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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to crash cushions and terminals used in highway applications to mitigate and preclude injuries to occupants of errant vehicles. 
     2. Description of the Related Art 
     Roadway crash cushions are widely used to absorb impacts and decelerate impacting vehicles in a controlled manner. Typically, crash cushions are positioned to shield fixed objects located within the roadway environment. Crash cushions are often positioned in front of obstacles such as concrete columns and abutments. Also, crash cushions are often located at the end of a guardrail installation to prevent the upraised end of the guardrail from spearing an impacting vehicle. 
     There are numerous crash cushion designs known that rely upon frangible members, or members that are intended to shatter or be destroyed upon impact, to absorb the energy associated with a vehicular impact. Examples are found in U.S. Pat. Nos. 3,768,781 issued to Walker et al. and 3,982,734 issued to Walker (both employing energy cells having internal frangible members of e.g., vermiculite). One problem with the use of frangible members is the crash cushion must be completely replaced after each collision. Thus, time and expense is incurred in replacing the frangible members. 
     A number of previous crash cushion designs rely upon the permanent deformation of plastics or steels to absorb the kinetic energy of errant impacting vehicles. A design of that nature suffers from the same drawbacks as those designs incorporating frangible members. The cost and time associated with replacing or repairing the deformed portions of the cushion is significant. 
     There have been a few attempts to provide reusable or restorable crash cushions. However, for the most part, these attempts have proven impractical or unworkable in practice. U.S. Pat. No. 4,452,431 issued to Stephens et al, for instance, describes a crash cushion wherein fluid filled buffer elements are compressed during a collision. It is intended that energy be absorbed as the fluid is released from the buffer elements under pressure. In practice, it is difficult to maintain the fluid-filled cylinders as they are prone to loss of fluid through evaporation, vandalism and the like. Also, after a severe impact, holes or punctures may occur in the buffer elements rendering them incapable of holding fluid. 
     U.S. Pat. No. 4,674,911 issued to Gertz describes a pneumatic crash cushion that is intended to be reusable. This crash cushion employs a plurality of air chambers and valve members to absorb and dissipate impact energy. This arrangement is relatively complex and prone to failure. In addition, the numerous specialized components used in its construction make it expensive. 
     The Reusable Energy Absorbing Crash Terminal (“REACT”)  350  is a crash cushion wherein a plurality of polyethylene cylinders are used to absorb impact energy. The cylinders are retained within a framework of side cables and supporting frames. This system is effective and reusable to a great degree due to the ability of the cylinders to restore themselves after impact. The cylinders typically return to 85%–90% of their original shape after impact. Unfortunately, the REACT system is also expensive to construct. The number of manufacturers producing large diameter polyethylene cylinders is limited and, as a consequence, prices for the cylinders are elevated. 
     An improvement that addresses the problems of the prior art would be desirable. 
     SUMMARY OF THE INVENTION 
     The present invention provides devices and methods relating to roadway crash cushions. An energy absorbing terminal is described that is made up of a plurality of cells partially defined by cambered panels made of thermoplastic. The panels are supported upon steel diaphragms. The cambered portion of the thermoplastic panels provides a predetermined point of flexure for each panel and, thus, allows for energy dissipation during a collision. The stiffness of the crash cushion is variable by altering material thicknesses and diaphragm spacing. 
     In operation, a vehicle colliding in an end-on manner with the upstream end of the energy absorbing terminal will cause the cambered panels to bend angularly at their points of flexure and, thus, cause the cells to collapse axially. The use of thermoplastic, such as polyethylene, results in a reversible, self-restoring collapse of the terminal; meaning the terminal is reusable after most collisions. 
     The invention provides a number of advantages over conventional crash cushions, including cost, ease of construction, and maintenance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of an exemplary crash cushion arrangement constructed in accordance with the present invention prior to impact from an errant vehicle. 
         FIG. 2  is a side view of the arrangement depicted in  FIG. 1 . 
         FIG. 3  is a plan view of the exemplary crash cushion depicted in  FIGS. 1 and 2  after being struck by an impacting vehicle. 
         FIG. 4  is a front view of an exemplary diaphragm used within the crash cushion shown in  FIGS. 1 ,  2 , and  3 . 
         FIG. 5  is a side view of the diaphragm shown in  FIG. 4 . 
         FIG. 6  is a plan view of the diaphragm shown in  FIGS. 4 and 5 . 
         FIG. 7  is a schematic depiction of an exemplary crash cushion shown prior to an end on impact by a vehicle. 
         FIG. 8  is a schematic depiction of the crash cushion shown in  FIG. 7  at approximately 0.18 seconds following an end-on impact. 
         FIG. 9  is a schematic depiction of the crash cushion shown in  FIG. 7  at approximately 0.27 seconds following an end-on impact. 
