape 11 to road 10. The compositions used for tape 11 and adhesive 15 may be the same for all embodiments. The body of tape 11, for example, must at least have some portions that are light-transmitting through which light reaches light-reflecting units 14 and may be fabricated from any suitable durable, weather resistant material, such as ceramic, glass, or synthetic resinous plastics materials, such as polyvinyl chloride, polycarbonates such as the polycarbonate of bisphenol A, and especially acrylates such as polymethacrylate and polymethyl methacrylate resins.

Such compositions may be glazed or pigmented, if desired, to impart colours.

Adhesive 15 may be any suitable material sufficiently adhesive to hold tape 11 with respect to road 10. Examples of suitable adhesives include natural materials like glue or bitumen, or synthetic resinous adhesives like epoxy, polyester, or polyurethane resins, Thermosetting cements such as rubber hydrochloride may also be used. Still other useful adhesives are described in U.S. Patents Nos.

3,585,415 and 3,935,365, to which reference should be made for further details.

The trihedral light-reflecting units 14, have three mutually perpendicular facets and may, for example, be any of those hereinafter more fully described. In the embodiments of Figure 2, units 14 extend continuously along the underside of tape 11. Because of the positioning of the marking tape on a road, that is appreciably below normal eye level, units 14 must be oriented to reflect light having relatively high angles of incidence with respect to the tape. This is illustrated in Figure 2 by an incident ray of light represented at 16 which strikes tape 11 at an angle of incidence A that becomes almost a glancing angle. Ray 16 is refracted by surface 13, retroreflected by units 14, and returned in substantially the same direction, as illustrated.

As used herein, the optical axis of a light-reflecting unit 14 is that axis of the trihedral configuration along which the unit is maximally retroreflective.

The optical axes of units 14 are preferably substantially parallel to the light they receive, that is to the direction ray 16 takes after being refracted by surface 13. However, satisfactory performance can still be achieved if the optical axes deviate from the direction of the refracted light, for example by up to about 15 degrees. To aid in their reflecting function, light-reflecting units 14, and the units of any embodiment, may be coated with metal or metallized in a manner known in the artto form a metallic layer 17.

Aluminum is the preferred metal for this purpose and can be applied by vapor deposition. Adhesive 15 fills the volume between the lower, embossed side of tape 11, having the light-reflecting units 14, and road 10 to secure tape 11 to the road.

The size of the tape is not critical. In one instance, a marking tape was 4 inches in width and about 2 mils to about 10 mils in thickness. In the embodiment of Figure 2, the body of tape 11 actually protects light-reflecting units 14 from wear and exposure to the elements. The tape is operative whether wet or dry.

Figures 3 to 8 illustrate modified forms of the marking tape which include the use of lightreceptors and optional positioning of the lightreflecting units. It is within the present invention to incorporate light-receptors which rise above the tape and are spaced apart along one direction. The light-receptors may have any polygonal or spherical cross-section and are used to garner rays of incident light and assist in focusing them toward the lightreflecting units. The latter can be placed at different stations below the light-receptors while still constituting an integral part of the tape and still being protected by an overlying segment of the tape.

For example, in Figure 3, a tape 12 has lightreceptors 19 which rise above intervening flat areas 20 so as to be spaced lengthwise of the tape.

Light-receptors 19 are triangular in cross-section in the longitudinal direction of tape 12. Light-reflecting units 14 extend continuously along the underside of tape 12 although, if preferred, they can extend along only that section of the underside which is substantially opposite light-receptors 19. An adhesive 15 secures tape 12to road 10. The use of a lightreceptor 19 permits a slightly different orientation of light-reflecting units 14 in aligning their optical axes with respect to the direction of the refracted light.

For example, an incident ray of light 21 may be refracted and retro-reflected as illustrated in Figure 3.

It is possible to place light-reflecting units at the interface of light-receptors and the body of the tape.

In this case, the light-receptors may be separately fabricated and secured to the body of the tape by an adhesive. Figure 4 illutrates a triangular lightreceptor 23 having a plurality of light-reflecting units 24 formed as by embossing along its flat base.

An adhesive 25 fills the cavities left by the light-reflecting units and secures the light-receptor 23 to a strip 26 defining the marking tape, and adhesive 15 secures strip 26 to road 10. Adhesive 25 may be any of those previously described for adhesive 15. The separate fabrication of lightreceptor 23 and strip 26 enables each to contan a different colouring pigment. For example, strp 26 can be coloured white for day-light marking, while light-receptor 23 can be tinted red or green or still other colours. In this case only light-receptor 23 need be light-transmitting.

