Active security barriers (also known as crash barriers) are large structures that are placed in roadways at entrance and exit points to protected facilities to control vehicle access to these areas. These barriers are placed perpendicular to traffic to block the roadway, so that the only way that traffic can pass the barrier is for the barrier to be moved out of the roadway. These types of barriers are typically constructed from sturdy materials, such as concrete or steel, such that vehicles cannot penetrate through them. They are also designed at a certain height off the roadway so that vehicles cannot go over them.

The key difference between active security barriers, which include wedges, crash beams, gates, retractable bollards, and portable barricades; and passive security barriers, which include non-moveable bollards, jersey barriers, and planters, is that active security barriers are designed so that they can be raised and lowered or moved out of the roadway easily to allow authorized vehicles to pass them. Many of these types of barriers are designed so that they can be opened and closed automatically (i.e., mechanized gates, hydraulic wedge barriers), while others are easy to open and close manually (swing crash beams, manual gates). In contrast to active barriers, passive barriers are permanent, non-movable barriers, and thus they are typically used to protect the perimeter of a protected facility, such as sidewalks and other areas that do not require vehicular traffic to pass them. These barriers are discussed in the Passive Security Barriers Product Guide.

Several of the major types of active security barriers are described below:

Wedge barriers are plated, rectangular steel buttresses approximately 2-3 feet high that can be raised and lowered from the roadway. When they are in the open position, they are flush with the roadway and vehicles can pass over them. However, when they are in the closed (armed) position, they project up from the road at a 45 degree angle, with the upper end pointing towards the oncoming vehicle and the base of the barrier away from the vehicle. Generally, wedge barriers are constructed from heavy gauge steel, or concrete that contains an impact-dampening iron rebar core that is strong and resistant to breaking or cracking, thereby allowing them to withstand the impact from a vehicle attempting to crash through them. In addition, both of these materials help to transfer the energy of the impact over the barrier's entire volume, thus helping to prevent the barrier from being sheared off its base. In addition, because the barrier is angled away from traffic, the force of any vehicle impacting the barrier is distributed over the entire surface of the barrier and is not concentrated at the base, which helps prevent the barrier from breaking off at the base. Finally, the angle of the barrier helps hang up any vehicles attempting to drive over it.

Wedge barriers can be fixed or portable. Fixed wedge barriers can be mounted on the surface of the roadway ("surface-mounted wedges") or in a shallow mount in the road's surface, or they can be installed completely below the road surface. Surface-mounted wedge barricades operate by rising from a flat position on the surface of the roadway, while shallow-mount wedge barriers rise from their resting position just below the road surface. In contrast, below-surface wedge barriers operate by rising from beneath the road surface. Both the shallow-mounted and surface-mounted barriers require little or no excavation, and thus do not interfere with buried utilities. All three barrier mounting types project above the road surface and block traffic when they are raised into the armed position. Once they are disarmed and lowered, they are flush with the road, thereby allowing traffic to pass. Portable wedge barriers are moved into place on wheels that are removed after the barrier has been set into place (see further description of these barriers under "Portable/Removable Barriers" below).

Installing rising wedge barriers requires preparation of the road surface. Installing surface-mounted wedges does not require that the road be excavated; however, the road surface must intact and strong enough to allow the bolts anchoring the wedge to the road surface to attach properly. Shallow-mount and below-surface wedge barricades require excavation of a pit that is large enough to accommodate the wedge structure, as well as any arming/disarming mechanisms. Generally, the bottom of the excavation pit is lined with gravel to allow for drainage. Areas not sheltered from rain or surface runoff can install a gravity drain or self priming pump.

Crash beam barriers consist of aluminum beams that can be opened or closed across the roadway. While there are several different crash beam designs (see below), every crash beam system consists of an aluminum beam that is supported on each side of the road by a solid footing or buttress, which is typically constructed from concrete, steel, or some other strong material. Beams typically contain an interior steel cable (typically at least one inch in diameter) to give the beam added strength and rigidity. The beam is connected by a heavy duty hinge or other mechanism to one of the footings so that it can swing or rotate out of the roadway when it is open, and can swing back across the road when it is in the closed (armed) position, blocking the road and inhibiting access by unauthorized vehicles. The non-hinged end of the beam can be locked into its footing, thus providing anchoring for the beam on both sides of the road and increasing the beam's resistance to any vehicles attempting to penetrate through it. In addition, if the crash beam is hit by a vehicle, the aluminum beam transfers the impact energy to the interior cable, which in turn transfers the impact energy through the footings and into their foundation, thereby minimizing the chance that the impact will snap the beam and allow the intruding vehicle to pass through.

