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Dier Road Safety Barrier Design Guide Part b Oct 07 | Vehicles | Road Traffic Safety
Dier Road Safety Barrier Design Guide Part b Oct 07
Uploaded by Ravi Valakrishnan
Dier Road Safety Barrier Design Guide Part b Oct 0...
Transport Infrastructure Branch
Hazard identification, evaluation, performance standards, crash test, end treatments, types of
barrier, design process, longitudinal barriers, end treatments, crash attenuators, transitions,
maintenance, temporary safety barrier systems.
The purpose of this guide is to provide guidelines for the identification of the need for a road safety
barrier, the selection of an appropriate type of barrier, and the design and location of longitudinal
The guide describes the processes used to identify hazards, test proposed safety barrier systems,
evaluate treatment options and to design a road safety barrier system, including the choice of end
treatments and transitions.
It outlines a set of guidelines rather than a prescriptive set of standards. Therefore, designers
should apply the recommended guidelines in conjunction with their own knowledge, experience
and judgement to develop the most appropriate treatment for the issue that they are considering.
However, every effort should the made to achieve the objectives of the guidelines whenever it is
BRIDGES AND BRIDGE APPROACHES............................................................................................3
TRANSITIONS BETWEEN BARRIER TYPES.....................................................................................4
END TREATMENTS AND CRASH ATTENUATORS ..........................................................................9
Introduction ..................................................................................................................................9
Types of End Treatments.............................................................................................................14
Selection of End Treatments........................................................................................................23
Terminal Approach and Placement Conditions ...........................................................................25
MAINTENANCE OF SAFETY BARRIERS...........................................................................................27
General.........................................................................................................................................4
Design Criteria –Physically Connected Barriers ..........................................................................4
Typical Interfaces between Barrier Types....................................................................................5
Introduction ..................................................................................................................................27
Routine Safety Barrier Maintenance ............................................................................................27
Repair and Reinstatement ...........................................................................................................28
Operational Monitoring.................................................................................................................29
TEMPORARY SAFETY BARRIER SYSTEMS ....................................................................................30
Introduction ..................................................................................................................................30
Purpose and Use of Safety Barriers at Roadwork Sites ..............................................................31
Types of Longitudinal Safety Barrier Systems for Temporary Use..............................................32
Operational Requirements for the Use of Barriers at Roadwork Sites ........................................33
Selection of Safety Barrier Type for Worksites ............................................................................36
REFERENCES ................................................................................................................................................39
Figure 7.1 — An Example of an Interface between Wire Rope Barrier and W-Beam Barrier ......................... 7
Figure 7.2 — Wire Rope Safety Barrier to Rigid Barrier.................................................................................. 8
Figure 8.1 — Gating and Non-Gating Systems............................................................................................... 11
Figure 8.2 — Redirective and Non-redirective Systems ................................................................................. 13
Figure 8.3 — General Arrangement Modified Eccentric Loader Terminal (MELT)...................................... 16
Figure 8.4 — Non-Proprietary Bull-Nose Treatment...................................................................................... 21
Figure 8.5 — Trailing Terminal ...................................................................................................................... 22
Figure 8.6 — Space Required for Crash Attenuators in Gore Areas............................................................... 24
Figure 8.7 — Grading for Flared Barrier End Treatment................................................................................ 26
Figure 8.8 — Grading for Non-flared Barrier End Treatment ........................................................................ 26
While AS 5100:2004 is focussed on bridge railings.6 BRIDGES AND BRIDGE APPROACHES Safety barriers for bridges shall be designed in accordance with AS 5100:2004 Bridge design. AS 5100:2004 also provides information on barriers for bicycle and pedestrian bridges and for some design elements for bicycle/pedestrian paths as they relate to bridges. It is based on risk assessment of the specific site and the cost of providing a barrier system of a specified performance level. 3 . the procedure could be applied to sites on the approaches to bridges and at other (non-bridge) locations along roads where similar conditions exist. Appendix B of AS 5100:2004 provides a procedure to assist in the selection of an appropriate bridge barrier performance level related to traffic conditions and the road environment. regular (TL4) or medium (TL5) performance level barrier descriptive advice on the assessment of individual medium-risk to high-risk sites. The selection method takes into account factors for road type. curvature. A significant difference between AS 5100:2004 and the former standard HB77 Australian Bridge Design Code is that clauses for both the performance level definition and selection and design of barriers have been completely replaced. deck height and under-structure conditions. using risk assessment and benefit cost analysis to determine whether a higher performance level barrier should be provided. commercial vehicle percentage. A procedure to assist in the determination of barrier performance levels that is based on recently developed AASHTO documentation has been provided. and speed environment. down grade. The procedure comprises: a selection method that leads to a recommendation for a low (TL2).
and/or transition to a heavier rail (e.2 Design Criteria –Physically Connected Barriers The following criteria are important when designing a transition section or connection (AASHTO 2002).1 General Transitions are used to provide a safe interface whenever it is necessary to change from one type of barrier to another.e. The Federal Highway Administration website provides details of crash tested transitions (http://safety. or where either of these semi-rigid barriers are to be connected to a rigid barrier (refer AS/NZS 3845:1999). The purpose of a transition section is to produce a gradual increase in stiffening between the barrier systems so that vehicular pocketing. The transition should be long enough to ensure that changes in deflection do not occur over a short distance. The latter situation typically arises on the approach to bridges that have rigid barriers.gov/fourthlevel/hardware/listing. The connection point of the two systems must be as strong as the approach barrier to ensure the connection will not fail on impact by pulling out. or an additional rail. A satisfactory interface may be achieved by: Providing a structurally designed and tested physical connection between the systems. but may also occur at other locations. over the transition length. The change in stiffness from the less rigid barrier to the more rigid barrier.fhwa. including one from the opposing lane on a two-way facility.e. 4 . snagging or penetration is prevented at any position along the transition. Transitions can be used only between semirigid systems (i. This may be used for any systems but is the only way of achieving a transition from wire rope barrier to a more rigid barrier. Overlapping the barriers by commencing the more rigid system behind the less rigid system.g. The use of a cast-in-place anchor or through-bolt connection is recommended. The overlapping of the barriers achieves a similar outcome by providing adequate lateral separation between them. Overlapping different types of barrier is only possible where adequate space is available. Thrie-Beam). Although AASHTO provides this guidance in relation to bridge approaches. The connections are facilitated through “transition sections” of barrier that are designed to provide gradually increasing lateral stiffness and hence continuity of protection for vehicles that impact the barrier in the vicinity of the interface. When providing a transition section to a bridge railing end.cfm?code=long). 7. This may be achieved by reducing the post spacing. should increase with a high degree of continuity. The transition must be designed to minimise the likelihood of snagging an errant vehicle. it is highly desirable to taper the bridge railing end behind the approach transition to prevent pocketing on vehicle impact.dot. strengthening the rail element or a combination of these techniques. steel to steel) or between semi-rigid and rigid systems (i.7 TRANSITIONS BETWEEN BARRIER TYPES 7. nested rail). steel to concrete). In practice transitions are achieved by: increasing the rigidity of a W-Beam system by decreasing the post spacing or by using a double thickness of rail (i. the principles apply where any semi-rigid barrier system is connected to a rigid barrier.e. Specially designed barrier sections or connections are used for situations where W-Beam is to be connected to Thrie-Beam.
3 W-Beam to Concrete W-Beams are connected to a concrete barrier either through the use of a Thrie-Beam transition (Figure F5 in AS 3845:1999) or by connecting the W-Beam directly to the concrete using an acceptable direct transition (Figure F9 in AS3845:1999).6 may be used as a guide to the lateral separation required in overlapping a rigid barrier with a WRSB. raised inlets or open drains should not be constructed in front of barriers or a transition area. 5 . The WRSB manufacturer should be consulted with regard to any proposed design transitions between WRSB and semi-rigid or rigid barriers to seek assurance that they have either been tested or have been otherwise demonstrated to be acceptable. kerb and slope features must be addressed. Table 4. as they may initiate vehicle instability and adversely affect the crashworthiness of the barrier or transition. Both treatments provide a structurally sound connection and a smooth and stiffened transition to prevent snagging and pocketing of impacting vehicles. kerb inlets. Such arrangements should enable the two systems to work independently while providing continuous shielding of hazards. Drainage features such as kerbs.3. These transitions have been tested or are deemed to comply with NCHRP (1993). This transition is 2 m between post centres and is illustrated in Figures F5 and F15 of Appendix F of AS 3845:1999.e. 7. 7. The slope between the edge of the road and the barrier should not be steeper than 1 on 10. However. Guidance on various transitions is provided in the following sections. The transitions are achieved through stiffening of the steel safety barrier by the use of special sections and connectors. Wire rope safety barriers (WRSBs) are not designed to be connected to semi-rigid or rigid safety barriers or bridge ends. 7.3. one inside the other) of the beams. reduced post spacing and nesting (i.3. two sections of rail.As with longitudinal barriers.2 W-Beam to Thrie-Beam The transition is achieved through the use of a product that bolts to the W-Beam at one end and to the Thrie-Beam at the other end.3 Typical Interfaces between Barrier Types 7. WRSB may be transitioned to more rigid barriers provided that the WRSB overlaps the more rigid barrier by an adequate longitudinal distance and the lateral separation is sufficient to accommodate the maximum likely deflection of the WRSB.1 General AS/NZS 3845:1999 provides detailed illustrations of transitions between semi-rigid and rigid barriers.