         FIG. 10  is a schematic depiction of the crash cushion shown in  FIG. 7  at approximately 0.345 seconds following an end-on impact. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1–3  illustrate an exemplary hybrid energy absorbing reusable terminal (“HEART”) crash cushion  10  that is constructed in accordance with the present invention. The crash cushion  10  is shown installed on a concrete pad  12  (visible in  FIG. 2 ) that has been placed within a section of ground  14 . Although not shown, it should be understood that the crash cushion  10  is typically installed adjacent a rigid obstacle, such as a bridge abutment, concrete post or other barrier. In addition, the crash cushion  10  may be located at the upstream end of a guardrail installation. 
     The crash cushion  10  includes a nose portion  16 , central body portion  18  and downstream end portion  20 . An approaching vehicle  22  is shown adjacent the nose portion  16  of the cushion  10  in  FIGS. 1 and 2 . The nose portion  16  consists of a sheet of plastic, or other suitable material, that is curved or bent into a “U” shape. The nose portion  16  may be painted with a bright color, such as yellow, or have reflective tape applied so that the cushion  10  may be easily recognized by drivers. The downstream end portion  20  includes a pair of upstanding rigid posts  24 ,  26  that are typically formed of concrete or steel and are securely anchored, either to the ground  32  or to an adjacent abutment, post or other barrier (not shown). 
     The central body portion  18  also includes a steel basetrack formed from a pair of parallel rail members  28 ,  30  that are attached to the ground  32 . Anchor members  19 , such as bolts, are typically used to secure the rail members  28 ,  30  to a concrete slab  21 . The central body portion  18  features a plurality of openings  34  that are arranged linearly along the length of the cushion  10 . In the described embodiment, the openings  34  are shown to be hexagonally shaped. While the hexagonal shape is presently preferred, it should be understood that other suitable shapes may be used, including, for example, octagonal, rectangular and square. The central body portion  18  incorporates two substantially parallel rows  36 ,  38  of cambered panels that are arrayed in an end-to-end manner along their lengths. The panel rows  36 ,  38  may comprise a single integrally formed sheet of plastic. Alternatively, they may be formed of a number of individual cambered panel members placed in an end-to-end, adjoining manner at each rectangular frame  40 . It is presently preferred that the rows of panel members  36 ,  38  be formed of polyethylene. A suitable polyethylene material for use in this application is PPI recommended designation PE3408 high molecular weight, high density polyethylene. A currently preferred thickness for the panel members  36 ,  38  is approximately 1¼″. It is noted that the panel members  36 , 38  are created so as to be substantially stiff and sturdy in practice and to possess substantial “shape memory” so that they tend to substantially return to their initial form and configuration following elastic deformation. Presently, panel members having a secured in place height of about 20 inches have provided suitable resistance to collapse and sufficiently return to original shape. It is noted that the thickness of a given panel member as well as its height may be adjusted as desired to increase or decrease resistance to expected end-on collision forces. For example, increasing the height of the panel members  36 ,  38  will increase the amount of panel material that would be bent by a colliding vehicle and would, therefore, be stiffer than a cushion that incorporated panel members of lesser height. 
     The crushable cells include rectangular frames or diaphragms  40  that join the parallel panle rows  36 ,  38  together. In the drawings, individual diaphragms are designated consecutively from the upstream end of the cushion  10  as diaphragms  40   a ,  40   b ,  40   c , etc. The diaphragms  40  are preferably formed of steel box beam members welded to one another. In a currently preferred construction, bolts or rivets  42  (visible in  FIG. 2 ) are used to affix the panel rows  36 ,  38  to the frames  40 . Referring now to  FIGS. 4–6 , a single exemplary diaphragm, or frame,  40 , is shown in greater detail. The diaphragm  40  includes a widened upper portion, generally shown at  50 , and a narrower lower portion  52 . The lower portion  52  includes a pair of generally vertically oriented support members  54  and a connecting cross-piece  56 . U-shaped engagement shoes  58  are secured to one side of each of the support members  54  and slidably engage the rail members  28 ,  30 . The upper portion  50  includes a pair of vertically disposed side members  59  with upper and lower cross-members  60 ,  62  that interconnect the side members  59  to form a rectangular frame. Additional vertical and horizontal cross-members  64 ,  66 , respectively, are secured to one another within the rectangular frame for added support. Plate gussets  68  are welded into each corner of the rectangular upper portion  50  in order to help maintain rigidity and stiffness for the diaphragm  40 . 
     Tension cables are used to provide the crash cushion additional strength and stability and, thereby, materially assist in the lateral redirection of side impacts into the cushion  10 . As shown in  FIGS. 1 and 2 , a pair of forward, or upstream, tension cables  72 ,  74  are disposed through a forward plate  76 , threaded through the upstream diaphragms  40   a ,  40   b  and are then secured to the third diaphragm  40   c . A currently preferred method of securing the tension cables to a diaphragm is to secure a threaded end cap (not shown) onto each end of each cable and then thread a nut onto the end cap after passing the end cap through an aperture in the diaphragm. In the exemplary construction shown, a pair of rearward tension cables  78 ,  80  are secured to the third diaphragm  40   c  and extend rearwardly through corresponding diaphragm apertures toward the downstream end of the central portion  18 . 
