Key takeaways
- A trailer mounted attenuator is a crash cushion on its own trailer, towed by a support vehicle, instead of being bolted to the back of a truck.
- Each accepted product lists its own conditions, such as test level, minimum support-vehicle mass and roll-ahead distance, and these differ noticeably between products.
- Trailer units can move between support vehicles and keep the attenuator off the truck chassis, but they are longer, harder to reverse and add coupling checks.
- Whether a trailer or truck unit is acceptable is set by the TGS, the road authority and the contract; in NSW, TS 05492 added crash attenuator requirements.
- The support vehicle, coupling, trailer and attenuator all need pre-start checks, and the whole combination is removed from service after any impact.
A trailer mounted attenuator is a crash cushion built into its own trailer and towed behind a support vehicle, rather than fixed to the back of a truck. Both types absorb the impact of an errant vehicle to protect workers ahead. Which one suits a job depends on the accepted product's conditions, the TGS, road authority requirements and your fleet.
Our guide to truck mounted attenuators covers when attenuators are required, operator requirements and the basic site routine. This article goes a step further for operators and fleet managers choosing between the two designs: how trailer units differ, what the acceptance documents tell you, the operating challenges, and the extra checks a trailer brings.
How a trailer-mounted attenuator works
The attenuator cartridge sits on a trailer frame with its own wheels, coupled to the support vehicle by a hitch. When a vehicle runs into the back, the cushion collapses in a controlled way and absorbs energy. The support vehicle provides the mass that resists the impact, and the combination rolls forward a distance known as the roll-ahead.
The trailer usually carries the warning devices too, such as an arrow board, lights and chevron markings, so the combination is highly visible from behind. Many trailer units are designed to work with a range of support vehicles, provided each vehicle meets the product's requirements.
A truck mounted attenuator, by contrast, has the cushion mounted on a frame at the back of the truck, usually folding up for travel. The truck itself is the support vehicle.
Key differences at a glance
| Factor | Trailer-mounted attenuator | Truck-mounted attenuator |
|---|---|---|
| Mounting | Own trailer, coupled by a hitch | Fixed frame on the truck |
| Flexibility | Can move between suitable support vehicles | Tied to one truck |
| Support vehicle downtime | Another suitable vehicle can tow the trailer | If the truck is off the road, so is the attenuator |
| Length | Longer combination | Shorter overall |
| Manoeuvring | Articulated; reversing and tight turns are harder | Rigid; easier to position precisely |
| Travel mode | Varies by product; follow the manual | Cushion typically folds for travel |
| Extra checks | Coupling, safety chains, trailer brakes, tyres and lights | Mounting frame, hydraulics, fold mechanism |
| Load on the truck | Less load on the truck's rear chassis | Attenuator mass carried on the truck's rear |
| Registration | Trailer registered separately | Part of the truck |
These are general differences. The specific product, its accepted configuration and the road authority's conditions decide what is acceptable on a given job.
What the acceptance documents tell you
Attenuators used on Australian roads are crash tested and assessed. Austroads coordinates national assessment of road safety barrier systems and devices against AS/NZS 3845, and road authorities decide which products they accept and on what conditions. You can find Austroads material on the Austroads website.
For any attenuator, trailer or truck, the acceptance documentation typically states:
- the test level, which relates to the impact speed and vehicles it was tested with
- the minimum support-vehicle mass, sometimes including ballast
- the roll-ahead distance expected on impact
- the accepted variants, with a warning that unlisted variants are not accepted
- conditions of use, such as how the support vehicle must be set when stationary
Two things stand out when you compare acceptance summaries. First, accepted products at the same test level list noticeably different roll-ahead distances and minimum support-vehicle masses. Second, the minimum support-vehicle masses we saw were several tonnes, well beyond a ute. That means you cannot treat attenuators as interchangeable: the roll-ahead gap you brief and the vehicle you couple to must come from the specific product's documents.
Choosing between trailer and truck-mounted units
Neither design is better for every job. These questions help a fleet manager or planner decide.
- What does the TGS and contract allow? Some authorities or contracts specify attenuator types for particular work. Start there.
- What test level is needed? The road speed and the TGS set this. Only consider products accepted at that level.
- What support vehicles do you have? A trailer unit is only as flexible as the number of vehicles in your fleet that meet its requirements and can tow it legally.
- How much mobile work is involved? On winding roads with frequent stops and starts, a rigid truck unit is easier to keep in the right position. On longer static shifts, the trailer's flexibility may matter more.
- How tight are the sites? Narrow shoulders, short work areas and sites where you must reverse into position favour the shorter, rigid truck unit.
- What happens when a vehicle breaks down? With a trailer, another suitable support vehicle can keep the attenuator working. Build this into your vehicle breakdown plan.
- Who will operate it? Reversing an articulated combination accurately is a skill. Plan training and induction on the specific unit.
If you hire attenuators rather than own them, ask the supplier for the acceptance documents, certification and support-vehicle requirements in writing. Our guide to traffic control equipment hire covers what to confirm.
