Patent Publication Number: US-7901155-B2

Title: Vehicle barrier

Description:
This application is a divisional of U.S. patent application Ser. No. 12/082,401, filed Apr. 10, 2008, now U.S. Pat. No. 7,690,859, which is a divisional of U.S. patent application Ser. No. 11/376,282, filed Mar. 15, 2006, now U.S. Pat. No. 7,374,362. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a vehicle barrier (and system and method) for absorbing energy of a moving vehicle in a captive net that stretches across a roadway, and particularly to a vehicle barrier in which the impact energy of the vehicle on the net is applied to shock absorbers via rotation of drums or spools coupled the ends of the net. Two of such vehicle barriers may be provided across a roadway on opposite sides of a railroad track to prevent vehicles from crossing the railroad track when a train is present. The invention may also be used in any other application to stop a moving vehicle, such as drawbridges, HOV traffic control, security gates, or crash cushion applications. 
     BACKGROUND OF THE INVENTION 
     The problem of vehicles improperly crossing railroad tracks is becoming more pronounced due to a rise in both the average speed of trains and in the number of vehicles on the roads. Traditional systems for preventing vehicles from crossing the tracks at inopportune times have proved less than fully satisfactory, and traditional gates can be bypassed by impatient drivers who do not yet see a train coming, and, in any event, will not stop a vehicle that is out of control. 
     Energy absorbing system have been developed for preventing vehicles from crossing a railroad track by automatically deploying a restraining barrier across a roadway adjacent to a railroad track upon an approaching train. For example, U.S. Pat. No. 5,762,443 describes a heavy-duty shock absorber system with two pairs of concrete bunkers on either side of a railroad track, and a retractable capture net extending across the roadway between each pair of bunkers. In each bunker, the net is coupled to two hydraulic shock absorber mounted in a rotatable structure about a stachion, i.e., a large concrete filled steel pipe embedded 4 feet deep in a concrete foundation and extending 5 to 6 about ground level. The shock absorbers each have a piston and a cylinder, and operate by compression of fluid by the piston being driven into the cylinder in response to vehicle impact on the net. 
     U.S. Pat. No. 6,843,613 and U.S. Published Patent Application No. 2003/0016996, published Jan. 23, 2003, describe another heavy duty shock absorber system also utilizing pairs of bunkers on either side of a railroad track and stachions, but has the advantage of mounting each pair of hydraulic shock absorber using rotational flanges to the stachions, thereby avoiding the large rotatable structure of U.S. Pat. No. 5,762,443 for orienting the shock absorbers for operation by compression. In each bunker, the net is coupled to two hydraulic shock absorbers that are in turn attached to a flange rotatable about the bunker&#39;s stachion. Each of the hydraulic shock absorbers operates by extension of their piston from an initial compressed position away from the cylinder in response to vehicle impact on the net. 
     U.S. Patent Publication No. 2005/0117967, published Oct. 6, 2005, describes a heavy duty shock absorber system similar to U.S. Pat. No. 6,843,613, but without bunkers in which the two hydraulic shock absorbers also operate in extension in response to vehicle impact on the net. Unlike U.S. Pat. No. 6,843,613, the shock absorbers are oriented perpendicular, rather than parallel, to the railroad track when no vehicle is present. The net is supported on either side of a roadway by pivotal supports that are rotatable to an upright position when the net is needed. 
     One major drawback of the vehicle energy absorbing systems describes in the above-cited U.S. Patents and Published Applications is that they require a large amount of square footage for installation along roadsides due to the large size of the shock absorbers required to absorb the momentum generated by a vehicle impacting the net. For example, the shock absorbers used in U.S. Patent Publication No. 2005/0117967 and U.S. Pat. No. 6,843,613 are 5-6 feet when compressed when no impact is present, and can extend 8-11 feet in response to impact. Thus, it would be desirable to reduce the size of shock absorbers used in these systems, while still providing the necessary energy absorption of an impacting vehicle. Such reduction in the overall size of the vehicle energy absorbing system can enable their installation along more railroad crossings where space about the roadside is limited. It would further be desirable if the stachions required in the above cited U.S. Patents and Published Applications were no longer required, thereby making installation easier and less costly. 