Figure 5 illustrates a light-receptor 27 having a quadrilateral cross-section and also a further modification in that the wide bottom side of receptor 27 is flat. In this case, light-reflecting units 28 are formed in an upper surface 30 of a strip 31 defining the tape.

An adhesive 32 fills the cavities of the light-reflecting units 28 to secure receptor 27 and strip 31 together, and adhesive 15 secures strip 31 to roadway 10.

However, since in this case adhesive 32 fills units 28 and is therefore interspersed between light-receptor 27 and units 28, it is necessary that adhesive 32 be light-transmitting. Light-transmitting adhesives which can be used for this purpose are known in the art and include transparent epoxy resins, transparent polyurethanes, solvent-based synthetic rubbers, and the like. In the embodiments of Figures 4 and 5, strips 26 and 31 need not be light-transmitting and can be opaque. For this purpose such strips can be coloured white, red, green, or still other colours.

It is also possible to make the marking tape bidirectional, that is to receive and retroreflect light from either or both of two opposite directions.

Figure 6 illustrates one embodiment of this modification in conjunction with a light-receptor and the use of light-reflecting units disposed only substantially opposite a light-receptor, although these structural features are not essential to bidirectional retroreflection. In the embodiment of Figure 6, a tape 33 has a triangular shaped light-receptor 34 with lightreflecting units embossed along the underside of the tape, as in the embodiment of Figure 3. However, in this instance the units extent along the underside a distance which corresponds generally to the length of receptor 34 along the tape and, further, are oriented so as to be bidirectional.Thus, lightreflecting units 35a on the left hand side as viewed in Figure 6 have their optical axes arranged to be preferably parallel orto be deviate by no more than about 15 degrees from light approaching from the right side of the Figure. The retroreflection of a ray of incident light 36 occurs therefore as illustrated in Figure 6. Light approaching from the left side receives little retroreflectivity from units 35a. Conversely, light-reflecting units 35b on the right side as viewed in Figure 6 have their optical axes arranged to be preferably parallel or to deviate no more than about 15 degrees from light approaching from the left,. The retroreflection of a ray of incident light 37 occurs therefore as illustrated in Figure 6.Light approaching from the right receives little retroreflection from units 35b. If desired, that portion of the tape contaning units 35a may be tinted one colour such as green, and that portion of the tape containing units 35b can be tinted another colour such as red.

Marking tapes, and especially those having upward projections like the light-receptors are subject to rugged wear and treatment because of being almost constantly over-ridden by vehicular traffic, such as automotive tyres, snow plows, and the like.

In one form, a light-receptor of the marking tape may be elastically deformable and thereby adapted to return substantially to its original position after deformation. In this manner, the light-receptor resiliently yields to over-rolling vehicular traffic and prolongs its useful life. Yet, after deformation, the light-receptor immediately returns substantially to its original position where it is useful as before. For this purpose, the light-receptor can be fabricated from various elastomeric, resinous synthetic plastics materials. Polyvinyl chloride is one such elastomer that may be so used.

Figures 7 and 8 illustrate a marking tape having an elastically deformable light-receptor as well as an adaptation of this form for bidirectional lightretroreflection. Preferably, the light-receptor is hollow and the light-reflecting units are disposed along an inner side of the light-receptor. In the illustrated embodiment, a marking tape 71 has a light-receptor 72 of triangular cross-section in the longitudinal direction of the tape. Light-receptor 72 is hollow and may be open from the underside of tape 71 as illustrated. Adhesive 15 secures tape 71 to road 10.

Light-reflecting units- are conventionally formed along an inner side of light-receptor 72.

The illustrated embodiment is bidirectional. Lightreflecting units 73a and 73b are formed along inner and opposite sides of light-receptor 72. Thus, an incident ray of light 74 approaching from the left as viewed in Figure 7 is refracted by the left hand side of the triangular shaped light-receptor 72 and retroreflected by light-reflecting units 73a as illustrated.

An incident ray of light 75 approaching from the right is refracted by the right hand side of triangular light-receptor 72 and retroreflected by light reflecting units 73b. In use, light-receptor 72 can be smashed flat and immediately bounce back to its original station. Alternatively, hollow-light-receptor 72 may be filled with a compressible material, such as foamed elastomeric material, which is secured inside the hollow-pocket formed by light-receptor 72 and/orto road 10.