Operating a trailer attenuator on site
The basic routine in our TMA guide still applies: briefing, positioning, deployment, monitoring and pack-down. Trailer units add some specific points.
Coupling and pre-departure
Check the hitch is the correct type and rating, fully engaged and locked, with safety chains, electrical connections and trailer brakes connected as the trailer requires. A coupling that is not fully latched is one of the most serious trailer failures, and it has nothing to do with the attenuator itself.
Positioning
Position the combination as the TGS shows, with the roll-ahead gap to the work area taken from the product's documents. Set the support vehicle as the acceptance conditions and your procedure require when stationary, including gear, park brake and wheel position. Avoid reversing near workers. Where reversing is unavoidable, use a spotter in a safe position with radio contact, and never let anyone stand between the trailer and the support vehicle.
Mobile works
On curves, the trailer tracks inside the support vehicle's path. Over crests and around bends, the following distance and the trailer's position relative to the lane change. Operators need to keep the attenuator where it protects the works, not just where the cab is. Our guide to mobile versus static traffic management covers mobile set-ups.
High-speed roads
On motorways, set-up and pack-down are the most exposed periods. Plan the attenuator's arrival so it is in position before the crew enters the lane, and stays until the last device is removed. See our guide to motorway traffic control.
Registration, licensing and heavy-vehicle compliance
A trailer attenuator is a registered trailer, so its registration, tyres, lights, brakes and coupling must meet vehicle standards, as well as the attenuator meeting its own certification requirements.
The licence class an operator needs depends on the support vehicle and the combination, so check with your state licensing authority. Operators also need appropriate traffic control training for the state and induction on the specific unit.
Where the support vehicle is a heavy vehicle, the Heavy Vehicle National Law applies in participating jurisdictions, including mass, dimension, maintenance and fatigue requirements, and the chain of responsibility duties that extend to schedulers, managers and the business. The National Heavy Vehicle Regulator has guidance. Rostering attenuator operators on long night shifts needs the same fatigue controls as any other heavy-vehicle work.
After any impact, however minor, take the whole combination out of service. The attenuator must be inspected and certified in line with the manufacturer's requirements and the relevant guideline before reuse, and the coupling, trailer chassis and support vehicle should be checked too. Help anyone injured, make the site safe, and report notifiable incidents to the WHS regulator, preserving the site except to help people or make it safe.
Trailer attenuator pre-start checklist
| Check | Done |
|---|---|
| Attenuator certification current and on board | ☐ |
| Product and configuration match the acceptance documents | ☐ |
| Support vehicle meets the product's minimum mass and requirements | ☐ |
| Hitch correct type and rating, latched and locked | ☐ |
| Safety chains, electrical and brake connections attached | ☐ |
| Trailer tyres, wheels, lights and brakes checked | ☐ |
| Cushion free of damage, cracks or deformation | ☐ |
| Arrow board, lights and markings working and clean | ☐ |
| Roll-ahead distance for this product briefed to the crew | ☐ |
| Radio working and channel confirmed | ☐ |
| Operator licence, training and hours checked | ☐ |
| Check recorded on the vehicle log | ☐ |
This is general information, not engineering advice. Attenuator selection and positioning come from the TGS, prepared by a suitably qualified person, the product's acceptance documents and the road authority's requirements.
Next steps
Pull the acceptance documents for every attenuator in your fleet and note each product's roll-ahead and support-vehicle requirements in your operator briefing. If you run a traffic control company, TrafWise helps you roster licensed attenuator operators within fatigue limits and record pre-start checks on digital dockets.
Frequently asked questions
What is a trailer mounted attenuator?
A trailer mounted attenuator is an energy-absorbing crash cushion built into a trailer that is towed behind a support vehicle. It is positioned behind a work area or slow-moving works to absorb the impact of an errant vehicle. It does the same job as a truck mounted attenuator, but the cushion is not fixed to the truck.
Is a trailer attenuator as safe as a truck mounted attenuator?
Both types can be crash tested and accepted for use, and performance depends on the specific product, test level and support vehicle. Each accepted product has its own conditions, including minimum support-vehicle mass and roll-ahead distance. Check the acceptance documentation and the road authority's requirements, and use the product exactly as accepted.
Can you tow a trailer attenuator with a ute?
Generally no. Accepted attenuator products specify a minimum support-vehicle mass, and in the acceptance documents we reviewed these were several tonnes, well above a typical ute. The support vehicle must meet the product's stated requirements, be rated to tow the trailer and carry any required ballast. Check the product's acceptance documents.
Why are trailer attenuators harder to use than truck mounted ones?
The combination is longer and articulated, so reversing, tight manoeuvres and accurate positioning take more skill. The coupling adds a set of checks, and on curves the trailer may not track exactly behind the support vehicle. These issues matter most during set-up, pack-down and mobile works on winding roads.
Does a trailer attenuator need to be certified after an impact?
Yes. After any impact, however minor, the attenuator must be removed from service and inspected and certified in line with the manufacturer's requirements and the relevant guideline before reuse. The coupling, trailer chassis and support vehicle should also be checked, because impact forces pass through all of them. Record the event and inspection.