Every automatic emergency braking (AEB) system on a commercial truck is, among other things, a recording device. The forward-facing radar, the cameras, the software that decides when to warn a driver and when to apply the brakes: all of it generates data. That data includes sensor readings, system status logs, warning activations, and braking interventions. When a truck equipped with AEB is involved in a crash, the system’s internal records can establish what the technology detected, what it did in response, and whether it was functioning at all.
AEB is designed to reduce the severity of rear-end collisions or avoid them entirely. A 2020 study by the Insurance Institute for Highway Safety (IIHS) found that AEB reduced rear-end crash involvement for large trucks by 41 percent, and that forward collision warning alone reduced rear-end crashes among large trucks by 44 percent.1 The National Highway Traffic Safety Administration (NHTSA) and the Federal Motor Carrier Safety Administration (FMCSA) jointly proposed a rule in 2023 that would mandate AEB on all new heavy vehicles with a gross vehicle weight rating above 10,000 pounds.2 The proposal itself reflects a regulatory conclusion that the technology works and that its continued absence on new trucks is no longer acceptable.
For trucks already on the road without AEB, or trucks where the system has been disabled or allowed to degrade, the question in any rear-end crash is direct: what role did the absence or failure of the system play?
How AEB Works on Commercial Trucks
AEB systems on heavy trucks use a combination of forward-facing radar and camera sensors to continuously monitor the road ahead. The radar measures distance and closing speed to objects in the vehicle’s path. The camera provides object classification, distinguishing a passenger vehicle from a motorcycle, a pedestrian, or a stationary object. The two inputs are processed by the system’s electronic control unit, which runs decision algorithms to determine whether a collision is imminent.3
When the system calculates that the truck is closing on an object at a rate that will result in impact, it responds in stages. The first stage is typically a forward collision warning, an audible and visual alert to the driver. If the driver does not respond within a defined time window, the system escalates to automatic braking. Depending on the system design and the speed differential involved, AEB may apply partial braking to reduce impact severity or full emergency braking to attempt a complete stop.4
The braking force AEB systems can deliver is significant. Under NHTSA’s proposed rule, AEB systems on heavy trucks must be capable of avoiding a collision with a stopped lead vehicle when tested at speeds up to 80 km/h (approximately 50 mph) without manual braking, and at speeds up to 100 km/h (approximately 62 mph) when manual braking is also applied.5 These are minimum performance standards. Some commercially available systems exceed them. The system operates continuously while the truck is in motion. It does not require the driver to activate it, and in most configurations, the driver cannot fully disable it through normal dashboard controls. Exceptions exist, though, and those exceptions matter in crash investigations.
What AEB Systems Record
The evidentiary value of AEB lies not just in what the system does but in what it documents. The electronic control unit logs time-stamped records of system events, including every forward collision warning issued, every automatic braking intervention, and the sensor inputs that triggered each event.
The depth of the recorded data varies by manufacturer. Systems produced by major suppliers, including Bendix (Wingman Fusion), ZF (OnGuardACTIVE), and Detroit Assurance, store event logs accessible through proprietary diagnostic tools.7 Some systems maintain a rolling buffer of recent sensor data, similar in concept to an event data recorder, capturing the seconds immediately before and after a triggering event.
What makes AEB data particularly useful in crash reconstruction is that it records what the system perceived, independent of what the driver later claims happened. If the AEB system logged a forward collision warning 3.5 seconds before impact and the driver took no evasive action, that sequence is preserved in the system’s memory. If the system attempted automatic braking and reduced the truck’s speed by a measurable amount before impact, the data will show the pre-intervention speed, the braking onset point, and the speed at the moment of collision.
The data also records when the system was not functioning. Fault codes stored in the AEB electronic control unit can document:
- When sensors were obstructed
- When radar calibration fell out of specification
- When the system entered a degraded operating mode
- When it was manually overridden or disabled
A truck that arrives at a crash scene with an AEB fault code logged days or weeks before the collision tells a fundamentally different story than one whose system was fully operational at impact.