1) involves a design that ensures that each barrier does not adversely affect the performance of the other. the principle of having each barrier separated by a distance that should enable them to operate independently under impact is considered to be a sound and safe practice.3. as shown in Figure F27 of AS 3845:1999.3.5 Wire Rope Safety Barrier to Semi-Rigid Barrier These transitions involve the wire rope safety barrier (WRSB) overlapping the W-Beam or ThrieBeam barrier by a nominal longitudinal distance based on site conditions. This transition has not been tested. 7. the latter being stiffened by circular hollow sections bearing on the face of the concrete at the rear of the beam. Details of the transition are shown in Figure F6 of AS 3845:1999. The W-Beam transition (directly to concrete barrier) involves the: W-Beam being recessed into the concrete barrier to provide a flush barrier face at the connection transition being strengthened by the post spacing being reduced progressively from the standard spacing (2 m) to 1 m and then 500 mm over the last 10 m of the beam transition being further strengthened by nesting of the W-Beam over the last 5 m concrete barrier being flared away from the W-Beam. 7.3. An alternative acceptable arrangement (refer Figure 7. However. 7.6 Wire Rope Safety Barrier to Concrete Barrier The transition between WRSB and concrete barrier also requires a longitudinal overlap and lateral separation adequate to accommodate deflections under impact. 6 .The Thrie–Beam transition involves: the use of a prefabricated product to connect the W-Beam to the Thrie-Beam (Figure F15 of AS 3845:1999) the post spacing being reduced from the standard spacing (2 m) to 1 m for five spaces and then to 500 mm for the two spaces prior to the concrete barrier nesting of the Thrie-Beam over the last 4 m prior to the concrete barrier the use of a structure connector to bolt the Thrie-Beam into a recess in the concrete barrier (Figure F27 of AS 3845: 1999). The Thrie-Beam is stiffened in the manner described in 7.2. which enables the Thrie-Beam to be bolted into a recess in the concrete barrier. An example of such a transition is shown in Figure 7.3.3.4 Thrie-Beam to Concrete The transition between Thrie-Beam barrier and concrete barrier is achieved through the use of a structure connector. Where space is available the barriers can be separated laterally so that they operate independently.
1 — An Example of an Interface between Wire Rope Barrier and W-Beam Barrier 7 .Figure 0.
(Source: VicRoads) Figure 0.2 — Wire Rope Safety Barrier to Rigid Barrier 8 . 2. The dynamic deflection “X” varies depending on the Wire Rope Barrier System used and the post spacing.6 for an approximate guide to deflections of WRSB. Refer to Table 4. X is the dynamic deflection of the WRSB related to vehicles travelling along the road adjacent to the WRSB should they impact it. A is the dynamic deflection for the WRSB for vehicle impacts from the opposite direction.Notes: 1.
No rigid backup is required for this type of system. This type of system is generally referred to as an “inertial barrier” (and may or may not be gating). ‘End Treatment’ (or terminal) is the term applied to devices specifically designed to ensure that the ends of barriers provide safe conditions for occupants of vehicles that may impact this area of a barrier. End treatments are used to terminate a safety barrier or to shield fixed roadside hazards such as bridge piers and the ends of road safety barriers in gore areas. (AASHTO. with serious consequences for occupants. End treatments can be classified as: Gating or Non-Gating. Some of the energy is also dissipated by the crushing of the front end of the colliding vehicle. This type of system is generally referred to as a compression system. When vehicles impact untreated or incorrectly treated barrier ends they stop abruptly or rollover. while others also function as an anchor for the system. This type of system requires a rigid back-up or support to resist the collision force of the vehicle. or both. depending on their behaviour when impacted on the side near the leading end. A crashworthy end treatment must be provided on both the approach and departure ends of barriers that: terminate within a clear zone are located in an area where they are likely to be hit head-on by an errant vehicle.8 END TREATMENTS AND CRASH ATTENUATORS 8. Some treatments function only to provide a safe terminal for the barrier. A barrier end treatment may fulfill its function by: permitting controlled penetration by the vehicle into an area behind the device decelerating a vehicle to a safe stop within a relatively short distance containing and redirecting the vehicle a combination of the above. The first is the kinetic energy principle whereby the kinetic energy of an impacting vehicle is absorbed by “crushable” or “plastically deformable” materials or by other energy absorbers. The second concept involves the conservation of momentum principle where the end treatment design involves the transfer of the momentum of an impacting vehicle to an expendable mass (usually sand) located in the vehicle’s path. 9 . or elements of the barrier may penetrate the passenger compartment.2 Introduction If not correctly installed barriers are a hazard to road users and this is particularly the case at the ends of barriers. usually in the form of a ground anchor or other linkage backup (such as part of the barrier). since the energy of the vehicle is not absorbed but transferred to other masses such as sand (AASHTO 2002). End treatments that stop a vehicle within a relatively short distance are called crash attenuators. The purpose of an end treatment is to protect occupants of a vehicle that impacts the end of a barrier from: excessive deceleration forces rails that may spear into the cabin of the vehicle consequences from possible loss of control of the vehicle. Commonly used crash attenuators use one of two concepts to absorb the energy of impacting vehicles at a controlled rate. The type of end treatment used depends on the barrier. 2002).
The kinetic energy of the vehicle is absorbed by “crushable” or “plastically deformable” materials or by the use of hydraulic energy absorbers placed in front of an obstacle (AASHTO. but this can vary depending on the specific terminal used. may also be designed to “gate”. Gating treatments for semi-rigid barriers comprise either a weakened section of W-Beam that hinges or moves out of the way on impact (e. they redirect vehicles back into their originally intended direction of travel. For flexible and semi-rigid barrier types. end treatments must be properly anchored so that the design operational requirements are achieved in practice. the ‘length of need’ usually starts about one panel of rail from the impact head of the unit.1 illustrates the behaviour of “Gating” and “Non-Gating” systems while Figure 8. or devices that cause the beam to deform or absorb the kinetic energy of the vehicle. They either arrest vehicles when they are impacted directly on the end. If an errant vehicle can pass through an end treatment beyond the nose and into an area behind the system it is classed as a “gating” terminal. It is therefore necessary to ensure that the lateral slope in this area is 1 on 4 or flatter and is free of any fixed hazards (e. Figure 8.Redirective or Non-redirective depending on their ability to redirect impacting traffic away from the hazard.g. MELT terminal). For gating end treatments. in narrow medians). these impact attenuators have the energy-absorbing ability to slowly bring the vehicle to a safe stop. when subjected to glancing or side angle impacts beyond the point of need. 2002). 10 . Non-Gating terminals do not allow vehicles to pass through the leading section of the terminal. Most impact attenuators are based on this concept and need a rigid support to resist the vehicle impact force as the energy-absorbing material is deformed. When impacted head-on. Vehicle that pass through a gating treatment are directed into the area behind the end treatment (i. on the side of the barrier opposite the travelled lane). A gating/redirective crash attenuator can function well in both head-on and side angle impacts. Any re-directive capability required by the design will only be achieved by the end treatment developing the same full tensile strength as the barrier upon impact. Crash attenuators.g. designed to be used as a terminal for concrete barriers or to shield other fixed objects.e. poles and trees).2 illustrates the behaviour of redirective and non-redirective systems. These end treatments are not suitable for use where there is a high potential that an errant vehicle may travel through the end treatment and into a hazard or into opposing traffic lanes (e. or redirect them along the travelled way when they are impacted at any point on the side of the barrier. Angle impacts in the leading section cause the device to “Gate” and. The length of need for a non-gating system is at the nose.g.
They are designed in accordance with the principle of conservation of momentum. detaching from the anchor block when hit and thus wire rope barriers have their end treatment as an integral part of the system. Therefore. whereby the kinetic energy of the impacting vehicle is transferred to the mass of sand. these devices require no rigid back-up or support. In a crash.g. and in side angle impacts is unable to redirect the vehicle back into its intended direction. Safety ropes are provided to ensure that uncoupled wire ropes are not a hazard to adjacent traffic. This is the only type of crash attenuator whose design can be analytically determined.A non-redirective crash attenuator performs most effectively when hit head-on. a non-redirective crash attenuator stops a vehicle in head-on impacts. The end anchors are frangible. These attenuators comprise barrels or containers. End treatments for WRSB may not have been crash tested in accordance with NCHRP 350 (1993).1 — Gating and Non-Gating Systems 11 . This limitation results in continued forward motion at a high speed.2(b). refer to Figure 8. with the consequence that the vehicle penetrates the attenuator. The impacting vehicle continues in the same direction until it either is arrested by the device or impacts an object (e. An errant vehicle running into the end of a WRSB straddles the cables and may be arrested by them as the vehicle progressively flattens posts and comes to rest. typically filled with variable masses of sand. Wire rope safety barrier (WRSB) end treatments are to be provided in accordance with the manufacturer's specification. near the rear end of the device. Figure 8. but may be deemed to be acceptable by the relevant road authority. and their performance is based on the transfer of momentum of a moving vehicle to an expendable mass of material located in the vehicle’s path.