Crash beam barriers can employ drop-arm, cantilever, or swing beam designs. Drop-arm crash beams operate by raising and lowering the beam vertically across the road. Cantilever crash beams are projecting structures that are opened and closed by extending the beam from the hinge buttress to the receiving buttress located on the opposite side of the road. In the swing beam design, the beam is hinged to the buttress such that it swings horizontally across the road. Generally, swing beam and cantilever designs are used at locations where a vertical lift beam is impractical. For example, the swing beam or cantilever designs are utilized at entrances and exits with overhangs, trees, or buildings that would physically block the operation of the drop-arm beam design.

Installing any of these crash beam barriers involves the excavation of a pit approximately 48 inches deep for both the hinge and the receiver footings. Due to the depth of excavation, the site should be inspected for underground utilities before digging begins.

In contrast to wedge barriers and crash beams, which are typically installed separately from a fence line, gates are often integrated units of a perimeter fence or wall around a facility. Gates are basically movable pieces of fencing that can be opened and closed across a road. When the gate is in the closed (armed) position, the leaves of the gate lock into steel buttresses that are embedded in concrete foundation located on both sides of the roadway, thereby blocking access to the roadway. Generally, gate barricades are constructed from a combination of heavy-gauge steel and aluminum that can absorb an impact from vehicles attempting to ram through them. Any remaining impact energy not absorbed by the gate material is transferred to the steel buttresses and their concrete foundation.

Gates can utilize a cantilever, linear, or swing design. Cantilever gates are projecting structures that operate by extending the gate from the hinge footing across the roadway to the receiver footing. A linear gate is designed to slide across the road on tracks via a rack and pinion drive mechanism. Swing gates are hinged so that they can swing horizontally across the road.

Installation of the cantilever, linear, or swing gate designs described above involve the excavation of a pit approximately 48 inches deep for both the hinge and receiver footings to which the gates are attached. Due to the depth of excavation, the site should be inspected for underground utilities before digging begins.

Bollards are vertical barriers at least 3 feet tall and 1 to 2 feet in diameter that are typically set 4 to 5 feet apart from each other so that they block vehicles from passing between them. Bollards can either be fixed in place, removable, or retractable. Fixed and removable bollards are passive barriers that are typically used along building perimeters or on sidewalks to prevent vehicles from passing them, while allowing pedestrians to pass them. These types of passive bollards are discussed in the Passive Security Barriers Product Guide. In contrast to passive bollards, retractable bollards are active security barriers that can easily be raised and lowered to allow vehicles to pass between them. Thus, they can be used in driveways or on roads to control vehicular access. When the bollards are raised, they project above the road surface and block the roadway; when they are lowered, they sit flush with the road surface, and thus allow traffic to pass over them. Retractable bollards are typically constructed from steel or other materials that have a low weight-to-volume ratio so that they require low power to raise and lower. Steel is also more resistant to breaking than is a more brittle material, such as concrete, and is better able to withstand direct vehicular impact without breaking apart.

Retractable bollards are installed in a trench dug across a roadway -- typically at an entrance or gate. Installing retractable bollards requires preparing the road surface. Depending on the vendor, bollards can be installed either in a continuous slab of concrete, or in individual excavations with concrete poured in place. The required excavation for a bollard is typically slightly wider and slightly deeper than the bollard height when extended aboveground. The bottom of the excavation is typically lined with gravel to allow drainage. The bollards are then connected to a control panel which controls the raising and lowering of the bollards. Installation typically requires mechanical, electrical, and concrete work; if utility personnel with these skills are available, then the utility can install the bollards themselves.

Portable/removable barriers, which can include removable crash beams and wedge barriers, are mobile obstacles that can be moved in and out of position on a roadway. For example, a crash beam may be completely removed and stored off-site when it is not needed. An additional example would be wedge barriers that are equipped with wheels that can be removed after the barricade is towed into place.

When portable barricades are needed, they can be moved into position rapidly. To provide them with added strength and stability, they are typically anchored to buttress boxes that are located on either side of the road. These buttress boxes, which may or may not be permanent, are usually filled with sand, water, cement, gravel, or concrete to make them heavy and aid in stabilizing the portable barrier. In addition, these buttresses can help dissipate any impact energy from vehicles crashing into the barrier itself.