     SUMMARY OF THE INVENTION 
     It is one object of the present invention to provide a vehicle barrier for absorbing energy of a vehicle in a net that stretches across a roadway which is more compact than the prior art net-based vehicle barriers. 
     It is another object of the present invention to provide a vehicle barrier having two shock absorbers, one on each side of a net, in which such shock absorbers are operable in compression and extension. 
     It is a further object of the present invention to provide a vehicle barrier having two shock absorbers, one on each side of a net, in which such shock absorbers move between parallel and angled orientations which respect to the railroad tracks during their operation. 
     Briefly described, the vehicle barrier embodying the present invention has a pair of platforms disposed on opposite sides of a roadway, and a capture net extending across the roadway between the platforms. Each of the platforms has a rotatably mounted spool (or drum) attached to a different end of the net, and a shock absorber pivotally mounted to the rotatably mounted spool, in which the shock absorber absorbs the impact force of a vehicle upon the net when conveyed to the shock absorber via rotation of the spool. 
     The shock absorbers of each of the platforms preferably are dual acting extension-compression hydraulic shock absorbers. The shock absorbers are mounted to their respective spool such that applied force to the net by an impacting vehicle is transferred via rotation of the spools to their respective shock absorbers, in which such rotation first operates the shock absorbers in compression, and when additional energy absorption is needed to stop the vehicle operates their respective shock absorber in extension. Two posts on either side of the roadway may be provided for supporting cables from the ends of the net to the platforms. The post may be part of, or separate from, the platforms. 
     In railroad crossing application, two of the vehicle barriers are provided on either side of the railroad track to prevent vehicles from crossing the railroad track. Preferably, the support posts are part of a net lowering and raising mechanism in which support posts are mounted to the mechanism for pivoting each of the posts between up and down positions, thereby raising and lowering the net. The net may be stored when in a down position in depressions in the roadway surface for receiving the net. In this manner, the net may be placed in a down position when no train is present to permit vehicle traffic flow, and the net is then raised when a train is detected. The mechanism may be operable in response to a typical railing crossing train detection system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing objects, features and advantages of the invention will become more apparent from a reading of the following description in connection with the accompanying drawings in which: 
         FIG. 1  is a perspective view which illustrates a railroad crossing for a roadway with two of the vehicle barriers according to the present application; 
         FIG. 1A  is a more detailed view of the circled portion of  FIG. 1  showing one of the pair of platforms of a vehicle barrier; 
         FIG. 2A  is an end view of one of the dual acting extension-compression hydraulic shock absorbers of  FIG. 1 ; 
         FIG. 2B  is a cross-sectional view along lines A-A of  FIG. 2A  in which the shock absorber is shown about midway between full extension and compression; 
         FIGS. 2C and 2D  are cross-sectional views similar to  FIG. 2B  in which the shock absorber is shown in full compression and full extension, respectively; 
         FIGS. 2E and 2F  are more detailed views of a portion of the piston head along the circle portion of  FIG. 2B  during compression and extension, respectively; 
         FIG. 2G  is an exploded view of one of the shock absorbers of  FIG. 1 ; 
         FIGS. 3A ,  3 B, and  3 C are perspective, plan, and side elevational views, respectively, of one of the platforms of  FIG. 1  prior to an impact by a vehicle on the net, in which  FIG. 3A  is a perspective view from the reverse side of the perspective view of  FIG. 1A ; 
