Guide to the Design of Workzone Barriers

 

This Guide outlines Main Roads approach to work zone barrier systems so that they are designed consistently and appropriately throughout Western Australia.

This Guide has been developed to be read as a supplement to the Austroads Guide to Road Design, Part 6: Roadside Design, Safety and Barriers (2009). A copy of this document can be purchased from the Austroads website (link).

This Guide contains additional sections and figures not covered in the Austroads’ document.

This guideline applies to all work zones managed by Main Roads.

Approved road safety barrier systems for work zones can be found in the List of Approved Barrier Systems.  Attached to this list are design sheets for each product approved which gives guidance on the use of it and the configuration that it is to be used in.

1. Work Zone Safety Barrier Systems

1.1 Introduction

This section provides guidance on the use, selection and location of work zone barriers (also referred to as temporary barriers) that are used in situations where a permanent barrier is inappropriate. Work zone barriers are used in situations where protection is required for a limited time. Examples include:

• roadworks sites (most common application);
• protection of infrastructure works on land adjacent to the road and associated work space; or
• special events where there is a need to control vehicle and pedestrian movements.

With respect to roadworks, occupational health and safety is an important aspect in the management of road systems. Apart from personal injury and grief, it has serious implications with respect to increasing litigation and insurance costs. Against this background there is a heightened awareness of Workplace Health and Safety requirements, and the potentially hazardous environment in which road workers have to perform their duties.

Contributing to these hazards are increasing traffic volumes, larger vehicles and in some cases higher speeds through and adjacent to work sites. Inappropriate speed limits or operating speeds can also increase the risk for road users, both during and after worksite operating hours. Where long-term construction sites are created, enforcement of speed can be a problem even with active police participation. While publicity campaigns and enforcement (often utilising radar devices and speed cameras) may assist, generally these measures are outside the control of the personnel on a construction site.

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). 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; and
• inadequate clearance to workers or hazards in order to cater for the dynamic deflection of the barrier under vehicle impact.

In some cases incorrect or inappropriate practices that have not led to any known problems on previous projects may be repeated. Construction and maintenance managers, engineers, supervisors and workers must therefore take an active role in ensuring that work sites are safe for workers and road users. It is particularly important that where safety barriers are necessary they are used in a consistent and appropriate way. This can only be achieved if relevant personnel are educated and trained to have knowledge of relevant guides and standards, and experience in their application. 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; and
• provision of effective and well-maintained signs and markings (including regulatory signs, warning signs, guide signs and delineation) that meet the requirements of AS 1742.3, Manual of uniform traffic control devices, Part 3: Traffic control for works on roads.

It is very important that work zone barriers are located so that they do not place road users at risk by restricting sight distance for traffic entering, crossing or moving through the worksite. Care must also be taken to ensure that drainage is adequate so that stormwater does not form ponds adjacent to the barrier or flow across the road at a depth that could cause vehicles to aquaplane.

The guidelines in this section cover the types of work zone barriers and how and when they should be used to enhance safety, both for workers and traffic passing through work sites. The principles also apply to barriers used for other works or activities adjacent to roads, and for traffic control for major events.

1.2 Purpose and Use of Safety Barriers at Roadwork Sites

The Australian Standard AS/NZS 3845.1:2015, defines a temporary road safety barrier system to be “a road safety barrier system used at roadworks, emergencies or other situations for limited durations”. It also notes that ”its purpose is to redirect an impacting vehicle so as to minimise damage to the vehicle and injury to the occupants, while providing protection for workers or other road users”.

Work zone barriers are therefore used to contain and redirect errant vehicles so as to prevent them from leaving the roadway and/or entering the worksite. Like permanent barriers, they should only be used if they reduce the severity and adverse consequences of potential crashes, as they are a hazard in themselves. As well as enhancing site safety, they may improve job productivity and reduce road user delays.

They are used in situations where it is considered that traffic volumes, traffic speeds, and the nature of the work (e.g. worksite/traffic separation and duration of the works) indicate that it is both desirable and practical to provide such additional protection. Only work zone barriers that have been successfully crash tested to the same test level (e.g. TL2, TL3) required of permanent barriers shall be used to shield worksites.

Guidance on the use of barriers at work zones can be found in AS 1742.3 and the Main Roads Traffic Management for Works on Roads - Code of Practice.

As work zone barriers and most permanent safety barriers are not designed to contain large trucks the use of barriers does not necessarily negate the need for reduced speed limits adjacent to construction areas. Reduced speed limits are often implemented on high-speed, high volume roads, particularly those carrying high numbers of large vehicles and where workers perform duties immediately behind barriers.

