Protection of Passenger Vehicles from Side Underrun with Heavy Trucks (SAE publication)


On several occasions, members of the trucking industry have expressed doubts about the ability of side guards to operate effectively in “real world” crash scenarios.

“We have yet to see evidence that side underride guards would be an effective safety countermeasure,” said American Trucking Associations vice president of public affairs and press secretary Sean McNally. “Until these devices can be proven reliable off the test track, we believe that Congress and regulators should focus on reducing accidents by addressing aggressive and distracted driving and investing in existing and proven safety technologies, including emerging connected vehicle technology.

The ATA also notes that trucking companies have opposed the legislation because it “promotes a solution that is neither data-driven nor proven effective in real-world road settingsand leads to potentially “dangerous” and “unforeseen” consequences. Congress takes a third bend with anti-side-embedding bill

For this SAE search, Protection of passenger vehicles against lateral underrun with heavy trucks, the authors performed a computer simulation to evaluate the operation of the side guards under various conditions. According to Garrett Mattos, the lead author,

  • We simulated angles (0 degrees, 30, 60 and 90 degrees).

  • We simulated a slide (like on ice).

  • We simulated a stationary and moving truck. Slightly worse results for the moving truck than for the stationary truck.

Garrett Mattos presented an overview of the research paper at the Side Guard Working Group Meeting on February 26, 2021. You can see his presentation at 1:46:21 on this YouTube video:


Impacts between passenger vehicles and heavy vehicles are particularly serious due to the aggressiveness of heavy vehicles; function of the difference in their geometry and their mass. Side collisions with heavy vehicles are a particularly serious type of collision due to the mismatch between the height of the bumper and the structure which often results in undercutting and significant intrusion into the passenger compartment. Underride occurs when part of a vehicle, usually the smaller vehicle, moves under another, rendering many passenger vehicle safety systems ineffective.

Heavy-duty vehicles in the United States, including single-unit trucks, tractor units, semi-trailers, and full trailers, are not currently required to have side underride protection devices. The NTSB, among other groups, has recommended that side underrun performance standards be developed and that heavy-duty vehicles be equipped with side underrun protection systems that meet these standards.

The work presented used virtual tests to evaluate the relative performance of examples of side underrun devices compared to a reference. Crash test results were used for validation purposes. A tractor-trailer, with or without side underride protection, was struck by a passenger car and an SUV under various impact conditions. Passenger vehicle intrusion parameters were calculated to provide an indication of relative risk for each impact condition. The results can support the development of recommended practices for lateral underrun protection.


The results of the analysis indicate that the available side underride guards are effective in significantly reducing passenger compartment intrusion (PCI) in often-fatal side collisions. Almost all cabin intrusions above the beltline have been mitigated, except in purely side impact conditions. When the intrusion extended above the beltline, for example in the purely side-slip condition, the amount of PCI was similar to the intrusion generated in a side impact at 56 km/h. a 5-star rated vehicle. Also, the average amount of PCI in the above tests was similar to the amount resulting from small overlap tests of the same vehicle. These results demonstrate that an underride protection can provide sufficient reaction surface to allow the vehicle’s passive and active safety systems to protect the occupant. The underride protection also causes the location of the ICP to move from near the occupant’s head and torso to the lower extremities, reducing the risk of serious injury or death..

In general, the results suggest that impacts with a moving truck/trailer combination are more severe than when the truck is stationary. The added speed of the truck/trailer combination results in greater bullet vehicle firewall intrusion as well as slightly higher top-end accelerations. Impact severity was also increased when the size of the gap between the end of the underride protection and the rear tires was increased. Increasing the size of the gap allowed the bullet vehicle to interact more with the rear tires. During impacts with a wide gap, the trailer tires almost engaged the driver’s side door. These results can help define a comprehensive test plan that can be used to assess the performance of an underrun protection.

The acceleration pulses for all impacts were within the range of frontal and side crash test pulses generated in similar tests of vehicles with 5-star safety ratings. This indicates that these impacts were all survivors. The most severe impact scenario was a sideways slide at 56 km/h in the trailer with underride protection.

As noted, there is an 80% or greater reduction in PCI for impacts with underride protection compared to the baseline condition. Additionally, the location of the PCI in underride protection impacts was generally found to be in the area of ​​the outer firewall rather than at or above the belt line as in the case of reference. Reducing the ICP and moving the ICP location away from the occupant’s head and torso greatly reduces the likelihood of serious injury. No adverse effects have been observed as a result of the underride protection.

These results indicate that tests used to assess the performance of underride guards should incorporate a moving truck/trailer combination, as this has been shown to increase crash severity. Additionally, the location and size of the gaps between an underride guard and the trailer’s tires and/or undercarriage should also be considered, as this affected the results. The results demonstrate, along with other work in the literature, that finite element analysis can improve physical testing to increase the number of impact scenarios in a cost-effective and rapid manner. Although additional impact conditions and test cases can be analyzed, results should further demonstrate the importance of trailer side underride guards in reducing passenger compartment intrusion in these crash conditions.

Side underride guards integrated into the trailer structure can further enhance the safety benefits associated with preventing the trailer from underriding and limiting additional weight. Exploration of these design alternatives should be explored in the future in conjunction with additional vehicles and configurations.


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