         FIG. 3D  is a cross-sectional view along lines  3 D- 3 D of  FIG. 3C ; 
         FIGS. 4A ,  4 B, and  4 C are perspective, plan, and side elevational views, respectively, similar to  FIGS. 3A-3C , respectively, showing the platform during initial impact of force by a vehicle on the net and operation of the shock absorber and the spool attached to the net in response to the impact; 
         FIGS. 5A ,  5 B, and  5 C are perspective, plan, and side elevational views, respectively, similar to  FIGS. 3A-3C , respectively, showing the platform when additional impact of force by a vehicle on the net requires more dampening than that illustrated in  FIGS. 4A-4C ; 
         FIGS. 6 ,  7  and  8  are perspective views of another embodiment of one of the platforms which may be used in  FIG. 1  showing the shock absorber at different angular orientation than  FIGS. 3A-3C , where the platform is shown in  FIG. 6  prior to an impact by a vehicle on the net, in  FIG. 7  shows the platform during initial impact, and in  FIG. 8  when more dampening is required, respectively; 
         FIG. 9  is a graph showing the stroke of each of the shock absorbers in response to applied force on the net of the vehicle barrier of the present invention for two different weight vehicles; and 
         FIG. 10  is a perspective view of the net extending between a pair of platforms of  FIG. 1 , in which the net is pivotal between up and down positions. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a typical railroad crossing for a roadway  10  is shown having two vehicle barriers  14  on opposite sides of railroad tracks  12 . Each vehicle barrier  14  has a vehicle capture net  16  stretched across roadway  10  between a pair of platforms  18 . Each platform has a rotatable spool or drum  24  attached by cables to a different end of the net  16 , and a shock absorber  26  pivotally mounted to spool  24 . The platforms  18  are identical, and a pair of platforms coupled to the same net  16  are in a mirrored orientation with respect to each other, as illustrated in  FIG. 1 . One of platforms  18  is shown in more detail in FIGS.  1 A and  3 A- 3 D when no impact by a vehicle on net  16  is present. Each platform  18  has a concrete pad  20  and a plate  22  attached to the pad. The plate  22  may be of stainless steel and attached to pad  20  by nuts  23   a  threaded onto anchor stubs  23   b  extending from the pad  20  through holes in plate  22 . Spool  24  is rotationally mounted on a fixed post  25  extending from the concrete pad  20  through an opening in the plate  22 . Bearings  25   a  may be present to facilitate rotation of the spool  24  about post  25 , as shown in  FIG. 3D . For example, the pad  20  may be 8 feet by 5 feet, and plate  22  may be 7 feet by four feet, but other dimensions may be used. 
     The shock absorber  26  of each platform  18  is a dual acting tension-compression hydraulic shock absorber having a cylinder  27  and a rod  46  ( FIGS. 2B-D ) having a piston head  48  ( FIG. 2B-F ) movable in the cylinder. Ends  26   a  and  26   b  of shock absorber  26  are coupled to cylinder  27  and rod  46 , respectively. Rod  46  is hidden from view in  FIG. 1A  by a cylindrical sheath  29  which extends over the rod, and such sheath moves in concert with the rod, but over the outside of cylinder  27 . A bracket  28  is attached, such as welded and/or bolted, to spool  24 . End  26   a  of the shock absorber  26  is pivotally mounted to bracket  28  by a shaft  30  extending through holes in each of the two flanges  28   a  of the bracket  28  and through an opening  73   a  ( FIG. 2B ) extending through shock absorber end  26   a . Another bracket  32  is attached, such as welded and/or bolted, to plate  22 , and end  26   b  of the shock absorber  26  is pivotally mounted in bracket  32  by a shaft  34  extending through openings in each of the two flanges  32   a  of bracket  32  and through an opening  73   b  ( FIG. 2B ) through shock absorber end  26   b.    
