Interpretation ID: NCC-230607-001 571.108 Automatic Activation of Hazard Warning Signal for Nonresponsive Driver Frooshani signed 7.22.25
U.S. Department of Transportation
National Highway Traffic Safety Administration
Office of the Chief Counsel
1200 New Jersey Avenue SE. Washington, DC 20590
July 22, 2025
Mr. John Frooshani
Vehicle Regulatory Manager, Government Relations
North American Subaru, Inc.
One Subaru Drive Camden NJ 08103
Dear Mr. Frooshani,
I write in response to your letter dated February 5, 2020, requesting an interpretation on whether identified advanced driver assistance system (ADAS) features would comply with the hazard warning signal requirements in Federal Motor Vehicle Safety Standard (FMVSS) No. 108; Lamps, reflective devices, and associated equipment. You also spoke with staff from the National Highway Traffic and Safety Administration's (NHTSA) Office of the Chief Counsel on October 29, 2024. You ask whether your system, after determining a driver is non-responsive, could automatically activate the vehicular hazard warning signal while decelerating the vehicle and bringing it to a stop. Our conclusion is that such a system is permissible in the circumstances described in your letter.
The contents of this interpretation letter do not have the force and effect of law and are not meant to bind the public in any way. This letter is only intended to provide clarity regarding existing requirements under the law at the time of signature.
Background
NHTSA is authorized by the National Traffic and Motor Vehicle Safety Act (Safety Act, 49 U.S.C. Chapter 301) to issue FMVSS setting performance requirements for new motor vehicles and new items of motor vehicle equipment. NHTSA does not pre-approve new motor vehicles, new motor vehicle equipment, or new motor vehicle technologies. In addition, NHTSA does not prohibit the introduction of new motor vehicles or motor vehicle technologies such as Automated Driving Systems (ADS) into the vehicle fleet, provided those vehicles and technologies meet applicable FMVSS and do not present a safety defect. The Safety Act requires manufacturers to self-certify that their vehicles and equipment conform to all applicable FMVSS in effect on the date of manufacture.
Two provisions of FMVSS No. 108 bear on the issue raised in your letter. First, the standard requires vehicles, except trailers and motorcycles, to be equipped with a vehicular hazard warning signal operating unit and a vehicular hazard warning signal flasher.1 A vehicular hazard warning signal operating unit is defined as a driver-controlled device which causes all required tum signal lamps to flash simultaneously to indicate to approaching drivers the presence of a vehicular hazard.2,3 Second, FMVSS No. 108 S6.2.1 states that no additional lamp, reflective device, or other motor vehicle equipment is permitted to be installed that impairs the effectiveness of lighting equipment required by FMVSS No. 108.4
Your letter states that Subaru is exploring deploying an ADAS with a feature intended to prevent a crash if a human driver has become nonresponsive, with the system assuming vehicular control when a driver is inattentive and non-responsive to system warnings. In the described scenario, the vehicle would be brought to a stop and would "activate the hazard warning lamps as the vehicle is decelerating and continue to activate the hazard lamps after the vehicle has been brought to a stop." You state that, in your opinion, the scenario described in your letter is like the one in our 2016 letter to Brian Latouf of General Motors, which allowed automatic activation of the hazard signal following detection of a nonresponsive driver and after the vehicle has been brought to a stop by advanced vehicle systems, and you seek our concurrence.
Discussion
NHTSA's interpretations of "vehicular hazard warning signal operating unit" read FMVSS No. 108 as broadly restricting automatic activation of the hazard signal.5 However, NHTSA historically has read FMVSS No. 108 to allow automatic activation on a case-by-case basis under limited circumstances consistent with the purpose of the hazard warning signal stated in
the standard. FMVSS No. 108 does not permit automatic hazard signal activations that lack any relationship to the original purpose of a hazard warning signal or could create confusion about the signal's meaning.6
1 49 C.F.R. § 571.108.
2 Id. When activated, sufficient turn signals must flash to meet the minimum photometric requirements in FMVSS No. 108 applicable to turn signals. For the purposes of this letter, we refer to these systems collectively as activating the "hazard signal."
3 Id. The standard also defines the vehicular hazard warning signal flasher as a device which, as long as it is turned on, causes all the required turn signal lamps to flash. This system must flash when actuated by the operating unit. See id Table I-a.