The light-reflecting units, such as those illustrated at 14,24,28, 35a, 35b, 73a, and 73b have been previously described as trihedral light-reflecting units having three mutually perpendicular facets. Figures 9 to 16 illustrate some of the configurations such trihedral units may take. These may be summarized as cube corners, rectangular parallelepipeds, truncated rectangular parallelepipeds, and truncated cube corners. In all of the Figures, it will be appreciated that the light-reflecting units are shown greatly oversize to facilitate their illustration and description.

Figures 9 and 10 illustrate light-reflecting units which are cube corners, sometimes referred to as triple reflectors, in which the light-reflecting units comprises three square faces which are mutually perpendicular. Since the sides are square and equal in size and thus simulate a cube, such a reflecting unit has come to be called a "cube corner". In Figures 9 and 10, the cooperating sides of a single cube corner unit are illustrated at 39,40, and 41.

Such a unit is repeated to form a row 42, and likewise such rows are repeated in parallel fashion to form a light-reflecting array along the underside of a marking tape 43. By the very nature of cube corners, it is difficult if not impossible to avoid optically inactive bands or blinds spots 44 (Figure 10) in forming the cube corners by pins. In a commercial production, rows of such pins are somewhat longitudinally offset with respect to each other unavoidably forming optically inactive bands 44 in the lightreflecting array. These bands represent loss of reflective area and accordingly diminish the effectiveness and light-reflecting intensity of the array.

Also, while a cube corner structure provides satisfactory reflectance to light striking perpendicularly against an array or strip of cube corners, that is, generally parallel to the axes of the cube corners, its performance falls off fairly rapidly as incident light strikes the array at angles away from the normal to the surface of the cube corner array.

A more preferred trihedral light-reflecting unit is illustrated by Figures 11 and 12 and is herein termed a rectangular parallelepiped. If a polyhedron is a solid bounded by planes, and a prism is a polyhedron of which two faces are congruent polygons in parallel planes, and the other faces are parallelograms having two of their sides in the two parallel planes, a parallelepiped may be broadly defined as a prism whose bases are parallelograms. A right parallelepiped, then, is a parallelepiped with edges perpendicular to the bases. The term "rectangular parallelepiped" can mean a right parallelepiped whose bases or sides are rectangles. However, some authorities broadly defined a rectangle as a parallelogram whose four angles are right angles. This definition generically includes a square as a rectangle.At least two of the three rectangular faces which form the present rectangular parallelepiped lightreflecting unit cannot be square. Each of the at least two faces must be rectangular under the more generally used definition that two adjacent edges of a rectangle are unequal in length. As used here and in the claims, the term "rectangular parallelepiped" means a right parallelepiped of which at least two faces or facets are rectangular in the sense that each such rectangle has two adjacent unequal length edges, The rectangular parallelepiped herein contemplated may be regarded as the volume occupied by a six-faceted box, all six facets being a quadrilateral with four right angles.In an actual light-reflecting unit, only three of the facets which meet in a point are actually exposed on the underside of the tape, at least two of those facets being rectangular in which two adjacent edges of each of the two rectangular facets are unequal. Preferably, the two rectangular facets, as defined, are equal in size of area, and the third facet is a square. The three facets of the rectangular parallelepiped may also be considered as forming the sides of a trihedral angle with two facets being rectangular as defined.

Referring to Figures 11 and 12, a marking tape 45 has a plurality of light-reflecting units 46 along its underside comprising three mutually perpendicular facets 47, 48, and 49 which define a trihedral angle of a rectangular parallelepiped as described. Facets 47 and 48 intersect each other in a direction toward refracting surface 51 of tape 45 to form an intersecting line 52 and define the two rectangular facets of the rectangular parallelepiped previously described.

Facet 49 is a square in the preferred embodiment. A light-reflecting unit 46 is preferably so positioned with respect to refracting face 51 that a body diagonal of the rectangular parallelepiped which is also its optical axis is preferably substantially parallel to incident light refracted by surface 51, although the body diagonal can deviate therefrom by up to about 15 degrees. A body diagonal connecting opposite corners of the rectangular parallelepiped of a unit 46 is shown at 53 in Figure 12. In the preferred form, facets 47 and 48 intersect facets of adjoining light-reflecting units 46 in a direction away from refracting surface 51 to form a second intersecting line 54. This line is not only substantially parallel to the first mentioned intersecting line 52 but, in the embodiment illustrated, is substantially aligned with intersecting line 52 of an adjacent row.