     The net  16  has a structure of a pair of horizontally extending cables  16   a  connected by a plurality of vertically extending cables  16   b . Cables  16   a  and  16   b  may be galvanized structural strands with a minimum breaking strength sufficient to withstand the force of an impacting vehicle. Vertical cables  16   b  may be connected to horizontal cables  16   a  by clamps or sockets, and spacers (not shown) may be present along horizontal cables between adjacent vertical cables  16   b  if needed to maintain spacing between vertical cables. Although only two vertical cables are shown, additional vertical cables may be provided, and may have shaped structures. The net 16 may be the same or similar to the net described in U.S. Pat. Nos. 5,762,443 or 6,843,613, or U.S. Published Patent Application No. 2003/0016996. 
     At the two ends of net  16  are side members  16   c  attached to the first and last vertical cable  16   b  of the net. Extending from each of the side members  16   c  are two net end cables  36  which are joined, such as by a clamp  37 , to a cable  38  that extends to the platforms  18  on either side of roadway  10 . In each of these platforms, the cable  38  in received in a channel  40  extends along the outer circumference of the spool. The end of cable  38  is captured by a swivel socket  42  and a shaft  41  extends through a hole in the socket and holes in two flanges  40   a  defining the top and bottom walls of channel  40 , such that end of cable  38  can swivel about shaft  41 . Other attachment mechanisms may be used for coupling end of cable  38  to spool  24  so long as the tension conveyed to cable  36 , via cables  38 , will rotate the spool when a vehicle impacts the net. When no impact is present, such tension is applied to maintain the net taught between net side members  16   c , but without initiating rotation of the spool  24 . 
     Each pair of platforms  18  has a fixed post  44  extending from concrete pad  20 . Each pair of net end cables  36  each extend through breakaway brackets  43  attached to the post  44  prior to joining cable  38  to each of the platforms  18  associated with the net. A rotational force in the direction of arrow  45  ( FIG. 3B ) on spool  24  will occur when cables  36  and  38  at the ends of the net are pulled due to impact of a vehicle on net  16 , as will be shown below, such the impact force or energy is dampened by shock absorbers  26  operable first is a compression mode, and if needed in an extension mode 
     Referring to  FIGS. 2A-2G , the hydraulic shock absorber  26  of each platform  18  is shown in more detail. One end  46   a  of the rod  46  is attached to piston head  48 , such end  46   a  being threaded to screw into a threaded bore  47  of the piston head  48 , so as to be movable within the cylinder between full compressed position, shown in  FIG. 2C , and full extended position shown in  FIG. 2D . Arrows  49   a  and  49   b  illustrate the direction of movement of the rod  46  with respect to the cylinder  27  when operating in compression and extension modes, respectively. The overlap of the threads of rod end  46   a  and piston head  48  is denoted by  48   c  ( FIGS. 2E and 2F ). 
     To couple the cylindrical sleeve  29  to rod  46 , the end  29   a  of sleeve  29  is attached, such as welded, to a sleeve adapter  50 , and rod end  46   b  is threaded, and extends through opening  50   a  of the sleeve adapter  50 , and screwed into a threaded bore  75   b  of a clevis  72   b . Clevis  72   b  is attached by screws or bolts  76  to sleeve adapter  50 . 
     At the shock absorber end  26   a , the ends  27   a  of the cylinder  27  is coupled to another clevis  72   a  by a threaded plug  77  screwed into threaded bore  75   a  of clevis  72   a  and then into threaded bore  27   d  of the cylinder. Each end  26   a  and  26   b  has a hex broach  51   a  and  51   b  ( FIG. 2A ), respectively, by which an Allen wrench may be used to tighten parts together. Each of the clevises  72   a  and  72   b  have an opening  73   a  and  73   b , respectively, having spherical bearings  74 . Openings  73   a  and  73   b , for example, may each be 2 inches in diameter. Such bearings facilitate pivoting of shock absorber ends  26   a  and  26   b  along shafts  30  and  34  ( FIG. 1A ), respectively, through brackets  28  and  32 , respectively. The rod  46  and piston head  48  may be made of stainless steel, and the cylinder  27  and cylindrical sleeve  29  may be of stainless steel, or plated steel. For example the dimensions may be as follows: the shock absorber  26  may be 66 inches in length when fully extended, as shown in  FIG. 2D , between the centers of openings  73   a  and  73   b ; rod  46  and piston head  48  when assembled may be 28⅝ inches in length from rod end  46   b  to front face  48   d  of piston head  48 ; sleeve  29  may be 29⅞ inches long and have an interior diameter of 6¾ inches; and cylinder  27  may have an outer diameter of 5⅝ inches. Other dimensions may also be used. 