Crash Reduction Data
The evidence supporting AEB effectiveness in commercial trucking is not speculative. The IIHS analyzed real-world crash data for large trucks equipped with forward collision warning and AEB and found that AEB reduced rear-end crash involvement by 41 percent, while forward collision warning alone reduced rear-end crashes by 44 percent.8 A separate IIHS analysis of passenger vehicles found that forward collision warning alone reduced rear-end crashes by 23 percent, suggesting that even the warning component changes driver behavior in ways that prevent collisions.9
Rear-end collisions involving large trucks are among the most dangerous crash configurations on the road. When a loaded tractor-trailer strikes a passenger vehicle from behind, the mass differential means the forces are absorbed almost entirely by the smaller vehicle and its occupants. NHTSA’s preliminary regulatory impact analysis for the proposed AEB rule estimated the rule would prevent approximately 19,118 crashes, save 155 lives, and reduce 8,814 non-fatal injuries annually once all covered vehicles are equipped with AEB and electronic stability control.10
The safety data also explains why the absence of functioning AEB on a truck involved in a rear-end crash carries investigative weight. If a technology demonstrably reduces a specific crash type by 41 percent, and a carrier elected not to install it, allowed it to degrade, or permitted drivers to disable it, that choice becomes a relevant fact in determining whether the carrier exercised reasonable care.
The Federal Mandate
In July 2023, NHTSA and FMCSA jointly published a notice of proposed rulemaking that would require automatic emergency braking systems on all new heavy vehicles with a gross vehicle weight rating above 10,000 pounds.11 The rulemaking responds to a mandate under the Bipartisan Infrastructure Law, enacted as the Infrastructure Investment and Jobs Act of 2021, which directed the Department of Transportation to prescribe an FMVSS requiring AEB on heavy commercial vehicles equipped with electronic stability control systems.12
The proposed performance standards under the new FMVSS are specific:14
- Stopped lead vehicle: The AEB system must prevent a collision at truck speeds between 10 and 80 km/h (~6 to 50 mph) without manual braking, and between 70 and 100 km/h (~44 to 62 mph) with manual braking.
- Slower-moving lead vehicle traveling at 20 km/h (~12 mph): The system must avoid a collision at truck speeds between 40 and 80 km/h (~25 to 50 mph) without manual braking, and between 70 and 100 km/h (~44 to 62 mph) with manual braking.
- Decelerating lead vehicle: A third test scenario evaluates performance when the lead vehicle decelerates from a shared speed of 50 or 80 km/h (~31 to 50 mph).
The proposed rule also addresses a known problem from voluntary AEB installations: driver deactivation.
The proposed regulatory text does not permit vehicle manufacturers to install a manual deactivation switch that would allow a driver to turn off the AEB system. Under the FMVSS framework, a standard that is silent on deactivation effectively prohibits it. NHTSA has sought comment on whether limited deactivation should be permitted under narrow circumstances, such as when aftermarket equipment obstructs sensors, but has proposed that the default position of any such switch would need to be "AEB ON" with each cycle of the starting system. Forward collision warning sensitivity may be adjusted by the driver, but the automatic braking function itself would be required to remain active.15
Before this proposed rule, no federal regulation required AEB on commercial trucks. Many large carriers had adopted the technology voluntarily, but a substantial portion of the fleet, particularly older vehicles and trucks operated by smaller carriers, remained unequipped. NHTSA’s market data indicates that while AEB is likely equipped on the majority of Class 8 vehicles and available on nearly all Class 3 and 4 vehicles, few Class 5 and 6 vehicles come equipped with any type of AEB system.16 The proposed rule would establish the regulatory floor for new vehicles, but NHTSA considered and decided against requiring retrofitting of in-service vehicles, citing the complexity and cost of integrating AEB with existing chassis, engine, and braking systems.
When AEB Fails or Is Disabled
AEB system failures in commercial trucks fall into distinct categories, each carrying different implications for crash investigation.
Sensor obstruction is the most common cause of degraded AEB performance. The forward-facing radar and cameras are mounted on the front of the cab, directly exposed to road spray, mud, ice, and debris. A radar unit covered in road grime may lose the ability to detect vehicles ahead at the correct range. A camera lens obscured by dirt, moisture, or condensation may fail to classify objects accurately. Manufacturers specify cleaning and inspection intervals for AEB sensors, and those requirements become part of the carrier’s maintenance obligations under 49 C.F.R. § 396.3, which requires every motor carrier to systematically inspect, repair, and maintain all motor vehicles subject to its control.17
Sensor misalignment is a subtler failure mode. AEB radar and cameras must be precisely calibrated to function within specification. A truck that has sustained a minor front-end impact, had its windshield replaced, or undergone any cab-area bodywork may have sensors that are physically intact but no longer properly aligned. A misaligned radar unit may track targets inaccurately, calculate incorrect closing distances, or fail to detect objects until they are too close for effective intervention. Recalibration after any event that could affect sensor positioning is a standard manufacturer requirement, and failure to perform it is a maintenance deficiency.