In determining whether a safety barrier is required, the designer should address the following questions:

• Can the speed of vehicles be maintained at such a value through the work site that, in combination with worker/roadside hazard clearance and the quality of the traffic arrangements (traffic control, road surface/alignment, etc.,) 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 work zone barrier has to cater for the highest speed environment that applies during its deployment, 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 barriers, is it practical to install 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, the speed of vehicles through the site, and the clearance between such traffic and workers/roadside hazards, would the use of safety barriers improve the safety of both workers and road users and should they therefore be provided?

1.3 Types of Longitudinal Safety Barrier Systems for Work Zone Use

The Australian Standard AS/NZS 3845.2:2017 sets out the requirements for Longitudinal Channelizing Devices (LCDs), including pedestrian fences, that are constructed from either steel fencing, barrier boards, standalone non-interconnected lightweight modules, interconnected lightweight modules or other channelizing devices used as traffic control devices. LCDs are a channelizing device only and should not be used where a road safety barrier is warranted.

The test level for work zone barriers shall be determined based on the speed zone imposed on the work sites. This requirement as well as warrants for the use of various temporary safety barriers is contained within the Main Roads Traffic Management for Works on Roads - Code of Practice.

Work zone barriers used on the Main Roads network include water filled barriers, steel barriers and precast concrete barriers.

Approved work zone barriers can be found in the List of Approved Barrier Systems.

Work zone barriers can be permanent type installations, or temporary installations that are more readily relocated. 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 or where the barrier is to be integrated into the permanent works.

The cost of installing and removing permanent concrete barriers may be prohibitive, but there may be a case for the use of semi-rigid permanent barriers depending on site and project characteristics.

Generally barriers used for road works will be of a temporary type to suit the relatively short duration that they are required, the need to move them in accordance with construction and traffic staging requirements, and because of the economic advantages.

Work zone barriers that are commonly used are:

• precast concrete safety shape units (e.g. Type F, single slope which may use public domain or proprietary connections);
• steel safety shape units (e.g. Barrierguard 800, Ironman)
• water filled plastic barriers.

Steel, 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.

Plastic barriers are light in weight and of modular design that makes them very portable. 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.

1.4 Operational Requirements for the Use of Barriers at Roadwork Sites

The principles that apply to permanent barriers are also applicable to the use of work zone barriers.  For example, it is recommended that work zone barriers not be installed in proximity to kerbing or where the slope on pavements, shoulders or batters is greater than 1 on 10.

All work zone barriers should be designed to the same requirements as a permanent barrier in terms of the length required to adequately shield the hazard.

Furthermore, when barriers are used at roadwork sites the following issues should be addressed.

1.4.1 Connection of Individual Barrier Units

Barrier units will act as a safety barrier only if they are properly connected to each other throughout the whole installation. The connections provide barriers with the continuity necessary to ensure that differential movement does not occur at the joints between units, and to resist displacement of the units. Joint movement may cause snagging and pocketing of impacting vehicles.

The method of connection will vary depending on the type of safety barrier but generally consists of steel pins, steel plates, concrete keys or a combination of steel pins and cables.

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. The plastic water filled safety barrier shell will have an internal or external steel frame to provide the required strength during an impact.

Only connections that are in accordance with the designer’s or manufacturer’s specification should be used.

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.

Installations of unconnected units do not form a safety barrier in any way. If impacted, individual units will either topple over or slide creating considerable risk to workers, the impacting vehicle and other road users.

1.4.2 Foundation

Work zone barriers must be founded on a base that enables proper alignment and is capable of supporting the barrier and other loads created during impacts. This may be a critical consideration for barriers located adjacent to trenches, deep pavement boxings, foundation excavations, etc.

Work zone concrete and plastic safety barriers will generally be free standing (not anchored) however some steel systems requiring anchoring in order to achieve the required deflections.

Sufficient clearance must therefore be provided between the back of the barrier and the work area to allow for dynamic deflection of the barrier as well as anchorage.

1.4.3 Minimum Length

The minimum length of work zone barrier installed shall not be less than the length of system recommended by distributors and based on successful crash tests. At a particular site the installation must:

• be adequate to shield the hazard (e.g. roadworks); and
• have sufficient strength to redirect an impacting vehicle.

The length of 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.

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.g. 2000 kg vehicle at 100 km/h, TL3 based on NCHRP 350).