2 — Redirective and Non-redirective Systems It should be noted that a number of end treatments that have been used in the past are no longer suitable for use because they enable the barrier to penetrate (or “spear”) the cabin space of light vehicles and/or cause vehicles to vault or roll. 13 . Sloped (turned down into the ground) ends on semi-rigid or rigid barrier (although a sloped concrete end treatment may be suitable where speeds are low e. 60 km/h or less and space is limited by right-of-way constraints or presence of other features preclude use of one of the tested end treatments). Break away cable terminals (non-slotted). A narrow double bull-nose end treatment on back-to-back W-Beam.Figure 8.g. These end treatments include: Splayed ends (fishtail ends) on W–Beam barrier.
4). Where a barrier is located some distance from the edge of the road it may be possible to flare the barrier and terminate it outside of the clear zone.e. For each type of end treatment the redirective characteristics will vary according to the design. 14 . A non-crashworthy end treatment should only be considered where a detailed assessment concludes that the likelihood of an end-on impact with the barrier is very low.8. material and construction of the treatment and these characteristics should be understood by designers as they must suit the requirements of particular sites. In such cases. and barrier is required between the cuttings.e.3 Types of End Treatments 8. public domain under AS/NZS 3845:1999) or proprietary products (i. These devices can be relatively expensive and in suitable situations it may be appropriate to terminate a barrier in a cutting face or a back slope. in deciding to adopt this technique designers should be confident that the batter will redirect an errant vehicle and not result in it travelling up the batter and behind the barrier.6.5H or steeper). This section describes the types of gating and non-gating treatments that are available.2) it will often be necessary to provide a proprietary end treatment. However.g.g. the cuttings are steep (e. say 1V: 0. smooth and able to redirect vehicles. This may require the use of a suitably designed end treatment/transition to anchor the barrier (e. These terminals have been successfully tested at test level TL3 (refer USA Federal Highways Administration website. This type of treatment may be appropriate where: A road passes through a series of cut to fill lines. MELT refer section 8.3.1 General In many cases safety barriers will have to be terminated relatively close to the edge of the road and end treatments that are non-proprietary products (i.g. it is preferable that a crashworthy end treatment is provided. A suitably designed flat-bottomed drain or V drain exists at a site and it is desirable that the safety barrier passes through the drain and is buried in a 1 on 4 back slope (refer section 2. These non-proprietary. FHWA approval letter CC-53 and CC-53A). As discussed previously. subject to patents) will have to be selected. end treatments are defined as being either gating or non-gating. because a significant percentage of errant vehicles may travel beyond the clear zone. perhaps in the shape of a concrete barrier). At sites that are unsuitable for a non-proprietary end treatment (e.2. that does not require significant disturbance of the cutting face. Key design considerations include: − the height of the W-Beam should remain constant relative to the roadway grade until the barrier crosses the flow line of the drain − a flare rate appropriate until the barrier reaches the flow line − adding a rubbing rail − using an appropriate anchor (concrete block or steel post) that is capable of developing the full tensile strength of the W-Beam rail. buried end treatments can be effective and may be used provided that they are designed and crash tested (including the anchor) to meet the requirements of the appropriate test level (usually TL3).
AS/NZS 3845:1999 gives the guidance that if a clear runout is not possible. These figures indicate grading requirements for a parabolic flared end treatment and for a parallel end treatment respectively. Designers should note that this is a minimum requirement and there are benefits in providing a longer and wider run-out area in the event of an impact with the gating portion of the end treatment.3. the offsets should be measured from the curve. this area should at least be similar in character to adjacent unshielded roadside areas. particularly for the smaller Australian passenger car. This requirement is based on results of a 97 km/h impact test under the FHWA. The run out area must be suitably graded as shown in Figures 8. It has therefore superseded the Breakaway Cable Terminal (BCT) which is no longer used. and are available in Australia include non-proprietary treatments such as the: Modified Eccentric Loader Terminal – MELT Leading Slotted Breakaway Cable Terminal – SBCT AS/NZS 3845:1999 gives further details on non-proprietary systems. MELT treatment) or a proprietary product (extruding head terminal). Modified Eccentric Loader Terminal .3 and AS/NZS 3845:1999). designers should assess the risk involved with the use of a gating end treatment and no suitable run out area versus other options such as the use of a non-gating end treatment.2 Gating End Treatments Gating end treatment systems are designed to breakaway. A gating terminal is considered to have functioned properly if the vehicle remains stable during and after impact and is kept away from the hard point of the barrier system. The MELT is considered to offer improved safety. 15 . Non-proprietary treatments Gating end treatments that are acceptable under AS/NZS 3845. They can be non-proprietary (e. rectangular-shaped run-out area behind the “gate” and the rail (parallel to the rail) in which vehicles can come to a safe stop (refer Figure 8. The general arrangement of the MELT and the run-out area is shown in Figure 8. Gating terminals must be provided with a hazard free. Consequently. hinge or pivot when impacted and therefore allow the errant vehicle to pass behind the barrier. In these situations. In constrained circumstances it may not be possible to provide the runout area.3) should not be measured from a tangent to the curve as this will lead to an exaggerated flare rate and high impact angles by errant vehicles.8.7 or 8. When these terminals are installed on curves the offset to the terminal (refer Figure 8.g.8. The FHWA also notes that the run-out size may not necessarily accommodate all crashes which may occur.MELT This end treatment is included in AS/NZS 3845:1999.3.
In some situations the BCT also resulted in the spearing of vehicles that impacted the system end-on. and the remaining two being steel C channel.3 — General Arrangement Modified Eccentric Loader Terminal (MELT) The MELT system in AS/NZS 3845:1999 consists of eight posts. and a strut is provided at ground level between the first and second posts to serve as an anchorage in order to develop the tensile 16 . testing with vehicles of 820 kg mass has shown that the BCT was too stiff to buckle readily under reduced energy crashes from this class of vehicle. the MELT is “deemed to comply” although it never passed the testing requirements of NCHRP 350 (1993). A cable is provided connecting the W-beam to the bottom of the first post. Under AS/NZS 3845:1999.The design of the BCT had been tested successfully with vehicles having a mass of 1020 kg and 2000 kg. even though it did not cause the rollover. the leading six posts being wooden breakaway posts. Figure 8. However. However. the MELT is not installed on new construction on the National Highway System in the USA. In the test the MELT “gated” but the test vehicle hit the rear of the barrier well downstream of the terminal causing it to roll. As a result. The vehicles of smaller mass did not develop sufficient kinetic energy to activate the pivoting mechanism and testing showed that this class of vehicle was more susceptible to rotational forces than the larger mass vehicles. The BCT is therefore no longer recommended for use. it did pass the NCHRP 230 testing and existing units can remain in place on US highways.
including in-service reports about crash attenuator features should also be used for the selection/design procedure. The first five posts of the barrier are weakened timber posts (4 x 20 mm holes 100 mm above ground level) whilst the end section of the barrier rail is slotted in the first two spans and curved on a 40 m radius away from traffic. Redirection along the travelled way for a side impact begins at the third post from the approach end. it being recognised that specific and highly controlled crash tests are not always adequate indicators of how crash attenuators will perform in different situations. The end of the MELT rail is prebent and a buffered end section is installed to prevent spearing.5 m x 6 m run-out area. and the designer must choose between them (AASHTO 2002). but in most cases two or more types will provide satisfactory protection to an errant motorist. In particular. comprising an 8 m section purchased as a MELT from the distributor and a 4 m panel required to connect from the last timber post to the standard W-Beam installation. The SBCT is similar to the MELT and is illustrated in AS/NZS 3845:1999. In all cases site preparation and the installation of the device must meet the manufacturers’ specifications. No blockout is provided on the first post and the blockout and backing plate are omitted on the first intermediate post. The MELT should only be used with the standard 1. Up-to-date information. Products that provide a “gating” terminal may use the following methods of operation. The standard length of a MELT is 12 m. It has not passed NCHRP 350 (1993) testing but is deemed to comply under AS/NZS 3845:1999. In addition. one crash attenuator will stand out as the most appropriate. including light cars. The number and complexity of factors that enter the selection process for crash attenuators preclude the development of a simple selection procedure. To further impede spearing. Proprietary Products Most crash attenuators including those that gate [refer Figure 8-1(a)] are generally patented proprietary products and the manufacturers’ specifications and representative should be consulted to establish the availability of new and improved products that have passed the required testing procedures. The maximum cross slope in the departure area should not be greater than 1 on 10. Slotted Breakaway Cable Terminal . This design results in the rail bending and folding away from the traffic lane under impact. although if struck by a vehicle travelling parallel to the traffic lane the impact may cause the curved rail of the MELT to bow towards the traffic lane. It is at this point that the "length of need" chord may cross the MELT. can pass through the end treatment with an acceptable level of severity.strength of the beam so that downstream impacts are redirected. This weakening of posts and rail enables the terminal to “gate” on impact so that vehicles. Each operational system has its own unique physical and functional characteristics. the designer should consult with the relevant road authority to determine specific acceptance criteria relating to new and improved products. The Federal Highway Administration (FHWA) web site and approval letters should also be consulted. the point of need may vary between products.SBCT The SBCT is also included in AS/NZS 3845:1999 and is an alternative to the MELT. The designer must therefore refer to manufacturers’ specifications and literature to develop a good understanding of the installation requirements of each device and its behaviour under impact. the rail is not bolted to the second through sixth posts. It also requires a traversable 22.25 m offset of the parabolic flare as any offset flare smaller than this may result in there not being enough kinetic energy in collisions by smaller vehicles to ensure that the terminal's pivot mechanism will activate for all collision angles. Displacement of Sand 17 . In some cases. Many attenuators provide a “gating” function and use various principles and mechanisms through which a safe end treatment is achieved. so that the most appropriate product can be selected for any given situation.