     The end  27   b  of cylinder is closed by a cylinder cap  52 , which has a central opening  53  through which the rod  46  can retract and extend. A cap nut  54  is screwed onto a threaded annular recess at cylinder end  27   b  to retain cap  52 . A guide ring  56  is positioned in cap  52  at one end of opening  53 , and the other end of the opening  53  is sized for insertion of a sealing member  58  and a ring  59  for retaining the sealing member  58  in cap  53 . The sealing member  58  may be of carbon steel ring press fit into cap opening  53 . 
     Opposite sides of the piston head  48  forms two chambers  60  and  61  in the cylinder  27 , and a fluid  62 , such as liquid silicone, is provided that can flow between the chambers in response to movement of piston head  48  in the directions of arrow  49   a  or  49   b . The fluid  62  is sealed in cylinder  27  by cap  28 , ring  59 , and sealing member  58 . The flow of fluid  62  is best shown in  FIGS. 2E and 2F . The piston head  48  has a number of drilled inner channels (or ports)  48   a  (e.g., between 1 to 10 inner channels) extending from its back face  48   d  through the piston head to its front face  48   e . An outer annular channel or gap  48   b  is defined by the outer surface of the piston head  48  and the inner surface  27   c  of the cylinder  27 . Each of the inner channels  48   a  extends through piston head  48  from chamber  60  to chamber  61  into an annular chamber  63  along back face  48   d . Annular chamber  63  has a movable ring flapper valve  64 , which is retained in chamber  63  by a fixed retaining ring  66 , while being movable in chamber  63  towards and away from channels  48   a  in response to the direction of fluid flow between chambers  60  and  61 . When the shock absorber  26  operates in a compression mode, the rod  46  and its piston head  48  are driven into the cylinder  27  (as indicated by arrow  49   a ), the piston head  48  moves along inner surface  27   c  of the cylinder  27  and its front face  48   c  forces fluid  62  to flow from chamber  60  to chamber  61  through inner channels  48   a  (as illustrated by arrow  62   a ), where the flapper value  64  is pushed by such flow (as indicated by arrow  64   a ) towards the retaining ring  66 , and fluid  62  to move from chamber  60  to chamber  61  around the piston head  48  through gap  48   b  (as illustrated by arrows  62   b ), until full compression is reached ( FIG. 2C ). When the shock absorber  26  operates in an extension mode, the rod  46  and its piston head  48  are driven away from the cylinder  27  (as indicated by arrow  49   b ) and the piston head&#39;s back face  48   d  forces fluid  62  to flow from chamber  61  to chamber  60 , but only around the piston head  48  through outer gap  48   b , as illustrated by arrow  62   c  ( FIG. 2F ), since the inner channel  48   a  is closed (or substantially closed) due to the flow pushing the flapper value  64  against inner channels  48   a , until full extension is reached ( FIG. 2D ). 
     The reduced fluid flow by closure of the inner channels  48   a  during extension results in the stroke of the shock absorber  26  being stiffer in extension than compression. This double acting shock absorber can be half the length of a conventional shock absorber operable in a single compression or extension mode, and the different stiffness of the extension stroke has advantages is stopping a moving vehicle, as will be shown below. In  FIG. 2B , the shock absorber is shown at a mid position between full compression and extension. 