The best light-reflecting unit found to date for the marking tape is that of a triangular based pyramid as illustrated by Figures 13, 14 and 15. This unit is well suited for light having high angles of incidence (grazing incidence) which is needed in a marking tape, and by virtue of its configuration such a unit can readily be nested with other like units to provide bidirectional retroreflection. More particularly, this type of unit is a truncated rectangular parallelepiped forming an asymmetrical triangular-based pyramid.

The three mutually perpendicular facets are those facets which meet the triangular base, one of the facets having a much smaller area than the other two. Such pyramids can be formed, for example, by cutting three parallel sets of grooves at specific spacings and angles, note Figure 13. The width and depth of the grooves may be varied depending on the retroflection desired. Each groove forms two adjacent facets of two different pyramids. The resulting reflector has two optical axes and therefore is bi-directional in light reflection, each axis being at the same angle to the surface normal but in opposite azimuthal directions.

Referring to Figures 13 to 15, a marking tape 56 has a plurality of light-reflecting units generally represented at 57. Each unit 57 has three mutually perpendicular facets indicated at 58, 59 and 60.

Facets 58 and 59 converge as viewed in Figure 13 to an intersecting line 61 and are perpendicular to facet 60. The pyramid of the three facets 58,59 and 60 define a unit 57 which projects from the plane of the bottom of underside of tape 56, such that the base of the pyramid from which facets 58, 59 and 60 extend actually merges with the body of tape 56. The triangular bases of the pyramids may, therefore, be contained in an imaginary plane indicated in Figures 14 and 15 at 62, plane 62 being essentially parallel to a refracting surface 63 of tape 56.

As shown in Figure 13, light-reflecting units 57 are placed in juxtaposed rows indicated at 64, each row being placed transversely to the direction of light to be retroreflected. Each row 64 consists of pyramidal units arranged in alternating order, such that the transversely disposed facets 60 and 60a of two adjacent pyramids are separated by a point 65 of an intervening third pyramid. This alternating positioning enables the array of units 57 to be bidirectional.

Thus, as viewed in Figure 13 and 14, facets 58,59 and 60 of one light-reflecting unit and facets 58a, 59a and 60a of another similarly disposed light-reflecting unit and still other units similarly disposed receive and retroreflect light coming from the right. In contrast, facets 67, 68 and 69 of one light-reflecting unit and facets 67a, 68a and 69a of another lightreflecting unit as well as other units similarly disposed receive and retroreflect light coming from the left. Each row 64 opticaliy behaves the same.

When the pyramidal units 57 are truncated rectangular parallelepipeds, the outlines of the triangular bases of the projecting pyramids are isosceles triangles. When the pyramidal units are truncated cube corners, the outlines of the triangular bases of the projecting pyramids are equilateral triangles. As a further guide in illustrating the pyramidal type of light-reflecting unit, Figure 16 illustrates a perspective view of truncated rectangular parallelepipeds.

Figure 16 is taken on the imaginary plane 62 of Figure 14.

In Figure 16, for instance, the plane of the sheet of drawing contains the triangular bases of the pyramidal light-reflecting units 57. Thus, points A, B, and C lie in the plane of the sheet of drawing and point D lies depressed below the plane of the sheet of drawing. The pyramid defined by points ABCD is a truncated rectangular parallelepiped, and the outline of base ABC is an isosceles triangle. Similarly, points A, C, and E lie in the plane of the sheet of drawing and point F lies depressed below the plane of the sheet of drawing. The pyramide defined by points ACEF is also a truncated rectangular parallelepiped, and the outline of base ACE is an isosceles triangle.

In the case of a truncated cube corner, the outline of base ABC or base ACE is an equilateral triangle.

It will be apparent that all light-reflecting units can be metallized to aid in their reflecting function as illustrated at 17 in Figure 2. In the other Figures, metallization has not been shown to facilitate illustration of the structures.

Claims (28)

1. A retroreflective marking material comprising a sheet having at least some light-transmitting portions, a light-refracting surface on one side and a plurality of trihedral light-reflecting units having three mutually perpendicular facets on the other side, said units being oriented to reflect light striking said sheet at relatively high angles of incidence, and being adapted to be adhered to a working site.

2. A material according to claim 1, in which said one side of the sheet has light-receptors which rise above said side and are spaced apart along one direction of the sheet, and said trihedral lightreflecting units are on said other side of the sheet substantially opposite the light-receptors.

3. A material according to claim 2, in which said light-receptors have a triangular cross-section disposed in said one direction of the sheet.

4. A material according to claim 2, in which said light-receptors have a quadrilateral cross-section disposed in said one direction of the sheet.