An obstacle free area for a distance of 22. It is essential that the sand used meets specific material grading requirements. Mixing rock salt (5 to 25% by volume) with the sand will help reduce the possibility of errant vehicles hitting barrels of frozen sand. These systems require sufficient width in the verge to accommodate the discarded rail sections and it is important to establish whether the rail is extruded onto the traffic side of the system or to the back of the system. The systems can be used as either a crash attenuator placed directly in front of the hazard. posts and cable anchorages. a system may lose its re-directive characteristics. or as a barrier end treatment. On impact. Some systems may also include containers filled with sand. They can be used on the left side of the road or in medians. 8.3 Non-gating End Treatments General Most non-gating end treatments are crash attenuators that do not allow a colliding vehicle to pass behind the terminal.5 m beyond the terminal (parallel to the rails) and 6 m behind the rails is generally necessary. The energy of the vehicle impact is transmitted to the weights of sand in the barrels. The force of impact is not transmitted through the barrels so backup structures or walls are not required for these systems. These systems can be used to protect hazards of any width and are particularly suited to gore areas. Collapsible Steel Beams and Posts These systems generally involve a structure of W-Beams. Deformation of a Steel Beam These end treatments employ a steel impact head mounted at the leading end of the system. The site must be well compacted and be able to accommodate a concrete or asphalt foundation pad and the transverse slope should not exceed 1 on 20. In this situation. they will not redirect some side crashes. the attenuators will not work as designed. the head is pushed along the W-Beam. their application is suited to: median barrier ends where it is important to prevent colliding vehicles from encroaching onto the opposite carriageway 18 .The containers are held together (e. particularly those occurring toward the rear of the installation. Damaged modules must be replaced after each impact. and nuisance crashes may result in a system not operating as it should. Designers should note that the water content (typically 3%) in the sand might freeze if systems are installed in mountainous regions and cold weather continues for several days. Because non-gating end treatments do not require a clear. thus dissipating the collision energy based on the principle of conservation of momentum. level area behind the barrier. An entire system or some of its components may not be able to be salvaged and used again after a major crash. Collision energy is dissipated by the breaking away of the posts and shearing as W-Beams telescope into each other. On colliding with the end of the terminal. causing the rail to deform. thus dissipating the collision energy. If the first post is damaged in any way. curl around or shred. by cables or other devices) and upon impact the deceleration of the vehicle is controlled through displacement of the sand or liquid. the vehicle will be redirected away from the barrier or be arrested by the barrier.3. However.g. Systems may be designed to suit straight or flared barrier alignments. liquid or other crushable material that contribute to the attenuation qualities of the end treatment.
Others may employ rubber components or crushable materials that are capable of being re-used after impact. the non-proprietary “bull-nose” treatment shown in Figure 8. The end treatment is suitable for use with W-Beam or Thrie-Beam barriers through the use of appropriate transition sections. Non-gating end terminals are appropriate for shielding: barrier ends. the rail in the nose section should not be bolted to the posts. guide cables) may be employed. A similar arrangement with splayed sides can be used to shield objects in gore areas of off-ramps. arresting the vehicle in the process.4). a sign support may be installed behind the bull-nose provided that it has a breakaway support. collapsible structures and other mechanical devices (e. The area within the bull-nose barrier system for a distance of 19. including those in medians exit ramp gore areas fixed objects located within the clear zone bridge rail ends bridge piers. Some systems may dissipate the energy of the impact through a braking mechanism and the nesting of barrier rails. The bullnose is constructed of a circular section of slotted Thrie-Beam supported by breakaway posts. It should also be noted that the original system was crash tested with timber breakaway posts.4 may be suitable. Non – proprietary End Treatments Where there is a need to install parallel semi-rigid barriers. 19 . All impact attenuation systems available at present are patented products and must be designed and installed in accordance with the manufacturer's specification.g.situations where a run-out area is not available. thus precluding the use of a gating terminal. for example to shield a bridge pier in a median. Non-gating terminals employ similar principles to “gating” terminals whereby crushable containers or cylinders. For the bull-nose to deform as intended under impact. This requirement is based on a 100 km/h test. Where necessary.0 m beyond the nose must be free of hazards (refer Diagram A in Figure 8. When a vehicle crashes into this bull-nose the posts in the nose break away and the rail deforms inward. and the bolt heads in the first section of rail at the sides should not be provided with washers.
In addition. They are generally patented and the manufacturers’ specifications and representative should be consulted to establish the availability of new and improved products that have passed the required testing procedures.Proprietary Products Non-gating crash attenuators are also available for use in situations where it is not desirable to allow vehicles to pass through the nose section of the attenuator.2. The point of need for non-gating attenuators is at the nose. This should ensure that the most appropriate product and design is used for any given situation.2) designers must refer to manufacturers’ specifications and literature for non-gating proprietary products to develop a good understanding of the installation requirements of each device and its behaviour under impact. 20 . As is the case for gating proprietary products (refer Section 8. the designer should consult with the relevant road authority to determine specific acceptance criteria relating to new and improved products.
4 — Non-Proprietary Bull-Nose Treatment 21 .Note: Original system was crash tested with timber breakaway posts (Source: DMR Qld) Figure 8.
Figure 8.5.8. on a wide duplicated road) where there is no possibility of an errant vehicle from the opposing traffic flow striking the barrier “end on”. it is not a crashworthy terminal and should only be used on installations (e. Therefore the Trailing Terminal should not be used within the clear zone for traffic from the opposing direction.3. Where the trailing end of an installation is located within the clear zone for opposing traffic a MELT or other appropriate terminal should be used.g.5 — Trailing Terminal 22 . Whilst this terminal will gate.4 Trailing Terminal This terminal that incorporates a cable anchorage and is shown in Figure 8. Its main function is to provide a cable anchorage at the trailing end of the system.
The selected barrier end should be suitable for the speed environment at the location.g. especially nuisance crashes. it is important that the installation complies with all of the manufacturers’ recommendations with respect to space. This is particularly important where the approach is on a tight curve. For instance. thereby reducing the maintenance that must be undertaken following an incident. Particular end treatments and different configurations of the same crash attenuator will be suitable for particular speed environments. a situation that may be encountered in construction zones. Some crash attenuators and end treatments such as the MELT may require a large run-out area free of hazards for gating of the end. Although the figure depicts a gore location. Consideration may be given to selecting a physically smaller system on the basis that a smaller size will reduce the number of crashes.1 General The selection of the most appropriate crashworthy end treatment for a barrier should take the following factors into account: end treatment’s gating characteristics end treatment’s re-directive characteristics speed environment space available for installation and deformation of the terminal width required for accommodation and deformation of the terminal capacity to absorb nuisance crashes compatibility with barrier type cost and maintenance factors.2 Speed Environment The end treatments and crash attenuators discussed may have been tested for different speeds. while others may require space to accommodate displacement of the attenuator clear of traffic.6 shows the area that should be made available for crash attenuator installation. The space requirements of crash attenuators to shield non-removable fixed objects should be considered during all stages of road design and construction (e. This will ensure compatibility between the final design and the crash attenuator that is to be installed. However. the information provided in the table is generic and should therefore only be used for planning purposes.4. Detailed product information should be used for design purposes. 8. The length of some crash attenuators can be varied depending on the speed environment and likely maximum impact speed.4. Manufacturers’ advice should be sought. However.4. 8. Factors that are important in selection of an end treatment are further discussed below. The speed limit is usually taken to represent the speed environment as it provides a practicable relationship to barrier test speeds.4 Selection of End Treatments 8. the same recommendations will generally apply to other types of fixed objects that require shielding (AASHTO 2002). preliminary design for new works or rehabilitation of existing roads). 23 .3 Space Availability The space available for the end treatment will also influence the type to be installed.8. Figure 8. some crash attenuators are more suited for use in narrow medians while others are suitable to shield wider hazards. Crash attenuators should be orientated so that they face the most likely direction of impact.
