Side underride accidents occur when a passenger vehicle strikes the side of a tractor-trailer and slides beneath the trailer instead of engaging a structure that can absorb crash forces. The danger is not simply that the smaller vehicle hits a large truck. The danger is that the side of the trailer can enter the windshield, A-pillars, roof, and occupant space of the striking vehicle. NHTSA describes side underride as a crash in which the front end of a smaller vehicle strikes the side of a larger vehicle and slides under the chassis; in severe cases, the side of the trailer enters the passenger compartment of the light-duty vehicle.[1]
A side underride guard is designed to close that open space along the lower side of a trailer. Its purpose is to give the striking passenger vehicle a structure to engage before the trailer edge reaches the occupant compartment. In a conventional frontal crash, the passenger vehicle's bumper, hood structure, crumple zone, seat belts, and airbags are all part of the survival system. In a side underride crash, those systems may be bypassed because the trailer rides above them. NHTSA's cost-benefit analysis states that side guards are placed along the lower left and right of combination trucks to mitigate side underride risk.[2]
Side underride guards should not be confused with aerodynamic side skirts. Side skirts are typically installed to improve fuel economy by reducing aerodynamic drag. They are not necessarily designed to withstand the forces of a passenger vehicle impact. NHTSA's side-underride analysis describes IIHS testing in which a fiberglass aerodynamic side skirt did not prevent underride. The car continued under the trailer, part of the roof was sheared off, and the vehicle became wedged under the trailer. In the comparison test using the AngelWing side underride guard, the guard bent but prevented the car from going under the trailer.[3]
The same distinction applies to city "side guards" or lateral protective devices. Volpe explains that lateral protective devices are designed mainly to protect pedestrians, bicyclists, and motorcyclists by covering the exposed space between the front and rear wheels so vulnerable road users are not swept under the truck. That is a different design problem than stopping a passenger car from underriding a semitrailer at highway speed.[4]
The Federal Gap: Rear Guards Are Required, Side Guards Are Not
Federal law treats rear underride differently from side underride. Rear underride guards are federally required on many trailers and semitrailers, and NHTSA's rear-impact standards have existed for decades. Side underride guards, by contrast, are not currently required by a Federal Motor Vehicle Safety Standard. NHTSA's 2025 denial of defect petition DP24-004 states directly that no FMVSS requiring side underride guards on semitrailers currently exists.[5] That does not mean federal regulators have ignored the issue. NTSB has urged action for years. In its 2014 safety recommendation letter, NTSB recommended that NHTSA develop performance standards for side underride protection systems for newly manufactured trailers with gross vehicle weight ratings over 10,000 pounds. NTSB also recommended similar side underride protection for newly manufactured truck tractors over 26,000 pounds.[6]
NTSB repeated that position in its comment on NHTSA's side underride guard advance notice of proposed rulemaking. The agency stated that its Safety Recommendation H-14-002 asked NHTSA to require side underride protection systems on newly manufactured trailers over 10,000 pounds GVWR, and H-14-003 called for comparable side underride protection on truck tractors over 26,000 pounds GVWR.[7] GAO reached a similar policy conclusion from a data and research perspective. In its 2019 report, GAO stated that current federal regulations require rear guards but do not require side or front underride guards. GAO also found that underride fatalities are likely underreported because police crash reports and state forms vary in how they define and capture underride.[8]
NHTSA's 2023 ANPRM confirmed the regulatory gap and asked for comment on side underride guard requirements, including possible performance standards, cost impacts, weight impacts, practicability, and safety benefits.[9] In 2024, NHTSA reported to Congress that it was continuing work on underride protection, data collection, and the Advisory Committee on Underride Protection.[10]
What Crash Testing and Federal Data Show
The strongest visual evidence supporting side underride guards comes from crash testing. IIHS reported that, in a 2017 test, an AngelWing side underride guard prevented a midsize car from sliding under a 53-foot semitrailer in a 40 mph impact. IIHS stated that the 40 mph test involved 30% more energy than its earlier 35 mph test, and the vehicle did not underride the trailer.[11] IIHS also placed side underride in context with fatal crash data. It reported that, among 1,542 passenger-vehicle occupant deaths in crashes with tractor-trailers in 2015, 301 involved the side of a tractor-trailer and 292 involved the rear. IIHS noted the regulatory contrast: federal rules require rear underride guards, but not side underride guards.[12]