1.4.4 Barrier Lateral Location

It is necessary to provide sufficient space between the barrier and the work area to accommodate the dynamic deflection of the barrier. The deflection of work zone barriers is a function of the speed and type of impacting vehicle, the angle of impact, and the design of the barrier system including its foundation and connections.

Barrier systems should also be located to allow for vehicle roll (also referred to as working width).

Work zone barriers should be installed so that the likely angle of impact is minimised as this will also minimise the dynamic deflection under impact.

The process and requirements to evaluate the suitability of a road safety barrier system are specified in Section 4 of AS/NZS 3845.1:2015.

Project management and supervisory personnel shall carefully consider the orientation of the barrier in relation to approaching traffic, as well as the speed environment that will operate at all times when the barrier is erected.

Shallower impact angles may be more applicable to a construction site as traffic may be more constrained through the use of various signing devices, and in many urban situations the speed of traffic can be controlled to 70 km/h or less. Water-filled barriers may be suitable in such circumstances. However, each site should be assessed on its merits and barrier requirements evaluated in accordance with the manufacturer's design criteria.

When determining the distance in front of hazards that barriers are located, designers are referred to the deflection values published in the Barrier Design Sheets.

Designers should be aware that the deflection values published are the values of test deflections recorded under controlled conditions. The vehicle mass, speed and angle of impact, along with the barrier length and condition are factors that are controlled as part of the testing. Designers should be aware that the deflection figure published as a test result may not be the deflection value achieved in the field for all impacts by errant vehicles.

Offset Between Barrier and Traffic Lane

Offset location of work zone barriers shall be in accordance with the Main Roads Traffic Management for Works on Roads - Code of Practice

The clearance between a work zone barrier and the edge of the traffic lane is important for driver safety, and to enable traffic flow and capacity to be maintained. At work sites it is necessary to accommodate the required number of traffic lanes, adequate clearance to barriers, and perhaps a shoulder. Wherever practicable, normal shoulder widths should be maintained between the outer edge of the traffic lane and the barrier. However, 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 width of traffic lanes required depends on the traffic mix and the alignment of any temporary sidetrack or diversion. Where there are few trucks in the traffic stream and/or speeds are relatively low (e.g. 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 important to provide adequate clearances to the barriers. On the other hand, a barrier erected on one side of the road adjacent to lanes that have desirable widths may require minimal clearance.

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, and the need for ongoing maintenance and the associated traffic disruption may be a significant issue. On the other hand, in order to keep impact angles to an acceptable value, 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.

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), or travelling adjacent to the site (e.g. stopping sight distance).

For driver safety and to maintain traffic flow conditions, 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.

1.4.5 Delineation

Work zone barriers should have adequate delineation installed. This will aid in guiding road users through the work site, particularly at night, and also alert road users to the presence of a barrier.

Delineation of work zone barriers is particularly important as barriers at road works sites frequently become covered in mud, dirt or grime because of site activities. 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.

To provide acceptable night time visibility, retroreflective devices or non flashing warning  lamps should be mounted along the safety barrier, generally perpendicular to the direction of traffic, and in a manner that will not endanger the occupants of impacting vehicles. Where a barrier is on a curved alignment, delineation devices should be orientated so as to reflect light back to drivers. In addition, a solid edge line and raised reflective pavement markers may be placed along the pavement adjacent to the barrier to improve delineation.

Retro-reflective delineators such as adhesive tape are easy to apply and, while their performance is diminished by dirt, they will generally reflect sufficient light from headlights to allow road users to see the path that they are required to follow.

If warning lamps are to be used, they should be deployed as shown in AS 1742.3, Part 3. 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.

In urban areas, acceptable visibility may be achieved through the public lighting system provided that the barrier is of a light colour. However, the existence of street lighting does not remove the need for effective delineation.

In order to achieve suitable daytime visibility, temporary safety barriers should be coloured white or red/orange. 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.

1.4.6 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, both of which can create a hazard to road users.

Some temporary barriers allow water to pass beneath or through the barrier.

1.5 Operational Monitoring

Monitoring of barriers in the field is the best way to determine the performance of a barrier under particular situations. These observations and feedback by maintenance contractors and staff will identify any problems that may occur with the system, and should ensure optimal performance for future installations.