5 0.5 21 1.5 7. In some instances a transition section will be required to ensure adequate stiffness is provided at the connection between the end treatment and the barrier.5 2.4.5 17 1 3.5 3.5 80 2 5 0.6 — Space Required for Crash Attenuators in Gore Areas.5 5 1.5 1 3. This stiffness is required to minimise vehicle snagging and pocketing of the barrier.5 130 2 11 0. The chosen system should be capable of performing satisfactorily following a number of these minor crashes without requiring repair.4 Susceptibility to Nuisance Crashes Like any part of a barrier.5 13. The unrestricted conditions represent the minimum dimensions for all locations except for those sites where it can be demonstrated that the increased costs for obtaining these dimensions (as opposed to those for restricted conditions) will be unreasonable. It is reasonable to suggest that a non-gating system would perform better than a gating system in this respect. and to limit the change in deflection occurring between the barrier and the end treatment.Dimensions for crash attenuators. Manufacturers’ specifications should therefore be consulted.5 (Source: AASHTO 2002) Figure 8.5 110 2 8.4.5 2. 8.5 2.5 1 3.5 10 1. 8.5 0.5 17 1. 24 .5 Compatibility to Barrier Type Designers should ensure that the chosen end treatment or crash attenuator is suitable for use with the proposed barrier type. end treatments and crash attenuators are susceptible to nuisance crashes. The preferred conditions dimensions should be considered optimum.5 1 3. reserve area (metres) Design speed on Preferred Minimum main carriageway Restricted conditions Unrestricted conditions N L F N L F N L F 50 2 2.5 2.
In the specific case of W-Beam barrier. (e.5 Terminal Approach and Placement Conditions As end treatments are designed and tested on flat and level terrain with a vehicle impacting at normal height. Figures 8-7 and 8-8 show desirable grading of the site for these types of treatments. in some narrow medians).g. The direct costs associated with worker safety and traffic management also need to be considered. make it frangible. The repair and replacement time for an attenuator system following an impact is also an important consideration as this can cause significant losses to road users through delays. end treatments can either be flared or non-flared. These features can cause a vehicle to become airborne and ride over the barrier or rollover on impact. life cycle costs for repairing or replacing an attenuator system may be a significant factor in the selection process. At locations where frequent hits are expected. so they are generally used only where it is likely that errant vehicles will hit a hazard with severe consequences. usually constructed of concrete. or realign the traffic path away from the hazard there is insufficient room for a normal barrier and its terminals. and either: it would be very difficult or costly to remove or relocate the hazard. Failure to do so may result in the device failing to perform as intended. it is imperative that these conditions be replicated in practice. 25 . Site preparation costs to accommodate some systems can be significant. Maximum crossfalls are recommended for various terminal installations. respectively.6 Cost and Maintenance Factors In selecting an end treatment for a barrier system a designer should take into account the whole of life cost associated with the treatment including: capital costs maintenance costs risks associated with maintenance repair times.8. such as excessive slopes or kerbs.4. 8. low maintenance area and uniform support for the sand barrels. Crash attenuators must therefore be placed on a relatively flat surface and the path between the road and the attenuator must be clear of any irregularities or obstructions. Energy absorbing attenuators must be placed on a hard. Crash attenuators are relatively costly to install and to repair after impact. Although a paved surface is not necessary for inertial systems it does provide an appropriate. smooth pad or surface. or normal barrier ends would form unacceptable hazards.
8 — Grading for Non-flared Barrier End Treatment 26 .7 — Grading for Flared Barrier End Treatment Figure 8.Figure 8.
Therefore maintenance personnel. In these situations the cost of traffic control and the risk of other collisions occurring during the maintenance operations are significant factors. Safety barriers including end treatments that have identified hazardous features should be replaced as soon as practicable. on barriers may easily exceed the initial installation cost. Safety barrier products continue to evolve within the service life of current installations. as well as construction personnel. are able to meet their original performance standards. high-volume urban freeways where the width available for works is often limited. particularly repair work. The level of maintenance required is often an important factor in relation to the occupational health and safety of road maintenance workers and the convenience and safety of road users during the period when barriers are being repaired. Provided that they are in an acceptable condition. This is particularly important at locations where impacts are likely to occur frequently or be severe.3. 27 . the severity of impact. should be fully briefed on the salient features and operational characteristics of the type of installation used. It follows that standardisation of details and components will help to minimise maintenance costs. and may be located in highly hazardous locations. especially in the more vulnerable locations. works are usually very disruptive to traffic flow. so that they do not inadvertently omit or alter some important component. semi-rigid barriers can remain in service after moderate impacts and rigid barriers require virtually no maintenance after significant impacts. Maintenance time and cost is an important factor. and do not include identified hazardous features. Capital cost. it is essential to include the potential maintenance costs in the economic assessment of alternative barrier designs and site-specific barrier types. but only one factor in selection of a barrier. and the continuing integrity of the barrier pending maintenance are also important.3 Routine Safety Barrier Maintenance 9. Although replacement of outdated components may be desirable. The cost of maintaining a barrier to preserve its functional integrity is influenced by the cost of replacement parts and the labour-intensive nature of the operation.9 MAINTENANCE OF SAFETY BARRIERS 9. This is particularly the case on high-speed. Therefore. Wire rope safety systems require maintenance after even minor impacts. 9. superseded components may be maintained until it is economically viable to replace them. The cost of maintenance work.1 General Routine maintenance is generally directed at the long-term upkeep of the barrier installation to ensure the retention of those properties of the system that allow it to fulfil its intended function. it is not practicable in terms of other safety priorities and budgetary constraints.2 Introduction The maintenance costs of barriers should be included in any economic assessment of the type of barrier to be installed.
galvanised components throughout. shrinking or splitting of blockouts treatment of the tops of posts checking post deterioration due to rotting.1 Cost Factors Impact repair costs for barriers can be significantly influenced by factors relating to the: type. Measures that may be adopted at the installation stage to minimise or simplify routine maintenance include: specifying an acceptable tolerance on anchor cable tension replacing timber posts and blockouts with corrosion resistant steel or using unpainted.4 Repair and Reinstatement 9.g.Routine maintenance includes: tensioning of cable anchors removal of rectangular washers from the heads of bolts holding rails to blockouts or posts (whilst this is not strictly a routine maintenance activity. of the installation struck by a vehicle 28 . This means that investigation of the locations of underground services is just as important for the determination of barrier position as it is for any other aspect of road design (e. and section. The coexistence of the barrier with other assets. free from oblique grain and gum veins protecting timber post tops with a durable moisture resistant coating (e. is relevant to maintenance operations if there is a risk of one of these assets being damaged while repairing the other. including the buried section.4. especially splice bolts correction of the rotation. such as underground cables and conduits. provided that the system with the steel posts has been suitably crash tested replacing the ground surrounding the post with an encasement of granular material that permits suppression of fungal attack using thoroughly seasoned high durability timber.g. 9. and longitudinal splitting correction of the effect of embankment settlement on the installation (i. or provides adequate space between the installation and fixed obstructions for workmen and/or machinery to operate effectively.2 Installation Measures All barriers must be installed in accordance with AS/NZS 3845:1999. and any relevant road authority specifications.g. petroleum jelly).3.3.e. drainage. signal foundations). While undertaking routine maintenance other remedial treatments (e. incorrect aspects of installation should be rectified when a section of barrier is being repaired) ensuring that all bolts have been installed correctly. These measures should also be incorporated when replacement or major servicing of safety barriers becomes necessary.3 Related Factors Maintenance work can be made simpler and cheaper if the barrier is located so that it allows access for machinery to maintain roadside areas. upgrading of end treatments) may be considered. 9. cross fall in front of the barrier and barrier height. the requirements of manufacturers of proprietary products. 9.
galvanised) avoidance of excessively curved lengths of rail which may require precise shop-curving. the following considerations should be addressed.crash? If not. 9.4. and should ensure optimal performance for future installations. as it was pre. impact speed and angle of impact location of the installation with respect to the adjacent roadway and the nature of the road degree to which terminals are affected availability of replacement items. which upgrade measures should be carried out to improve the safety of the hazard? It is noted that AS/NZS 3845:1999 suggests that an action plan for maintenance of safety barrier systems should include these assessment criteria. These observations and feedback by maintenance contractors and staff will identify any problems that may occur with the system. After crashes into barrier systems.g.5 Operational Monitoring Monitoring of barriers in the field is the best way to determine the performance of a barrier under particular situations. as a minimum: Did the system function as designed? Should the system be repaired. The Australian Standard on safety barriers (AS/NZS 3845:1999) requires that post-crash evaluations be carried out.2 Installation Measures Characteristics that facilitate the progress of repair work are: correct installation in accordance with relevant specifications use of standard.vehicle type. 9. components post embedment in non-bound materials proper embedment of posts to the correct depths replacement using correct posts that also ensure adequate strength and quality use of the correct bolts and nuts (e. or matchable. 29 .