NHTSA's cost-benefit analysis used federal crash databases and police crash reports to estimate the safety problem. It found that, from 2008 through 2017, the annual average number of light passenger vehicles involved in side crashes with combination trucks was 192, and about 23% resulted in side underrides. The same analysis found an annual average of 212 light-passenger-vehicle occupant fatalities in combination-truck side crashes, with about 24% attributed to side underride crashes.[13] The data is incomplete. NHTSA compared FARS coding with police crash report reviews and found underreporting. In one review of 184 front-to-side light-passenger-vehicle/combination-truck trailer crashes from 2017, police reports identified 92 vehicle-underride cases, while FARS coded only 52 as underride. NHTSA stated that the difference ranged from 77% to 94%, depending on how unknown underrides were classified.[14]
NHTSA's 2024 report to Congress later estimated that, on average, 89 fatalities annually fell within the Bipartisan Infrastructure Law directive—light-passenger-vehicle occupant fatalities in two-vehicle crashes with tractor-trailers where a light passenger vehicle strikes the side of a trailer or semitrailer and underrides it.[15]
Cost, Weight, and the Reason Federal Action Has Been Slow
The side-underride debate is not only about whether the technology can work in a controlled test. It is also about how much it costs, how much weight it adds, how it affects trailer operations, and how many real-world crashes occur within the speed range the tested guard can address. NHTSA's 2023 cost-benefit analysis used the AngelWing side guard as the primary available design for estimating cost and weight. The report identified an off-the-shelf weight range of 450 to 800 pounds, depending on configuration. It also estimated the hardware cost at $2,897, labor at about $93, and total average installation cost at $2,990 per trailer. Equipping 260,000 eligible new trailers in 2022 would cost about $777.5 million in hardware and installation.[16]
Fuel cost was another major factor. NHTSA estimated that the lifetime incremental fuel cost associated with side guard weight would range from about $245 million to $426.3 million at a 3% discount rate, depending on whether the guard weighed 450 or 800 pounds. Total lifetime costs for the 260,000-trailer cohort were estimated between about $1.02 billion and $1.20 billion at a 3% discount rate.[17]
NHTSA did not conclude that side underride crashes are unimportant. It concluded that, under the assumptions used in its analysis, the costs exceeded the measurable benefits. The agency estimated that full adoption of side guards across the combination-trailer fleet would mitigate about 17.2 fatalities and 69.1 serious injuries annually under its central underreporting assumption and effectiveness assumptions. But when the analysis was limited to all eligible new trailers entering the fleet in 2022, NHTSA estimated an annual reduction of about 0.7 fatalities and 2.7 serious injuries from that new-trailer cohort.[18]
The speed limitation is central to the debate. NHTSA treated 40 mph as the relevant maximum tested impact speed for the AngelWing design. The report acknowledged that side underride occurs predominantly at impact speeds above 40 mph and that protective effects above 40 mph could increase estimated benefits, but the agency stated that it did not have information available to estimate the degree of safety improvement beyond the 40 mph test limit.[19] That is why cost-benefit conclusions should be described carefully. The analysis does not say side guards are ineffective. It says that, based on available data, tested designs, estimated costs, weight, fuel impacts, and the target crash population, NHTSA did not find measurable benefits exceeding costs across the scenarios it evaluated.[20]
City Side Guards Are a Different Safety Tool
Municipal side-guard programs are often discussed alongside side underride guards, but they address a different crash population. City programs usually focus on pedestrians and bicyclists, not passenger vehicles striking trailers at highway speed. Volpe's lateral protective device materials explain that LPDs cover the exposed space between wheels so pedestrians, bicyclists, and motorcyclists are less likely to be swept under the truck. Volpe also reports that after the United Kingdom required LPDs, cyclist fatalities in relevant side-impact collisions dropped 61% and pedestrian fatalities dropped 20%.[21]
Boston adopted side-guard requirements for certain city-contracted vehicles. The city's side-guard materials state that Boston requires contractors to install side guards, crossover mirrors, and safety decals on motor vehicles over 10,000 pounds and semitrailers with total weight over 26,000 pounds.[22] New York City adopted a similar safety rule for heavy-duty trade waste vehicles, requiring certain large vehicles to be equipped with safety-enhancing side guards by January 1, 2023.[23]
These city rules are important because they show that side protection has moved from concept to procurement policy in some jurisdictions. But they should not be used as proof that the same devices would prevent high-speed passenger-vehicle side underride. A pedestrian-focused LPD and a crash-tested passenger-vehicle side underride guard are not necessarily designed for the same impact forces, crash geometry, or injury mechanism.
Litigation: Absence of Technology Is Evidence, Not the Whole Case
In litigation, the absence of side underride guards can become important evidence, but it is not enough by itself. The central questions are usually crashworthiness, feasible alternative design, causation, and admissible expert proof. A plaintiff must show not only that the trailer lacked a guard, but that a feasible guard design existed and would have reduced injury in the actual crash.
