Safety Barrier Deflection and Working Width in Work Zones

Barrier deflection is how far a road safety barrier moves sideways when hit. Working width is the total space it needs: from the traffic face before impact to the furthest point any major part of the barrier or the vehicle reaches during and after the impact. In work zones, keep workers, plant and hazards outside that space.

This is one of the most misunderstood parts of temporary traffic management. Crews see a solid line of barrier and assume everything behind it is safe. In a crash, the barrier moves, the vehicle leans over the top, and the space behind the barrier is exactly where the energy goes. This guide explains the terms, why the numbers vary so much, where to find them, who decides, and what the site team must do to keep the working width clear.

This is general information, not engineering advice. Barrier selection, test level and working width are decisions for a suitably qualified designer.

Deflection, working width and the parts that make them up

Austroads technical advice on working width for temporary safety barriers quotes the definition in AS/NZS 3845.1: in summary, working width is the distance between the traffic face of the barrier system before impact and the maximum lateral position of any major part of the system or the vehicle during and after the impact. Austroads also clarifies that, because some temporary barriers have feet that extend beyond the traffic face, the measurement starts from the outermost point on the traffic side.

Working width is recorded during full-scale crash testing and is made up of three parts:

The difference between deflection and working width catches people out. A barrier with modest deflection can still have a much larger working width once its own width and vehicle lean are counted. Always plan to working width, not deflection alone, unless the designer has specifically said otherwise.

Why working width varies so much

There is no single working width for "a barrier". The value depends on:

• Test level. Test levels describe the vehicle mass, speed and impact angle the barrier was tested with. A higher test level generally means higher impact energy and often a different working width. Austroads notes that working width varies with test level and the behaviour of the barrier system.
• The product. Water-filled plastic, steel and concrete systems absorb energy differently. Lighter free-standing systems generally move more. Our comparison of water-filled and concrete barriers covers selection in more detail.
• Configuration. Length of the installation, how units connect, and whether the system is free-standing, pinned or anchored all change performance. Many products have several accepted configurations with different values.
• Foundation. A barrier tested on one surface may behave differently on another. Some acceptance documents note that deflection achieved in the field may differ from the test result because of foundation conditions and roadside geometry.
• Site geometry. Curves, crossfall, kerbs in front of the barrier and the approach angle all affect how a vehicle meets the barrier.

That is why a working width from another job, a brochure or a colleague's memory is not good enough.

Where the numbers come from

In Australia, Austroads coordinates national assessment of road safety barrier products through its barrier assessment panel, which assesses technical performance against AS/NZS 3845. Road authorities then decide which products they accept on their networks and on what conditions. Acceptance of a product does not oblige anyone to use it, and states can apply their own limits.

For a specific job, the values should come from:

1. The product's acceptance documentation for the configuration and test level being used.
2. The manufacturer's installation manual, which sets out connections, minimum lengths, anchoring, end treatments and surfaces.
3. The road authority's conditions for that product.
4. The TGS or barrier design, which should state the working width the designer has used.

You can find Austroads material through Austroads, and AS/NZS 3845 through Standards Australia. In NSW, check accepted products and current requirements with Transport for NSW.

Who decides: the designer and the site

When briefing a designer, give them the real working footprint: where plant will sit, where materials will be stored, where people will stand. Our traffic management designer brief covers what to include, and our guide to lane closure traffic plans explains buffers and layout.

Keeping the working width clear on site

Once the barrier is installed, the working width is a no-go zone for anything that would be hurt or would make a crash worse. In practice, the things that creep into it are:

• Workers, especially when the job gets busy or a task is "only for a minute".
• Plant, including excavators slewing, trucks reversing and parked utes.
• Stockpiles and materials, such as pipes, pallets and spoil.
• Excavations and drop-offs, where a deflecting barrier or a vehicle could fall in.
• Site facilities, such as lighting towers, generators and sheds.
• Pedestrian routes, if the plan places them behind the barrier.

Three practical habits make a big difference:

1. Mark it on the ground. Paint or mark a line behind the barrier at the working width offset. People respect a line they can see.
2. Brief it at every pre-start. Tell the crew and the construction team the distance and why it matters.
3. Inspect it. Check the working width at each inspection, not just the barrier itself. In NSW practice, work-site traffic control is inspected at the start and end of every shift and at suitable intervals during it.

Our guide to work zone intrusion covers the wider question of how vehicles enter work areas.

When the space is not there

Sometimes the work cannot be done outside the working width. That is a design problem and needs a design answer. Options a designer may consider include:

• a different barrier system or accepted configuration with a smaller working width
• closing an extra lane so the barrier can move further from the work
• staging the work so plant and people are not within the working width
• changing the work method or the times of work
• using other protection, such as a truck mounted attenuator or a full closure, where appropriate

What is not acceptable is quietly pushing the barrier closer to the work, removing units to fit plant in, or deciding that "nobody will hit it". Changes to a barrier layout need review by someone qualified to change the design.

An illustrative planning example

Picture a kerb-side drainage job on a multi-lane arterial road. The TGS shows a temporary barrier along the closed kerb lane with a stated working width behind it. On the first morning, the excavator arrives and the operator needs to sit closer to the barrier than the marked line to reach the pit. The supervisor has three honest choices: stop and call the designer, re-stage the job so the excavator works from a position outside the line, or keep the plant where it is and hope. Only the first two are acceptable. A designer might respond by specifying an accepted configuration with a smaller working width, closing a second lane, or changing the dig method. Whatever the answer, record the problem, the instruction received and the revision, and brief the crew before the excavator moves. This is a hypothetical scenario to show the decision, not a reported incident.

Installation details that change performance

A barrier installed slightly wrong can behave very differently from the tested system. Check these at installation and at every inspection:

After any impact, however minor, the barrier may have moved, connections may be damaged and the working width may have changed. Help anyone injured, make the site safe and follow the notifiable incident requirements where they apply, then have the system inspected and reinstated as the manufacturer requires before work resumes.

Next steps

Ask your designer for the working width on every barrier job and paint it on the ground. If you run a traffic control company, TrafWise records pre-start briefings and inspections on digital dockets, so working width checks are documented on every shift.