and experience in their application. Temporary barriers are used in situations where protection is required for a limited time. It is particularly important that where safety barriers are necessary they are used in a consistent and appropriate way. Inappropriate speed limits or operating speeds can also increase the risk for road users. Against this background there is a heightened awareness of Workplace Health and Safety requirements. Examples include: roadworks sites (most common application) protection of infrastructure works on land adjacent to the road and associated work space special events where there is a need to control vehicle and pedestrian movements. selection and location of temporary barriers that are used in situations where a permanent barrier is inappropriate. it has serious implications with respect to increasing litigation and insurance costs. While publicity campaigns and enforcement (often utilising radar devices and speed cameras) may assist. Apart from personal injury and grief. This can only be achieved if relevant personnel are educated and trained to have knowledge of relevant guides and standards. With respect to roadworks. occupational health and safety is an important aspect in the management of road systems.10 TEMPORARY SAFETY BARRIER SYSTEMS 10. and the potentially hazardous environment in which road workers have to perform their duties. generally these measures are outside the control of the personnel on a construction site. enforcement of speed can be a problem even with active police participation. In some cases incorrect or inappropriate practices that have not led to any known problems on previous projects may be repeated. It is equally important that designers focus on guiding traffic safely through work sites by: the development of effective temporary traffic management plans including adequate standards for sidetracks and deviations for traffic within sites 30 . Audits of construction sites have highlighted deficiencies that exist in work zone barrier systems and indicated a lack of knowledge about safety barriers and their fundamental design (Muthusamy and Kumar 1995). Where long-term construction sites are created. Construction and maintenance managers. Some of the problems identified included: installations that are too short to shield errant vehicles from the hazard embankment approaches to barriers that are too steep to ensure that the barrier will be effective ineffective end treatments inadequate clearance to workers or hazards in order to cater for the dynamic deflection of the barrier under vehicle impact. both during and after worksite operating hours. supervisors and workers must therefore take an active role in ensuring that work sites are safe for workers and road users. engineers. Contributing to these hazards is increasing traffic volumes. larger vehicles and in some cases higher speeds through and adjacent to work sites.2 Introduction This section provides guidance on the use.
as they are a hazard in themselves. both for workers and traffic passing through work sites. In determining whether a safety barrier is required. and for traffic control for major events. the designer should address the following questions: 31 . Road Safety Barrier Systems defines a temporary road safety barrier system to be “a device designed to be erected and dismantled quickly. The principles also apply to temporary barriers used for other works or activities adjacent to roads. It is very important that temporary barriers are located so that they do not place road users at risk by restricting sight distance for traffic entering. they may improve job productivity and reduce road user delays. Only temporary barriers that have been successfully crash tested to the same level required of permanent barriers shall be used to shield worksites.g. They are used in situations where it is considered that traffic volumes. Care must also be taken to ensure that drainage is adequate so that stormwater does not form ponds adjacent to the barrier and or flow across the road at a depth that could cause vehicles to aquaplane. 10. Part 3: Traffic control devices for works on roads. temporary safety barriers may be used to: provide positive protection for workers from vehicles entering the worksite protect critical construction works such as bridge false work from vehicle impact prevent traffic from entering work areas where hazards such as trenches. guide signs and delineation) that meet the requirements of AS 1742. crossing or moving through the worksite. The guidelines in this section cover the types of temporary safety barriers currently available and how and when they should be used to enhance safety. Like permanent barriers. Reduced speed limits are often implemented on high-speed. As well as enhancing site safety. Specifically. they should only be used if they reduce the severity and adverse consequences of potential crashes. while providing worker protection”. Its purpose is to redirect an impacting vehicle so as to minimise damage to the vehicle and injury to the occupants. warning signs.3. and the nature of the work (e. traffic speeds.3 Purpose and Use of Safety Barriers at Roadwork Sites The Australian Standard AS/NZS 3845:1999. particularly those carrying high numbers of large vehicles and where workers perform duties immediately behind barriers. used to prevent vehicular access into construction or maintenance work zones. material stockpiles or construction plant could endanger road users separate opposing traffic where temporary traffic diversions or sidetracks may lead to potential vehicle conflict minimise road user delays by negating the need for worksite speed limits.provision of effective and well-maintained signs and markings (including regulatory signs. highvolume roads. Temporary safety barriers are therefore used to contain and redirect errant vehicles so as to prevent them from leaving the roadway and/or entering the worksite. worksite/traffic separation and duration of the works) indicate that it is both desirable and practical to provide such additional protection. Manual of uniform traffic control devices (MUTCD). As temporary safety barriers and most permanent safety barriers are not designed to contain large trucks the use of temporary barriers does not necessarily negate the need for reduced speed limits adjacent to construction areas.
and because of the economic advantages. etc. what is the traffic speed likely to be outside of construction hours? Bearing in mind the duration of the particular works and the space available to locate safety barriers. Generally barriers used for road works will be of a temporary type to suit the relatively short duration that they are required. road surface/alignment. The appropriate arrangement of sand filled barrels should be designed to suit the width of the hazard and the speed of approaching traffic.. Empty units weigh 25 to 60 kg (depending on manufacturer) and they are therefore able to be lifted and positioned by two workers without the need for cranes or special equipment.2. The profiles of the F Type and Single Slope concrete barriers are shown in Figure 4. Details of proprietary products deemed acceptable by the relevant road authority are available from manufacturers. Plastic barriers are light in weight and of modular design that makes them very portable. Permanent systems may be used where the particular road works site requires shielding for a relatively long period and it is cost effective to provide a barrier. single slope. the speed of vehicles through the site. would the use of safety barriers improve the safety of both workers and road users and should they therefore be provided? 10. is it practical to install safety barriers? Is the consequential effect of a vehicle striking construction features such as bridge false work such that protection must be provided? In view of the nature and duration of the particular work.g. 32 .4 Types of Longitudinal Safety Barrier Systems for Temporary Use Work site safety barriers can be permanent type installations.Can the speed of vehicles be maintained at such a value through the work site that. Temporary safety barriers that are commonly used are: precast concrete safety shape units (e. or temporary installations that are more readily relocated. but there may be a case for the use of semi-rigid permanent barriers depending on site and project characteristics. F Type. and the clearance between such traffic and workers/roadside hazards. in combination with worker/roadside hazard clearance and the quality of the traffic arrangements (traffic control. The following section outlines some types of temporary barrier systems that are available. the need to move them in accordance with construction and traffic staging requirements. The cost of installing and removing permanent concrete barriers is prohibitive. The concrete and water filled plastic barriers comprise units that must be properly connected over the minimum length required in order to perform in the appropriate manner. Other types of temporary systems may be available or under development.) the risk of injury to either workers or road users is consistent with good practice and the requirements of health and safety legislation? As a temporary barrier has to cater for the highest speed environment that applies during its deployment. or proprietary units) water filled plastic barriers (proprietary products) sand filled barrels that may be used to shield narrow individual hazards.
1 Connection of Individual Barrier Units (Precast Concrete and Water Filled Plastic Systems) Barrier units will act as a safety barrier only if they are properly connected to each other throughout the whole installation. etc. individual units will either topple over or slide creating considerable risk to workers. steel plates.3 Minimum Length The minimum length of temporary barrier installed shall not be less than the length of system recommended by distributors and based on successful crash tests. it is recommended that temporary barrier systems not be installed in proximity to kerbing or where the slope on pavements.5. Barriers of different profiles and materials should not be used in the same installation unless an approved transition is provided as 'pocketing' of impacting vehicles could occur due to the different stiffness and/or shapes. concrete keys or a combination of steel pins and cables. roadworks) have sufficient strength to redirect an impacting vehicle. The plastic water filled safety barrier shell will have an internal or external steel frame to provide the required strength during an impact. stiffen and connect temporary barrier units must comply with the designer’s or manufacturer’s drawings and specifications. Sufficient clearance must therefore be provided between the back of the barrier and the work area to allow for dynamic deflection of the barrier. The length of temporary barrier required to shield the hazard should be determined from the length of need for the particular site plus the additional lengths necessary to provide end treatments 33 .10. Furthermore. Temporary concrete and plastic safety barriers will generally be free standing (not anchored). For example. the impacting vehicle and other road users. The method of connection will vary depending on the type of safety barrier but generally consists of steel pins.5. and to resist displacement of the units.g.5. All materials and components used to construct. If impacted. 10. This may be a critical consideration for barriers located adjacent to trenches. At a particular site the installation must: be adequate to shield the hazard (e. foundation excavations.2 Safety Barrier Foundation Temporary barriers must be founded on a base that enables proper alignment and is capable of supporting the barrier and other loads created during impacts. Installations of unconnected units do not form a safety barrier in any way. deep pavement boxings. Plastic water filled safety barriers have interlocking knuckles at the ends of the units to enable them to be joined with a pin and swivelled to follow the required alignment. Only connections that are in accordance with the designer’s or manufacturer’s specification should be used. shoulders or batters is greater than 1 on 10. Joint movement may cause snagging and pocketing of impacting vehicles. when barriers are used at roadwork sites the following issues should be addressed.5 Operational Requirements for the Use of Barriers at Roadwork Sites The principles that apply to permanent barriers are also applicable to the use of temporary barriers. The connections provide barriers with the continuity necessary to ensure that differential movement does not occur at the joints between units. 10. 10.