Wilden v. Laury Transportation illustrates the point. The plaintiff's vehicle underrode the side of a Great Dane trailer, and the plaintiffs argued that a telescoping side guard would have prevented or reduced the injuries. The Sixth Circuit affirmed exclusion of the plaintiffs' expert opinions and summary judgment for Great Dane because the proposed telescoping side-guard design had not been built or tested in the real world, and the experts had not designed or built such a guard before litigation.[24] The district court decision in Wilden framed the same issue as a design-defect crashworthiness claim requiring proof of a practical, safer alternative design. Without reliable expert testimony supporting the proposed design and its ability to mitigate the crash, the claim could not proceed.[25]
NHTSA's 2025 defect-petition denial adds another layer. The petitioner asked NHTSA to open a defect investigation into van-type or box semitrailers lacking side underride guards. NHTSA denied the petition, stating that the issues were better addressed through rulemaking and ongoing Infrastructure Investment and Jobs Act actions rather than a defect investigation. The agency also stated that no FMVSS requiring side underride guards on semitrailers currently exists.[26]
That does not remove side underride evidence from litigation. It means the evidence must be specific. Investigators need the crash angle, impact speed, overlap, vehicle height, trailer geometry, whether the passenger vehicle underrode the trailer, whether there was passenger compartment intrusion, and whether a tested side guard would have engaged the striking vehicle. They also need evidence about available designs, industry testing, manufacturer knowledge, prior petitions, regulatory history, and whether the trailer's configuration made side guard installation feasible.
The side underride issue sits in the gap between known safety risk and incomplete federal mandate. IIHS testing shows that a properly designed side underride guard can prevent passenger compartment intrusion in controlled tests. NHTSA's analysis recognizes measurable safety benefits but questions whether current designs justify nationwide installation costs. NTSB and GAO have identified continuing safety and data concerns. Courts require reliable proof that a feasible alternative design would have changed the outcome.
Side underride guards are not federally required, but the absence of a federal requirement does not end the safety analysis. In a crash involving side underride, the missing guard becomes one piece of a broader investigation into crash mechanics, feasible technology, industry knowledge, regulatory delay, and whether the occupant compartment was invaded by a hazard that available engineering could have reduced.
Sources
- [1] NHTSA, DOT HS 813 404, Side Impact Guards for Combination Truck-Trailers: Cost-Benefit Analysis (Apr. 2023).↩
- [2] Id.↩
- [3] Id.↩
- [4] Volpe National Transportation Systems Center, Lateral Protective Devices.↩
- [5] Denial of Motor Vehicle Defect Petition, DP24-004, 90 Fed. Reg. 14409 (Apr. 1, 2025).↩
- [6] NTSB, Safety Recommendations H-14-001 through -007 (2014).↩
- [7] NTSB, Comment on NHTSA Advance Notice of Proposed Rulemaking, Side Underride Guards (June 6, 2023).↩
- [8] GAO, Truck Underride Guards: Improved Data Collection, Inspections, and Research Needed, GAO-19-264 (Mar. 14, 2019).↩
- [9] NHTSA, Advance Notice of Proposed Rulemaking, Side Underride Guards (Apr. 2023).↩
- [10] NHTSA, Report to Congress: Underride Protection (June 2024).↩
- [11] IIHS, Side Guard on Semitrailer Prevents Underride in 40 MPH Test (2017).↩
- [12] Id.↩
- [13] NHTSA, DOT HS 813 404, Side Impact Guards for Combination Truck-Trailers: Cost-Benefit Analysis.↩
- [14] Id.↩
- [15] NHTSA, Report to Congress: Underride Protection (June 2024).↩
- [16] NHTSA, DOT HS 813 404, Side Impact Guards for Combination Truck-Trailers: Cost-Benefit Analysis.↩
- [17] Id.↩
- [18] Id.↩
- [19] Id.↩
- [20] Id.↩
- [21] Volpe National Transportation Systems Center, Lateral Protective Devices.↩
- [22] City of Boston, Side Guard Requirement Flyer (2019).↩
- [23] City of New York, Side Guards on Heavy-Duty Trade Waste Vehicles.↩
- [24] Wilden v. Laury Transportation, LLC, 901 F.3d 644 (6th Cir. 2018).↩
- [25] Wilden v. Laury Transportation, LLC, No. 3:13-CV-784-DJH-CHL, 2016 WL 4522670 (W.D. Ky. Aug. 29, 2016), aff'd, 901 F.3d 644 (6th Cir. 2018).↩
- [26] Denial of Motor Vehicle Defect Petition, DP24-004, 90 Fed. Reg. 14409 (Apr. 1, 2025).↩