The giant orange crash cushion is something you’ve likely seen on the back of a work truck, but you may have wondered what it is for. Does it have to come to a halt before a vehicle? Or so it’s just there to look frightening?

The short answer is that a TMA (truck mounted attenuator) will not prevent a crash. It manages one. But how much impact it absorbs is determined by speed, weight and what kind of engineering is packed inside that big foam-and-metal cushion.

So, let’s do some math and understand how much impact a TMA has, why it’s important and how engineers determine the “safe enough” when a car slams into the back of a truck at highway speed.

Exactly What is a Truck Mounted Attenuator?

A truck mounted attenuator, or crash cushion, is a heavy support vehicle that has a crash cushion installed on the back. It is used as a shadow vehicle behind road workers, arrow boards, or slow moving equipment by the crews.

It has a very easy job. The TMA crumples in a controlled manner if a distracted or speeding vehicle collides with the truck in the rear. The controlled crumple is the whole idea as it spreads and dissipates the impact energy both in front of the roadway crew and in the striking vehicle. A TMA is designed to spread and dissipate the impact energy in a controlled manner, protecting the roadway workers in front and occupants of the striking vehicle.

Imagine a boxer rolling, rather than standing flat-footed, with a punch. The energy still exists but it is absorbed slowly as opposed to all at once.

How Strong is the Impact It Absorbs?

You won’t find one number here, one article that provides a single number is guessing. There are three moving parts, which affect how well the impact is absorbed.

Samantha Schwartz-Lenhart of TMA supplier Royal Truck & Equipment says that there are three key factors to consider when assessing the effectiveness of a TMA truck: the effectiveness of the attenuator, the braking force of the truck, and the weight of the ballast used to add weight to the truck if needed. If you take out any one of those three, the whole system underperforms.

Engineers instead measure attenuator performance in terms of the government’s crash-test standards, rather than one single number. These numbers are the real numbers.

The Standard That Decides Everything: MASH

In the USA, TMAs are evaluated against the Manual for Assessing Safety Hardware (MASH) issued by AASHTO which replaces the previous NCHRP Report 350 standards for roadside safety hardware, including truck mounted attenuators.

MASH testing categorizes attenuators into Test Levels, and that is where the “how much impact” begins to receive a good answer.

• TL-2 attenuators are designed to reduce an impact vehicle to approximately 45 mph (70 km/h) and are qualified and tested for that purpose.
• The highway speed range is where TL-3 attenuators come in: A TL-3 attenuator is rated for 62 mph (100 km/h).</cite> It’s at the forefront of highway-grade TMAs on the market today.

When asked how much impact a TMA absorbs, the real answer is it depends on which test level you’re looking at, since the TL-3 is designed to absorb a lot more of the kinetic energy than the TL-2 is.

It is important to note that MASH is not an out-of-the-nowhere standard; it was developed to address the increased risks highway workers encounter in today’s changed environment of larger, heavier vehicles and higher highway speeds.

The Human Body is Affected by a Number Known as G-Force

Speed is the indicator of a car’s speed prior to impact. However the force in G’s that determines the difference between the driver walking away and being seriously injured are deceleration.

Now is the time to get serious about crash testing. If it passed the test with headroom, it’s a 20.49 at the absolute most, otherwise it failed.

For comparison, a well-designed TMA cartridge can reduce the deceleration of a great-looking car to near the 15 G level. The official crash testing of one of the TMA’s approved by MASH took place with a recorded longitudinal ridedown of 15.0 G and a peak value of 20.49 G. That’s a product right on the limits of “excellent” and entirely within the legal safety design envelope.

What is the significance of this? The human body has limitations, too. A sudden 30 or 40 G stop (which is equivalent to slamming into an unprotected steel truck bed) can often be fatal. If it can be reduced to the mid teens, that’s the difference between a bruised driver and a fatality. That’s not marketing jargon. That’s why this equipment is there.

How the Cushion ACTUALLY Consumes the Energy!

Then how does a TMA make a violent hit a survivable one? Not with magic. By combining physics, aluminum and some ingenious sequencing.

Modern attenuators employ energy-absorbing cartridges, typically made from stacked aluminum or composite materials, that buckle in stages rather than all at once, like a soda can. One popular design is the use of 12 internal blades that buckle in a precisely programmed manner upon impact.

The key to that staged collapse is that trick. One dramatic impact, and the whole cushion would come to a head-stopping stop. The total energy is being used across a greater area – and a longer time.

The engineers’ objective is to decelerate the impacting vehicle to a stop within the designed length of the attenuator, which is usually 18 to 22 feet, without crushing the occupant compartment of the struck vehicle or allowing the striking vehicle to ride underneath or vault over the truck.

Why It’s Just as Important as the Cushion to Have the Truck

There’s one thing that people don’t think of; the attenuator cartridge is only half the safety system. The truck it is bolted to needs to be stable.

Structural anchoring relies on the host truck’s mass, and TL-3 systems require a minimum gross vehicle weight rating of 26,000 pounds, to prevent the TMA from being pushed forward into the work zone crew.

If the truck is too light, it is not an anchor, but a battering ram! That is why it’s important for fleet operators to not simply attach any cushion to any pickup truck and declare it compliant. The attenuator’s rated test level must be matched by the weight, braking capacity and ballast.

But, Do TMAs Really Save Lives?

This sounds like the numbers and G-forces are helpful, but the question is, are any of these injuries real injuries that are prevented by any of this? The evidence is in, and it says yes.

It’s not optional, it’s a mandate. Any TMA that is used in highway work zones on federally funded projects using federal dollars must be certified as MASH compliant by the FHWA since 2020. It is policy for a reason: the old standards were allowing substandard equipment onto “real” highways.

A Quick Sanity Check to Select the Right TMA

If you are not simply playing with equipment but have a specific project in mind, then consider a few of these practical questions:

• What are the posted speed limits on the roads that your crew drive on? This will determine which TL-2 or TL-3 to use.
• Does your host truck’s weight meet the minimum requirement for that test level?
• Has the cartridge been tested by MASH 2016 and has a FHWA eligibility letter been submitted?
• Is the brake system and ballast the correct combination for the attenuator or merely bolted on it?

If a question is not answered, this is an expensive “false sense of security” investment regarding safety.

The Bottom Line on truck mounted attenuator

So, how much energy does a truck mounted attenuator absorb? So much so as to make a difference if it is the correct unit for the correct road. When installed on a large enough truck, a TL-3 attenuator is designed to decelerate a car up to 62 mph to a stop, but not to exceed that desired 15 G force on the occupants.

It’s not going to make highway driving easy. Nothing does. The difference between a controlled 15 G stop and an uncontrolled 40+ G impact can be the difference between a bad day and a funeral, however. That’s all the business case for this unglamorous, orange-and-black piece of equipment that is rolling behind work crews every day.