4 Barrier Lateral Location Offset between Barrier and Work Area/Hazard It is necessary to provide sufficient space between the barrier and the work area to accommodate the dynamic deflection of the barrier.0 m).fhwa. However. TL3 test results for temporary F Type and New Jersey barriers show that the deflection of barriers ranged from 1. TL3). The lengths of the systems tested ranged from about 40 m to 68 m.0 m to 4. the angle of impact. and consider this information together with the experience of the relevant road authority with respect to particular devices. each site should be assessed on its merits and barrier requirements evaluated in accordance with the manufacturer's design criteria. The US Federal Highway Administration (FHWA) website (www.5 m. The FHWA acceptance letters show that the deflections of water filled plastic systems varied substantially depending on the design of the system. Deflections for barrier systems passing TL2 ranged from 2.0 m. The lengths of the systems tested ranged from about 40 m to 80 m.0 m and those passing TL3 ranged from 4.dot. 10. Designers and project personnel should seek test results and recommendations from manufacturers or distributors regarding barrier deflections under impact and required clearances.5.3 m to 6. Shallower impact angles may be more applicable to a construction site as traffic may be more constrained through the use of various signing devices. curved alignment or multi-lane carriageway). 2000 kg vehicle at 100 km/h.9 m. Temporary safety barriers should be installed so that the likely angle of impact is minimised as this will also minimise the dynamic deflection under impact. 3. 34 .7 m and 6.The barrier should meet the appropriate test level and hence be capable of redirecting the appropriate design vehicle travelling at the likely speed of traffic (e. Water-filled barriers may be suitable in such circumstances. The deflection of temporary barriers is a function of the speed and type of impacting vehicle.0 m to 2.gov) provides a listing of acceptance letters containing summaries of test results of various temporary barriers and practitioners should utilise this information in considering the design of temporary barrier systems. The test results contain barrier systems that have several different segment lengths (3. as well as the speed environment that will operate at all times when the barrier is erected. and the design of the barrier system including its foundation and connections. For this reason water-filled barriers are generally not suitable for many high-speed environments where the work area is close to the existing road and impact angles of errant vehicles may be high (e.g. Project management and supervisory personnel carefully consider the orientation of the barrier in relation to approaching traffic.g. and in many urban situations the speed of traffic can be controlled to 70 km/h or less. It is noted that deflections of water filled barriers (as determined during crash tests) may be quite large.
available space) a clearance as small as 200mm from the edge of the traffic lane may be acceptable. At work sites it is necessary to accommodate the required number of traffic lanes. On the other hand. The width of traffic lanes required depends on the traffic mix and the alignment of any temporary sidetrack or diversion. Depending on site conditions (e. Regular cleaning of delineators by site personnel before nightfall enhances night-time safety in general and for workers if work is being carried out at night. Where there are few trucks in the traffic stream and/or speeds are relatively low (e. clearance and shoulder width (if any) for any given construction environment. However. particularly at night. 10.g. in order to keep impact angles to an acceptable value. 35 . and the need for ongoing maintenance and the associated traffic disruption may be a significant issue. dirt or grime because of site activities. the desirable minimum clearance should not be less than 500 mm. On the other hand. road alignment. a barrier erected on one side of the road adjacent to lanes that have desirable widths may require minimal clearance. However. This will aid in guiding road users through the work site. and perhaps a shoulder. or travelling adjacent to the site (e. Delineation of temporary barriers is particularly important as barriers at road works sites frequently become covered in mud.Offset Between Barrier and Traffic Lane The clearance between a safety barrier and the edge of the traffic lane is important for driver safety.5 Delineation Temporary barriers should have adequate delineation installed.g. and also alert road users to the presence of a barrier. For driver safety and to maintain traffic flow conditions. normal shoulder widths should be maintained between the outer edge of the traffic lane and the barrier. It should be noted that barriers placed very close to the edge of the traffic lane are more likely to sustain damage due to minor impacts. roadworks speed limit. it is desirable that safety barriers placed parallel to the pavement should not be located more than 4 m from the edge of the travelled lane. 60 – 70 km/h) it may be acceptable to reduce traffic lane widths in order to provide adequate clearance to barriers. Where safety barriers are provided on both sides of a temporary traffic arrangement it is particularly important to provide adequate clearances to the barriers. in restricted situations a traffic engineer or road designer should be consulted to determine the most appropriate balance between traffic lane width. Wherever practicable. As a general rule. adequate clearance to barriers.5. and to enable traffic flow and capacity to be maintained. stopping sight distance). It is also very important to ensure that placement of barriers does not impede sight distances for all drivers entering and leaving the work site near barriers (intersection sight distance). when temporary barriers are installed alternately on both sides of traffic it is desirable that the ends of the barriers be staggered by a minimum of 30 m.g. the width available for “staging” traffic during works is often constrained and project managers and supervisors therefore need to determine how best to use the available space with respect to the various cross section elements.
the existence of street lighting does not remove the need for effective delineation.1 Concrete Barriers Concrete barriers should be used at sites where the consequence of errant vehicles striking critical construction works (e. Some brands of plastic temporary safety barrier have integral fittings that allow them to be lit internally whilst others have moulded sockets to accommodate standard workplace warning lights. a solid edge line and raised reflective pavement markers may be placed along the pavement adjacent to the barrier to improve delineation. they will generally reflect sufficient light from headlights to allow road users to see the path that they are required to follow. 10. temporary safety barriers should be coloured white or red/orange.5. In urban areas.6. 36 .3. In order to achieve suitable daytime visibility.To provide acceptable night time visibility.4 also applies to temporary barrier systems. acceptable visibility may be achieved through the public lighting system provided that the barrier is of a light colour. retroreflective devices or non flashing warning lamps should be mounted along the safety barrier. 10.5. Retro-reflective delineators such as Class 1 adhesive tape are easy to apply and. The use of a higher grade material (e. delineation devices should be orientated so as to reflect light back to drivers. Part 3 of the MUTCD. they should be deployed as shown in AS 1742. If warning lamps are to be used. However. Class 1A) should be considered where the delineators have to compete with other background information such as signs and lights. with observation and feedback being provided by construction and contract personnel. 10. Where barrier is on a curved alignment. the following approach should be adopted.g. Yellow reflectors provide greater luminance than other colours except white.7 Operational Monitoring The discussion relating to operational monitoring for permanent barriers in section 9. The use of barrier segments having alternating colours is also an effective way of improving the visibility of the barrier and the alignment of the travelled way. Some temporary barriers allow water to pass beneath or through the barrier. In addition. Yellow is the recommended colour for roadworks warning purposes.6 Drainage Drainage from the uphill side of barriers needs to be provided to avoid ponding against and/or concentrating flows at the ends of the barrier. while their performance is diminished by dirt. generally perpendicular to the direction of traffic.6 Selection of Safety Barrier Type for Worksites In considering what type of safety barrier to use. Yellow is also readily identified with warning signs and lamps and is the logical colour to use in defining the alignment of any temporary safety barriers. bridge false work) could have major flow-on effects.g. For this reason the dynamic deflection of temporary concrete safety barriers must be accommodated between the barrier installation and the work zone. 10. both of which can create a hazard to road users. and in a manner that will not endanger the occupants of impacting vehicles.
as a more substantial concrete barrier than a TL3 barrier may be warranted. particularly semi-trailers. The deflection of water filled barriers varies according to the model used and it is most important that all relevant information is obtained from the manufacturer and considered in conjunction with other information such as test results and the prior experience of road authorities and contractors. Temporary concrete barriers should allow surface drainage water to pass beneath the barrier or between the barrier segments at the joints. repair activities would cause severe traffic problems.4) Attention also needs to be given to drainage of water from the units as wetting of the pavement could create a slippery surface and therefore be an unexpected hazard to road users. if installed as such as a temporary barrier. Advantages of water filled plastic barriers may be seen as: low or nil maintenance. Empty units weigh 25 to 60 kg (depending on manufacturer) and are therefore able to be lifted and positioned by two workers without the need for cranes or special equipment. This is an important factor for critical sites where protection needs to be maintained at all times. the angle of impact and the characteristics of the particular barrier and impacting vehicle.4. Worksite characteristics should be considered to ensure that the barrier will perform satisfactorily when impacted at the speeds and angles likely to occur at the site.6. out of normal working hours).2 Water Filled Plastic Barrier Only barriers that meet Test Level 3 should be used where impact speeds will exceed 70 km/h whereas a barrier meeting Test Level 2 requirements may be used only where impact speeds will be less than 70 km/h. If the units cannot be drained at the site then the water may be siphoned out or the units moved by machinery and drained at a suitable location.Concrete barriers generally meet Test Level 3 (note that a low profile TL2 barrier exists) and provide a high level of protection and generally continue to remain functional after being struck. 10. Some systems can be more easily moved to allow for short-term traffic management of incidents within the site. The deflection of water filled plastic barriers and their performance under impact is related to the speed of impact. Where impact speeds will not exceed 50 km/h a barrier that meets test level TL0 or TL1 may be used depending on the number of cars in the traffic stream that are heavier than 1600 kg. 37 . Consideration should also be given at critical sites to the traffic volume and mix of commercial vehicles. modular design of water filled barriers makes them very portable. either as a delineator device for traffic guidance or. apart from maintaining water levels easy re-positioning (compared with fixed barrier) when it is desired to alter traffic flow or allow equipment access colour differential for high visibility in adverse weather impact force transmitted longitudinally throughout an interlocked system versatility of use.g. The light weight. They may therefore be seen to have an advantage at sites where safety barriers have to be moved frequently to comply with traffic staging or works requirements. geographical location. Water filled plastic barriers may only be used where the deflection of them under impact can be accommodated between the barrier and the work area. or maintenance response would leave the site unprotected for an unacceptable period (e. Test results have indicated that the deflection of some barriers can be substantial (refer 10.