Because these barriers are not anchored into the roadway, they do not require excavation or other related construction for installation. In contrast, they can be assembled and made operational in a short period of time. The primary shortcoming to this type of design is that these barriers may move if they are hit by vehicles. Therefore, it is important to carefully assess the placement and anchoring of these types of barriers to ensure that they can withstand the types of impacts that may be anticipated at that location.

Because the primary threat to active security barriers is that vehicles will attempt to crash through them, their most important attributes are their size, strength, and crash resistance. Other important features for an active security barrier are the mechanisms by which the barrier is raised and lowered to allow authorized vehicle entry, and other factors, such as weather resistance and safety features. These attributes and features are discussed in more detail below.

One of the major design principles behind security barriers is their size. They are designed so that they will impede the path of vehicles, and so they must be of sufficient size (both in bulk and in height) so that vehicles cannot drive over or around them. Most security barriers are at least 3 feet high, which places them at least at the grill level of most vehicles. Because they are at the grill level, these barriers will penetrate through to the engine block, or become lodged underneath the vehicle and destroy its transmission, if vehicles attempt to drive through/over them. This ensures that, even if the vehicle manages to pass the barrier, it will no longer be drivable, and the vehicle will only travel as far past the barrier as its momentum will take it.

Crash resistance is dependent on the strength of the barrier, which is in turn determined by the construction of each type of barrier, its shape and dimensions, and the depth to which it is anchored into the roadway. The material from which each type of barrier is constructed is an important factor contributing to the strength of the barrier. The typical construction materials for barriers discussed in this document are steel and aluminum, although some of these barriers may be constructed from concrete. Steel and aluminum are the preferred materials for crash-resistant applications due to their ability to transfer the force of any impact throughout the entire barrier, thereby lessening the force imposed on the barrier at any one point. In contrast, concrete is brittle and can shatter upon impact, and thus must be reinforced with an iron or steel rebar core to add crash resistance. Thus, barriers using concrete as part of their design are often built around an impact-dampening iron rebar core that can spread the force of the impact over the entire structure and hold the concrete together even after impact. Portable barriers constructed from plastic filled with sand, cement, concrete, gravel, or water, and those equipped with wheels, although physically strong, may not provide the same level of resistance to a vehicle hitting them as a fixed barrier. Additionally, all materials from which barriers are constructed may need to be corrosion resistant so that they can withstand the elements and not deteriorate.

The shape and dimensions of a barrier are a significant component to the overall strength of the barrier. The larger the barrier in both height and width, the greater stopping power it will have against a potential threat vehicle. For example, gates are typically large and block the entire roadway, while wedges block the roadway from the base of the road to a height of several feet. In contrast, crash beam only block a small space above the roadway, but are typically deployed at a height such that vehicles cannot go over or under them. The shape of the barrier can also help in stopping vehicles. As discussed above, the shape of a wedge barrier causes vehicles to be hung up if they attempt to drive over the barrier, while the shape of a crash beam causes all of the force of the impact of the vehicle to be concentrated in its grill area, which can allow the beam to penetrate into the engine block of the vehicle.

The shape of the barrier also helps to keep it from breaking off on impact. Barriers are designed and configured to dampen the impact energy over the entire foundation. This ability to distribute impact energy assures the user that the barrier will not break apart upon impact and allow the threat vehicle to pass through. For example, as described above, the shape of a wedge barrier spreads the force of any impact over its entire volume, lessening the force at the point of impact and therefore minimizing the chance of the wedge shearing off. Similarly, the steel cable in the interior of a crash beam also distributes the energy of the crash away from the point of impact, reducing the chance that the beam and cable will break.

The depth to which a barrier is anchored into the roadway is important in determining the barrier's ability to withstand vehicular impact. The deeper a barrier is anchored into the roadway, the more base it will have to distribute the force of the impact over the entire foundation. This is true both for barriers that are anchored directly into the roadway, such as retractable bollards and wedges, and barriers that are anchored at the side of the roadway, such as crash beams and gates. As described above, each of these barriers is designed such that the crash impact is distributed throughout the barrier and its anchor, whether that anchor is at the base of the barrier or at the side of the road.

The ability of the barrier core to successfully withstand an impact is referred to as its impact resistance or crash test rating. Crash test rating specifications are usually given in terms of the barrier's ability to withstand an impact from a vehicle of a certain weight traveling at a certain speed. For example, a barrier may be designed to stop a 15,000-lb vehicle traveling at 50 miles per hour. The primary federal standard for crash rating is the Department of State Specification SD-SDT-0201 (Specification for Vehicle Crash Test of Perimeter Barriers and Gates, April, 1985, updated March 2003). ASTM standard WK2534 is equivalent to this rating. This standard is based on the distance past a barrier that can be penetrated by a 15,000-LB vehicle traveling at different speeds. The vehicle speed is designated as "K," and the distance of penetration past the barrier is designated as "L." These designations are summarized in the tables below. Thus, a barrier with a SD-SDT-0201 crash rating of K12/L3 could keep a 15,000 vehicle traveling at 50 mph from penetrating more than 3 feet past the barrier. Tables 6 and 7 below summarize the components of the Department of State's crash resistance rating system.