5. A material according to claim 2, in which some of said trihedral light-reflecting units are disposed to retroflect light in one direction and some of said trihedral light-reflecting units are disposed to retroflect light in the opposite direction, whereby said marking material is bidirectional with respect to retroreflecting light.

6. A material according to claim 1, in which said one side of the sheet has light-receptors which rise above said side and are spaced apart along one direction of the sheet, said light-receptors being elastically deformable and adapted to return substantially to their original positions after deformation.

7. A material according to claim 6, in which said light-receptors are hollow and said light-reflecting units are disposed along an inner side of the follow light-receptors.

8. A material according to claim 6, in which said light-receptors are hollow and open from said other side of the sheet and said light-reflecting units are disposed along inner and opposite sides of the light-receptors to render them bidirectional with respect to retroreflecting light.

9. A material according to claim 1, in which said trihedral light-reflecting units are so disposed that their optical axes are within 15 degrees of the direction of the refracted light.

10. A material according to claim 1, in which said trihedral light-reflecting units are so disposed that their axes are substantially parallel to the direction of the refracted light.

11. A material according to any preceding claim, in which said light-reflecting units are metallized.

12. A material according to any preceding claim, in which said trihedral light-reflecting units- are cube corners.

13. A material according to any of claims 1 to 11, in which said trihedral light-reflecting units are rectangular parallelepipeds.

14. A material according to any of claims 1 to 11, in which said trihedral light-reflecting units are triangular-based pyramids.

15. A material according to claim 14, in which the outlines of the bases of said pyramids are isosceles triangles.

16. A material according to claim 14, in which the outlines of the bases of said pyramids are equilateral triangles.

17. A material according to any preceding claim, in which said sheet is a tape adapted to be secured to a roadway.

18. A retroreflective marking tape comprising a light-trasmitting strip having one side provided with light-receptors which rise above said one side and are spaced apart lengthwise of the strip, and lightreflecting units formed at the interface of said light-receptor and said one side of the strip, the other side of said strip being adapted to be adhered to a working site.

19. A tape according to claim 18, in which said light-reflecting units are formed in a light-receptor, and an adhesive substantially fills said units and adheres said light-receptor to said one side of the strip.

20. Atape according to claim 18, in which said light-reflecting units are formed in said one side of the strip, a light-receptor seats over said lightreflecting units, and a light-transmitting adhesive fills said light-reflecting units and adheres together said light-receptor and said one side of the strip.

21. A bidirectional retroreflector comprising a light-transmitting body having a light-refracting surface and a plurality of light-reflecting units adapted to receive and retroreflect light refracted by said surface, said light-reflecting units comprising juxtaposed rows of units disposed transversely to the direction of the light to be reflected, each row of units comprising three faceted, triangular-based pyramids, said three facets being mutually perpendicular, and said pyramids being arranged in alternating order, such that the transversely disposed facets of two pyramids are separated by the point of an intervening third pyramid, the bases of said pyramids merging with and forming a part of said body of the retroreflector.

22. An intergral, retroreflective tape for marking roads and the like, comprising a light-transmitting strip having a light-refracting surface on one side and a plurality of trihedral light-reflecting units on the other side, said other side being adapted to be adhered to a working site, said trihedral lightreflecting units comprising juxtaposed rows of units disposed transversely to the direction of the light to be reflected, each row of units comprising threefaceted, triangular-based pyramids, said three facets being mutually perpendicular, and said pyramids being arranged in alternating order, such that the transversely disposed facets of two adjacent pyramids are separated by the point of an intervening third pyramid.

23. Atape according to claim 22, in which said one side of the strip has light-receptors which rise above said side and are spaced apart lengthwise of the strip, and said pyramidal light-reflecting units are on said other side of the strip substantially opposite to said light receptors, the bases of said pyramids merging with and forming a part of said strip.

24. A tape according to claim 22 in which said one side of the strip has Iight-receptswhich rise above said one side and are spaced apart lengthwise of the strip, and said pyramidal light-reflecting units are formed at the interface of said light-receptors and said one side of the strip.

25. Atape according to claim 22 in which saidpyramidal light-reflecting units are metallized.

26. A tape according to claim 22 in which the outline of the bases of said pyramids are isosceles triangles.

27. Atape according to claim 22 in which the outlines of the bases of said pyramids are equilateral triangles.

28. A retroreflective marking material substantially as herein described with reference to Figures 1 to 4, or Figure 5, or Figure 6, or Figures 7 and 8, or Figures 9 and 10, or Figures 11 and 12, or Figures 13 to 15, or Figure 16 of the accompanying drawings.