Separate end treatments are not required for temporary safety barriers if the barrier can be suitably flared so that the exposed end is located outside of the clear zone. The most appropriate crashworthy end treatment for a barrier should be selected following consideration of: crash attenuator characteristics redirective characteristics design speed of the road space available for installation of the terminal capacity to absorb nuisance crashes compatibility with barrier type cost and maintenance factors.6. or (in some instances) plastic barrier is used it must be protected by a suitable end treatment. The speed zone value adopted must also be consistent with the physical restrictions and general driving environment at the site. Manufacturer’s/distributor’s specifications should be consulted to establish the minimum acceptable length and the appropriate point of need. Some plastic water-filled barriers may be provided with additional segments at the leading end that provide a form of ‘gating’ terminal. precast concrete. The speed value selected to determine clear zone values and suitable temporary barriers must be consistent with the 24-hour operation of the road and not only the temporary daytime worksite speed zones that may be employed.3 End Treatments The ends of temporary safety barriers must be appropriately treated as they can be a major hazard to road users if struck end on. Temporary concrete barriers may be provided with a crash attenuator designed for the appropriate speed environment or a suitable gating non-directive crash attenuator. this may not often be the case for temporary systems. Where a temporary W-Beam. 38 . Where W-Beam is used as a temporary barrier a conventional MELT system or any other suitable W-Beam end treatment should be used depending on site conditions (refer Chapter 8). While sloped end treatments are not acceptable for permanent barriers because of the propensity for vehicles to vault or roll.10. However. a 6 m long sloped end may be adopted for temporary concrete barriers where project staff are confident that impact speeds will not exceed 60 km/h.
. Australian/New Zealand Standard. Truedsson and Tingvall of Monash University Accident Research Centre (MUARC). ATSB (2000b) Motorcycle and Safety Barrier Crash – Testing: Feasibility Study. AASHTO (1996) Roadside Design Guide American Association of State Highway and Transportation Officials. New South Wales. Austroads. Victoria. ARRB (2005). Part3: Traffic control devices for works on roads Standards Australia. Austroads (2004) Guide to Traffic Engineering Practice Series. Second Edition. Austroads (2002) Road Safety Audit. Australia. AP-G15. Sydney. Australia.gov.1. Publication Number AP-G11.4/04. USA Ajluni. Sydney. Australia.3 – 2002. Publication Number AP-G1/03. Austroads. No 4. Corben. Public Roads. Australia. Austroads (1999) Guide to Traffic Engineering Practice. Australian/New Zealand Standard. ATSB (2001) Road Fatalities Australia 2001 Statistical Summary Australian Transport Safety Bureau. AS 1742. Part 13 – Pedestrians. Austroads (1996) Benefit Cost Analysis Manual.standards. Australia. Part 15 – Motorcycle Safety. Austroads (2003) Rural Road Design: A Guide to the Geometric Design of Rural Roads. Road Safety Risk Manager User Guide.1/04 and CD ROM. USA. Part 14 – Bicycles. Sydney. AS/NZS 3845 – 1999 Road Safety Barrier Systems.C. Australia. http://www. AS 5100:2004 Bridge design. Publication Number AP-G30/02. 2000. Sydney. Standards Australia. Washington D. Sydney. Austroads. — 39 — . Publication Number AP-42. Karen K. Vol 52. New South Wales. Sydney. Publication No. Pages 107-13. Section 1.au/catalogue/script/Details. Austroads. Part 4: Treatment of Crash Locations. Vermont South.au Austroads (1992) Bridge Design Code. Austroads. New South Wales. Austroads (1993) Guide to Traffic Engineering Practice. Austroads (2002a) Urban Road design Guide: A Guide to the Geometric Design of Major Urban Roads. Austroads. Prepared for ATSB by Duncan. Australia. Sydney. www. New South Wales. Sydney. New South Wales. Washington D.asp?DocN=AS713422603459. (1989) Rollover Potential of Vehicles on Embankments. Austroads (1999) Guide to Traffic Engineering Practice. ATSB Working Party Report. June. Standards Australia. Australia. Austroads. Sydney. Publication Number AP-G69/02. New South Wales.. Publication Number AP-15. Australia.C. Sydney. ARRB Group. Sideslopes and Other Roadside Features. March 1989. AS/NZS 4360 – 2004 Risk Management. Austroads. Standards Australia. ATSB (2000a) Review of Wire Rope Safety Barriers. Sydney. New South Wales.REFERENCES AASHTO (2002) Roadside Design Guide American Association of State Highway and Transportation Officials. New South Wales.atsb.com. Sydney. Manual of uniform traffic control devices.
gov/hq/traffops/signtech/signdel/trafficmanual. Washington D. (2000).1. Australia. http://www. Roads and Traffic Authority of New South Wales. Transportation Research Board. NCHRP 350 (1993) National Cooperative Highway Research Program. pp173-8. http://safety.htm CEN (1998). June 6. Proceedings of The Road Safety Conference. Volume 63. (NHI Course No. (1987) Reduction of injury severity involving guardrails by the use of additional WBeams.qld. 65-71. Publication No. 1966.C. Canberra. June 2000. Sala & Astori (1998) New concepts and materials for passive safety of motorcyclists. Australia. www. National Research Council. (1966) Medians of Divided Highways – Frequency and Nature of Vehicle Encroachments.mainroads. ACT. University of Illinois Engineering Experiment Station. Eleventh International Technical Conference on Experimental safety Vehicles. Guide Rail. P. 1993..fhwa. Sections 8.F. NY (2003) Highway Design Manual.be/catweb/93. Recommended Procedures for the Safety Performance Evaluation of Highway Features. and Kennedy.April 2000. Road Work Sites – Are Motorists and Road Workers Really Safe? Proceedings of The Conference: Accident Investigation. 1981. Bulletin 487. September 1998. National Academy Press. Brisbane.30.gov/roadway_dept/road_hardware/barriers/pdf/b110. National Road Safety Summit. Vol. Australia. T. Public roads. Department of Main Roads. no. Muthusamy and Kumar (1995). Canberra. Transportation Research Board. pp878-883). Queensland University of Technology.pdf Hutchinson. NCHRP 230 (1981) National Cooperative Highway Research Program. Federal Highway Administration. 5. Washington D.dot. (1987) Crash testing bridge rails and transitions United States. Chapter 10 Roadside Design. Interpretation and the Law. pp 957-964. 51. Chapter 7. Proceedings of the IRCOBI Conference. Public Roads. pp333344. Kloeden & McLean (1999) Roadside Hazard Involvement in Severe and Fatal Car Crashes. J. (1998).au) FHWA (1997). March . 3 (December 1987).W. Australia. Recommended Procedures for the Safety Performance Evaluation of Highway Appurtenances. Australia. Queensland.ca. Australia. Brisbane. Koch. April 1998.FHWA-HI-97-026. McDevitt C. (Limited Revisions). Queensland. Office of Road Safety. and Appurtenances. Reconstruction. Report 350. McDevitt C. H & Schueler. F. No.gov. Traffic Safety Systems. pp.080. Gold Coast. May 1996. impact attenuators and ‘sigma-posts’ as a contribution to the passive safety of Motorcyclists. Basics of concrete barriers. Sydney.38034). Report 230. Mount. Australia.F.Parts 1 & 2. 2003 RTA (1996) Draft Road Design Guide. Office of Safety and Traffic Operations Research and Development.cenorm. ACT. — 40 — .C. Safety Barriers for Roads and Bridges. Road restraint systems . Revision 41. Future Safety: the changing face of motorcycling. Canberra. Approval letter HSA-B110 Federal Highways Authority. European Committee for Normalisation.W.dot.htm. FHWA (2002). reference numbers EN 1317-1:1998 and EN 1317-2:1998. Washington DC. Canberra. ACT. May12-15. Design Construction and Maintenance of Highway Safety Features and Appurtenances Federal Highways Authority.Caltrans (2003) Traffic Manual. (www. Australia. Section 6. DMR QLD (2000) Road Planning and Design Manual. Goteborg. ACT. Australian Road Safety Bureau.
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