As discussed above, these barriers must be opened and closed across a road in order to control vehicular access. Depending on the security needs at the site, the frequency with which the road is used, and the response time of the barrier, the barriers may be either normally open or normally closed. In either case, opening and closing the barriers requires some action either by people or equipment. Barriers can be operated either by physically stationing an operator at the barrier site, or by enabling authorized drivers to disarm the barrier upon driving up to it. Barriers that are disarmed by authorized drivers may require the use of a card or keypad to enter information to lower the barrier, or it may be necessary for the driver to physically remove the barrier by hand.

The mechanisms for opening and closing the barriers are an important part of their design, and can range from requiring manual operation to being fully automated. Mechanisms for raising and lowering these types of barriers are discussed in more detail below.

It is important to consider local climate conditions and geography when placing a barrier at a facility. For example, in cold weather conditions, freezing temperatures, snow, and ice buildup may affect barrier operation. Specifically, hinges or motorized parts may freeze or become encrusted with snow. Surface-mounted wedge barriers may need protection against water and/or slush forming a solid mass of ice in the barricade, thus preventing the barricade from moving in and out of position. It may be necessary to use heaters and/or snowmelt systems to mitigate these conditions. For example, installing strip heaters in the foundations of surface-mounted wedge barriers to aid in keeping the temperature above 32 degrees Fahrenheit will help alleviate the buildup of ice under the barrier. In contrast, in warm climates it may be necessary to protect the barrier from excessive heat and humidity. For instance, a hydraulically-powered barrier located in an area with high ambient temperatures may require a heat exchanger on the oil tank to avoid overheating and subsequent failure of the hydraulic system. Mechanized barriers (i.e., hydraulic, pneumatic, and electromechanical) located in urban areas may require protection from dirt and debris so that the motors do not become disabled. Along coastal areas, barriers may need protection from salt water, sand, and a high water table. For example, it may be necessary to analyze the water table to determine whether installation of a below-grade barrier with a sump pump would be feasible, or a surface-mounted barrier would be most appropriate.

The installation of safety devices can prevent unintended activation of a vehicle barrier (caused either by operator error or equipment malfunction), and can therefore prevent serious injuries to operators or vehicle occupants. There are many options that will enhance the safety and performance of the system, and most can be interfaced with any of the barrier types discussed in this document. Safety locks may be added to a barrier to prevent the unit from accidentally lowering during servicing. These safety locks can also be used to secure the barrier into position for extended periods of time. Tamper switches ensure that intruders are not able to access and disable the barrier, and can be helpful in securing barricades that are not under continuous supervision. For example, installing tamper switches on the doors of hydraulic pump units will ensure that a hydraulically operated barrier can not be disabled by unauthorized personnel.

Vehicle safety loops can be used to prevent a barrier from moving into the active position in front of, or under, an authorized vehicle. These loops are generally applied in high traffic cycle operations where guards can make errors by accidentally pressing barrier control switches at the wrong time. They consist of vehicle-sensing devices that are placed in the roadway directly in front of and behind the barrier. When these loops detect a vehicle over them, they disable the barricade, thereby preventing its accidental activation. In the case of an unauthorized intrusion, the guards can use an emergency override switch to activate the barrier even when a vehicle is in the loop. With the threat of forced entry and the possibility of tailgating, this emergency fast operation mode can be crucial. Note: this emergency fast feature is used only on an as-needed basis due to the wear and tear it creates on the system from regular use.

Warning signs, traffic safety paint, face-mounted LED lights, traffic arms, traffic lights and audible alarms can be installed and operated in conjunction with the barrier to identify the location of the barrier and make it more visible to oncoming traffic. An infrared safety beam that alarms when a pedestrian enters the protected area can be employed to protect pedestrians from being hit by closing barriers. Sidewalks, fencing, and landscaping may also be used to channel pedestrian traffic away from the barrier.

The following table shows cost ranges for each barrier type. With most of these security barriers, the major factors affecting cost will be the size of the barrier and the material from which it is constructed. There will be no installation costs for portable barriers because they are not anchored into the ground.