     Longitudinal tension in the cushion  10  is provided by the side panels  36 ,  38  that tend to want to remain in a substantially flattened (unfolded) configuration due to shape memory. As noted, prebending of the panels is done to provide a point of planned bending for the panels  36 ,  38  at the cambered portions  44 . 
       FIGS. 7–10  are schematic representations of a crash cushion constructed in accordance with the present invention and illustrate the mechanics of collapse over time. In  FIG. 7 , the cushion  10  has not yet been collapsed by an end on impact. Thus, the cushion  10  is at rest, and in a fully extended position. In  FIG. 8 , an end on collision has taken place. The cushion  10  has been impacted by a vehicle (small car), shown schematically as load  82 , traveling at approximately 62 mph. The cushion  10  is shown at approximately 1.8 seconds into the collision in  FIG. 8 . As can be seen, the cushion  10  has begun to collapse at two primary locations along its length. One of the locations  84  is proximate the upstream end of the cushion  10 . The second location  86  is proximate the downstream end of the cushion  10 . In  FIG. 9 , the cushion  10  is shown approximately 0.27 seconds after the impact. By this time, a third location  88  of axial collapse has begun to form. This third location  88  is proximate the central point along the length of the cushion  10 . In  FIG. 10 , the cushion  10  is essentially completely crushed or collapsed. 
     There are significant advantages to a system that provides for separate collapsing portions spread out in terms of location upon the cushion as well as time. These advantages include efficient use of material and aid in self-restoring nature of cushion. A collapse concentrated in one point along the length could cause that portion of the cushion  10  to be inelastically damaged. 
     As noted, the cells  34  may be hexagonal, octagonal, rectangular or square in shape, being formed between two adjacent frames  40  and the two panel rows  36 ,  38 . As shown in  FIG. 1 , the cells  34  need not all be the same size. The different lengths of the cells provides for differing resistances to collapse. The frames  40  have rollers or shoes (not shown) that engage the rails  28 ,  30  in a manner known in the art so that the frames  40  may move longitudinally along the rails  28 ,  30 . During an end-on collision with the crash cushion  10 , there is a dynamic wave that propagates through the cushion  10  and may collapse sections other that the lead sections (defined between the upstream frame  40   a ,  40   b ,  40   c , and  40   d ). Additionally, some inertial properties can be used by collapsing the segments in varying order. 
     It is noted that each of the panel segments, such as segment  43  of each row  36 ,  38  are cambered at a point  44  approximately midway between adjacent frames  40 . This cambered portion  44  forms a point of flexure and preplanned weakness for the panel segment  43 , thereby permitting the segment  43  to collapse upon the application of an end-on force. The bend also prevents large acceleration spikes from being needed for initial column buckling of the segments  43 . Currently, it is preferred that the amount of bend at the cambered point  44  be about 5–10 degrees, as this amount of bend has been found to provide enough eccentricity to assure proper buckling. The bend at the cambered point  44  may be formed by using a press device of a type known in the art. 
     In operation, the cells  34  are substantially, reversably compressed during an end-on impact by a vehicle  22 . The use of a resilient, thermoplastic material, such as polyethylene, ensures that the terminal  10  will be self-restoring after minor end-on impacts. The nose  16  maybe crushed during the impact, but should be easily replaceable. The posts  24 ,  26  serve as a reinforcement portion at the downstream end of the terminal  10 . The central portion  18  is compressed against the posts  24 ,  26 . 
     The terminal  10  of the present invention provides a number of advantages over prior art terminals. The first is cost. As compared to systems that incorporate polyethylene cylinders, suitable sheets of polyethylene may be obtained readily and inexpensively from a number of suppliers. Secondly, if it becomes necessary to replace one or more of rows  36  or  38 , or individual panels  43  within those rows, this may be accomplished quickly and easily, requiring only removal and replacement of the fasteners  42  used to secure them to the frames  40 . 
     Those of skill in the art will recognize that many changes and modifications may be made to the devices and methods of the present invention without departing from the scope and spirit of the invention. Thus, the scope of the invention is limited only by the terms of the claims that follow and their equivalents.

Summary:
An energy absorbing terminal is described that is made up of a plurality of cells partially defined by cambered panels made of thermoplastic or another suitable material. The panels are supported upon rectangular frames. The cambered portion of the panels provides a predetermined point of flexure for each panel and, thus, allows for energy dissipation during a collision. The stiffness of the crash cushion may be varied by altering material thicknesses and diaphragm spacing. In operation, a vehicle colliding in an end-on manner with the upstream end of the energy absorbing terminal will cause each of the cambered panels to bend angularly at its point of flexure and, thus, cause the cells to collapse axially. The use of thermoplastic, such as polyethylene results in a reversible, self-restoring collapse for the terminal, meaning that the terminal is reusable after most collisions.