The Australian Standard for road safety barrier systems (AS/NZS 3845.1:2015) states that in-service performance evaluations shall be undertaken, including post-crash investigations. After crashes into barrier systems, the following considerations should be addressed, as a minimum:

• Did the system function as designed?
• Should the system be repaired, as it was pre- crash?
• If not, which upgrade measures should be carried out to improve the safety of the hazard?

It is noted that section 8.8 of AS/NZS 3845.1:2015 identifies specific maintenance and inspection requirements for temporary barriers.

1.6 Selection of Safety Barrier Type for Worksites

The deflection of non-rigid barriers and their performance under impact is related to the speed of impact, the angle of impact and the characteristics of the particular barrier and impacting vehicle. 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.

In considering what type of safety barrier to use, the following approach should be adopted.

1.6.1 Concrete Barriers

Concrete barriers should be used at sites where the consequence of errant vehicles striking critical construction works (e.g. bridge false work) could have major flow-on effects. For this reason the dynamic deflection of work zone concrete safety barriers must be accommodated between the barrier installation and the work zone.

Concrete barriers generally provide a high level of protection and generally continue to remain functional after being struck. This is an important factor for critical sites where protection needs to be maintained at all times, repair activities would cause severe traffic problems, or maintenance response would leave the site unprotected for an unacceptable period (e.g. geographical location, out of normal working hours).

Consideration should also be given at critical sites to the traffic volume and mix of commercial vehicles, particularly semi-trailers, as a more substantial concrete barrier than a TL3 barrier may be warranted.

Work zone concrete barriers should allow surface drainage water to pass beneath the barrier or between the barrier segments at the joints. Some systems can be more easily moved to allow for short-term traffic management of incidents within the site.

1.6.2 Water Filled Plastic Barrier

Water filled plastic barriers generally provide an adequate level of protection in low speed situations.

They may have an advantage at sites where safety barriers have to be moved frequently to comply with traffic staging or works requirements. 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. 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.

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 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.

Advantages of water filled plastic barriers may be seen as:

• low or nil maintenance, 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; and
• versatility of use, either as a delineator device for traffic guidance or, if installed as such as a temporary barrier.

1.6.3 Steel Systems

There are steel work zone systems available and advantages of these may be seen as:

• low or nil maintenance
• higher test level protection (some systems are TL 4)
• lighter than concrete
• come in different configurations which change the deflection of the system which enable the system to be tailored to the specific site requirements.

1.7 Terminals

The ends of work zone barriers must be appropriately treated as they can be a major hazard to road users if struck end on.

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; and
• cost and maintenance factors.

Approved road safety barrier terminals for work zones can be found in the List of Approved Barrier Systems.  Attached to this list are design sheets for each product approved which gives guidance on the use of it and the configuration that it is to be used in.

Separate terminals are not required for work zone barriers if the barrier can be suitably flared so that the exposed end is located outside of the clear zone. However, this may not often be the case for work zone systems and the lack of a terminal may increase the deflection of the system or alter the location of where it becomes directive from.

Work zone concrete barriers may be provided with a crash attenuator designed for the appropriate speed environment or a suitable gating non-directive crash attenuator, providing the transition is adequate.

Where W-Beam is used as a work zone barrier a suitable W-Beam terminal should be used depending on site conditions.

Some plastic water-filled barriers may be provided with additional segments at the leading end that provide a form of ‘gating’ terminal. Manufacturer’s/distributor’s specifications should be consulted to establish the minimum acceptable length and the appropriate point of need.

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. The speed zone value adopted must also be consistent with the physical restrictions and general driving environment at the site.

References

1. Austroads Guide to Road Design, Part 6: Roadside Design, Safety and Barriers (2009)
2. Muthusamy and Kumar (1995). Road Work Sites – Are Motorists and Road Workers Really Safe? Proceedings of The Conference: Accident Investigation, Reconstruction, Interpretation and the Law, Gold Coast, Australia. Queensland University of Technology, Brisbane, Queensland, Australia, pp333-344.
3. AS 1742.3, Manual of uniform traffic control devices, Part 3: Traffic control for works on roads
4. Main Roads Traffic Management for Works on Roads – Code of Practice
5. AS/NZS 3845.1:2015, Road safety barrier systems and devices Part 1: Road safety barrier systems
6. AS/NZS 3845.2:2017, Road safety barrier systems and devices Part 2: Road safety devices
7. Transportation Research Board, NCHRP Report 350, Recommended Procedures for the Safety Performance Evaluation of Highway Features (1993)
8. American Association of State Highway and Transportation Officials (AASHTO), Manual for Assessing Safety Hardware (MASH), 2009