     Within the closed end  27   a  of the cylinder  27  is an accumulator  68  mounted in a can  70  having wall abutting the inner surface of the cylinder  27 . The accumulator  68  may be of foam blocks, and the wall of the can  70  facing piston head  48  has a small orifice  71 . When the piston head  48  is pushed to its full extent into cylinder  27 , as shown in  FIG. 2C , fluid  62  can pass to the accumulator  68  through this orifice  71 . 
     Abutment of the sleeve adapter  50  to cylinder end  27   a  defines the full compression of the rod  46  and its piston head  48  into the cylinder  27  ( FIG. 2C ). Abutment of the piston head  48  against the cylinder cap  52  defines the full extension of rod  46  from the cylinder ( FIG. 2D ). The shock absorbers  26  are shown in full extension in  FIG. 1 , as well as in FIGS.  1 A and  3 A- 3 C. 
     Prior to impact by a vehicle  15  on net  16 , the pair of platforms between net  16  have their shock absorbers  26  at their full extended position and are disposed between brackets  28  and  32  at an angle (e.g., approximately 45 degrees) with the railroad track  12  and roadway  10 , as shown in  FIGS. 1 ,  1 A, and  3 A- 3 C. When a vehicle impacts the net  16 , the applied force on the net  16  rotates the spool  24  (as shown by arrow  45   a ) along a first degree (e.g., up to 90 degrees), as shown in  FIGS. 4A-4C . This breaks away the cables  36  from break away brackets  43  of post  44  of each of the platforms  18 , in which brackets  43  have tension and fragility properties enabling such break away response. The broken brackets are indicated by reference number  43 ′. The shock absorber  26  of each platform  18  pivots about its end  26   a  in bracket  28  attached to spool  24  which pushes rod  46  into cylinder  27  of the shock absorber, thereby operating the shock absorber in compression. This dampens the force (energy) applied to the net by the impact, which for a light vehicle impact may not require full compression via rotation of the spool. If needed to further absorb the impact energy, the spool  24  continues to rotate (as shown by arrow  45   b ) a second degree (e.g., up to another 90 degrees) in response to any additional applied force (energy of the impact), which pivots the shock absorber at its end  26   a  in bracket  28  pulling the rod  46  away from the cylinder  27  of the shock absorber, thereby operating the shock absorber in extension (or tension). This further dampens the force applied to the net which, if needed for a heavy or high momentum vehicle, such as a truck, may require full extension of the shock absorber. The end  26   b  of the shock absorbers  26  also pivots in bracket  32  to facilitate pivotal motion of end  26   a  in bracket  28 . The shock absorber  26  with the rod fully extended is approximately parallel with the long side of the railroad tracks  12 , and hence substantially perpendicular to the sides of roadway  10 . Thus, the impact force or energy of the vehicle is absorbed by shock absorbers  26  in the pair of platforms  18  coupled to the net  16 , thereby lessening adverse effects of the impact forces acting on vehicle and its occupants and preventing encroachment of the vehicle onto tracks  12  when a train passes through. 
     Referring to  FIGS. 6-8 , one of the platforms of another embodiment of platforms  18  is denoted by reference numeral  18   a . Like numbers reference the same elements as in other figures. The only difference in platform  18  is that the shock absorbers  26  in the vehicle barrier  14  are shown in another orientation with respect to the spools  24 . In this orientation, the shock absorbers  26  are in full extension approximately parallel with the railroad tracks  12  and substituted perpendicular to roadway  10  prior to impact of a vehicle on the net ( FIG. 6 ). The shock absorber  26  in response to impact on the net by the vehicle rotates the spool  24  pivot to operates the shock absorber in compression ( FIG. 7 ), and then in extension ( FIG. 8 ) in the same manner as described above, wherein full extension the shock absorbers  26  are at an angle (e.g., approximately 45 degrees) with respect to the railroad tracks  12  or roadway  10 . 