Deliberate deactivation is the most straightforward failure and often the most consequential in litigation. Some earlier AEB installations permitted drivers to fully disable the system through dashboard controls. Drivers who found the system’s warnings intrusive (particularly in congested, stop-and-go traffic) sometimes turned it off and never turned it back on. Carriers that were aware of this practice, or that had access to telematics data showing system status but never monitored it, face questions about why trucks under their control were operating without functioning safety equipment. Under 49 C.F.R. § 396.7, a motor carrier shall not permit a driver to operate a motor vehicle that is likely to cause an accident or a breakdown.18 A truck with a deliberately disabled collision avoidance system operating in rear-end crash conditions fits within that prohibition.
What Investigators Examine After a Crash
When a rear-end crash involves a truck equipped with AEB, the system’s data is among the first evidence investigators pursue. The initial step is determining whether the system was operational at the time of the crash. This requires downloading the fault codes and event data from the AEB electronic control unit using the manufacturer’s proprietary diagnostic tools. The download should be performed before the vehicle is moved, repaired, or powered down in a way that could overwrite volatile data.19
The event log reveals the sequence leading to impact: whether a forward collision warning was issued, how many seconds before the collision it activated, whether the system escalated to automatic braking, and what speed reduction the braking achieved. It also documents whether the driver independently applied the brakes and, if so, when relative to the system’s own intervention.
Investigators cross-reference AEB data against other available sources: the engine control module, the event data recorder if separately installed, GPS and telematics records, and any dashcam or forward-facing video. Consistency across these data streams reinforces the evidentiary narrative. Discrepancies, such as AEB data documenting a warning three seconds before impact while the driver’s statement claims there was no time to react, identify areas requiring further examination.
For trucks where AEB was not functional at the time of the crash, the investigation shifts to why. Maintenance records should reflect sensor inspections, calibrations, and any diagnostic fault codes that were identified and either repaired or left unaddressed. Fleet management telematics may preserve historical AEB system status, including periods of degraded operation or full deactivation. The carrier’s internal policies regarding AEB maintenance, driver training on the system, and monitoring of system status are all subject to discovery.
The carrier’s knowledge is the central question. Federal regulations impose an affirmative duty on carriers to maintain vehicles in safe operating condition and to refrain from dispatching vehicles with known deficiencies.20 A carrier that knew or should have known that AEB systems across its fleet were routinely disabled, improperly calibrated, or operating in degraded modes, and that took no corrective action, faces an inquiry that extends beyond the individual crash to the carrier’s broader safety management practices. A finalized AEB mandate would reinforce this obligation by establishing a regulatory baseline for the technology itself. Even in the absence of a final rule, the NPRM and its underlying safety data provide a framework for evaluating whether carriers have adopted available and proven safety technology.
Sources
- [1] Insurance Institute for Highway Safety, Study shows front crash prevention works for large trucks too (Sept. 3, 2020).
- [2] National Highway Traffic Safety Administration and Federal Motor Carrier Safety Administration, Heavy Vehicle Automatic Emergency Braking Notice of Proposed Rulemaking (June 2023).
- [3] NHTSA/FMCSA, Heavy Vehicle AEB NPRM (June 2023).
- [4] National Transportation Safety Board, Special Investigation Report: The Use of Forward Collision Avoidance Systems to Prevent and Mitigate Rear-End Crashes, SR-15/01.
- [5] NHTSA/FMCSA, Heavy Vehicle AEB NPRM (June 2023).
- [6] NTSB, Special Investigation Report SR-15/01.
- [7] ZF Group, ZF OnGuardACTIVE product announcement (Nov. 2023).
- [8] Insurance Institute for Highway Safety, Study shows front crash prevention works for large trucks too (Sept. 3, 2020).
- [9] Insurance Institute for Highway Safety, Front crash prevention slashes police-reported rear-end crashes (Jan. 28, 2016).
- [10] NHTSA/FMCSA, Heavy Vehicle AEB NPRM (June 2023).
- [11] Id.
- [12] Id.
- [13] Id.
- [14] Id.
- [15] Id.
- [16] Id.
- [17] 49 C.F.R. § 396.3.
- [18] 49 C.F.R. § 396.7.
- [19] NTSB, Special Investigation Report SR-15/01.
- [20] 49 C.F.R. § 396.3; 49 C.F.R. § 396.7.