       FIG. 9  is a graph showing the stroke of each of the two shock absorbers  26  of platform  18  in response to applied force on the net  16  for two vehicles of different weight. The lower line illustrates the example of an 1800 lb vehicle traveling at 45 MPH crashing in net  16 . The force or energy applied to the net is almost fully dampened by the shock absorbers operating in full compression ( FIG. 4A-C ) from their initial full extended position ( FIGS. 3A-C ), and needs only partial extension to fully stop the vehicle. The upper line illustrates the example of a 4500 lb vehicle crashing into net  16  at 45 MPH in which full compression ( FIGS. 4A-C ) of the shock absorbers first occurs, and then almost full extension ( FIG. 5A-C ) is needed to fully stop the vehicle. As the heavier vehicle illustrates, the stiffer stroke of the shock absorbers in extension enable stopping of vehicles applying up to 50,000 lbs of force on the captive net. The dampening functions shown in  FIG. 9  will differ in the case of  FIGS. 6-8 , but will have similar different energy dampening characteristics in extension than compression due to the stiffer extension stroke. 
     Once the vehicle barrier has been used to capture a vehicle, such as one about to crash into a moving train, the vehicle barrier can be reset to that shown in  FIG. 1  by removal of the vehicle from the net, reversing rotation of spools  24 , and placement of cables  36  through replaced breakaway brackets  43  on posts  44 . 
     Preferably, the net  16  is present across the roadway  10  when a train is detected by typical train detection system, such as commonly used to control gates at railroad crossings, and otherwise is lowered to allow vehicles to cross the railroad tracks  12 . The net  16  may be raised and lowered as shown in  FIG. 10  from it upright position to a down position for storage in a recessed grid  78  in the roadway surface  10  shaped for receiving the net. In this embodiment, the posts  44  are not fixed in pad  20 , rather each of the posts  44  are attached to a motor driven actuator  80 . Normally, the posts  44  are is down position as illustrated by dashed lines  44   a  locating the net  16  and grid  78 . When a train is detected by typical train detection system, in addition to flowing gates (not shown) or initiating flashing lights (not shown) typically used, signals to the motor driven actuators  80  pivot the post  44  upward to their upright position. After the train is no longer detected, signals to the motor driven actuators  80  pivot the post  44  back down to its recess location in the roadway  10 . Arrows  82  illustrate the pivotal motion of the net  16  between up and down positions. Optionally, springs may be provided which compress when the posts  44  is down to facilitate upward force on posts  44 . Other devices for raising and lowering a net may also be used, such as the spring-based systems described in U.S. Patent Publication No. 2005/0117967, published Oct. 6, 2005. In other applications, the net lowering or raising of the mechanism provided by actuators  80  to each posts  44  may be raised or lowered by security personnel, e.g., via switch, button, or other wired or wireless signaling device, to control signals to each actuator  80 . 
     Other shock absorbers may also be used than the dual acting shock absorbers described above. For example, the liquid spring unit of U.S. Pat. No. 4,611,794 may be used by providing channels or ports in and around a piston head enabling the response illustrated in  FIG. 9 . Although less preferably due to their large size, the shock absorbers operable from an extended state to a compressed state, such as those similar to those of U.S. Patent Publication No. 2005/0117967 and U.S. Pat. No. 6,843,613, may be used by their mounting to a accommodating pad  22  and to spool  24  similar in the manner of shock absorber  26  for pivotal movement in response to rotation of spool  24  in each pair of platforms  18  for the same net  16 . 
     Although described for capturing a moving vehicle, such as a car or truck, the vehicle barrier of the present invention may be used at the end of a runway to stop an errant moving airplane. 
     From the foregoing description, it will be apparent that an improved vehicle barrier for absorbing energy of a vehicle in a net that stretches across a roadway has been provided. Variations and modifications of the herein described system and other applications for the invention will undoubtedly suggest themselves to those skilled in the art. Accordingly, the foregoing description should be taken as illustrative and not in a limiting sense.