4 49 C.F.R. § 571.108 S6.2. l. Under the impairment analysis NHTSA typically considers four aspects of supplemental lighting: activation pattern, brightness, color, and lamp location. In addition, the impairment analysis is applicable to supplemental lighting (i.e., lighting not required by the standard) and is closely related to the requirement that a manufacturer not "knowingly make inoperative" a compliant device or element of design. 49
U.S.C. § 30122(b). See, e.g. Letter to Paul Schaye, Feb. 9, 2019, available at
https://www.nhtsa.gov/inter.pretations/571108-ama-schaye-front-color-changing-light.
5 In letters to Mark Steele and Eric Reed in 1999 and 2000, NHTSA interpreted "a driver controlled device," to mean that that the hazard signal may not automatically activate. See Letter to Mark Steele, Feb. 25, 2000, available at https://www.nhtsa.gov/inter.pretations/2ll7lztv : and Letter to Eric Reed, Feb. 29, 2000, available at https://www.nhtsa.gov/inter.pretations/reedztv.
6 Letter to Sen. Orrin G. Hatch, Aug. 5, 1999, available at https://www.nhtsa.gov/inter.pretations/20l80ztv.
Per FMVSS No. 108, the purpose of the hazard warning system is "to indicate to approaching drivers the presence of a vehicular hazard." Recent interpretations have noted that "the purpose of the hazard warning is to indicate to approaching drivers that the vehicle is stopped or is proceeding at a slower rate than surrounding traffic."7 Several previous agency interpretations referenced FMVSS No. 108 S6.2.1 and emphasized that there is no ambiguity about the meaning or purpose of the hazard signal following a crash.8 Although S6.2.1 applies only to equipment not required by FMVSS No. 108, in our 2016 letter to Mr. Latouf, we clarified that the same analysis applies to automatic activation of the hazard signal.9,10
Several letters of interpretation have discussed situations in which we believe automatic activation of the hazard signal would be inconsistent with FMVSS No. 108, such as during braking events,11 after "sudden release of the accelerator pedal,"12 or as a "deceleration warning system"13 where it would conflict with the purpose of the stop lamps.14 Most recently, on March 22, 2023, NHTSA denied a petition for decision of inconsequential noncompliance filed by
7 Letter to Brian Latouf, Nov. 18, 2016. https://www.nhtsa.gov/interpretations/16-1289-gm-hazard-innovative-28-apr-16-rsy. This statement is based on a previous statement made in our letter to Sen. Richard Lugar on May 9, 2000, that drivers in general activate the hazard signal to "indicate either that [they are] proceeding at a slower rate than surrounding traffic, or that the vehicle is stopped on or off a roadway." https://www.nhtsa.gov/interpretations/21478ztv. This was a description of typical hazard signal use and was not intended to state the purpose of the hazard warning. The letter stated the activation should depend on the driver's evaluation of the driving environment and should not occur involuntarily simply because the vehicle is decelerating. 8 Letter to Mark Steele, Feb. 25, 2000, available at https://www.nhtsa.gov/interpretations/2ll71ztv ; Letter to Timothy Bartlett, Jan. 28, 2002, available at https://www.nhtsa.gov/interpretations/23695ztv; Letter to Stephen Powers, Jan. 19, 2021, available at https://www.nhtsa.gov/interpretations/571108-help-system-powers.
9 Letter to Brian Latouf, Nov. 18, 2016, available at https://www.nhtsa.gov/interpretations/16-1289-gm-hazard innovative-28-apr-16-rsy. This conclusion is based in part on our longstanding position that the "use of required lighting equipment for other than its original purpose" constitutes equipment not required by the standard such that it can result in impairment. See, e.g. Letter to Sen. Orrin G. Hatch, Aug. 5, 1999, available at https://www.nhtsa.gov/interpretations/20180ztv. Automatic activation could be understood as allowed supplemental lighting (in addition to the manually controlled hazard signal) if the specific system does not impair the effectiveness of any required lighting. Because it does not influence our conclusion in this letter, we do not determine whether automatic activation of the hazard signal in the manner you describe is or is not supplemental lighting or other motor vehicle equipment.
10 NHTSA interprets FMVSS No. 108 to require that all auxiliary lamps be "steady burning," with the sole exception being auxiliary lamps that supplement required lamps that flash, such as turn signals. Letter to Donald Lane, Nov. 6 2024, available at https://www.nhtsa.gov/interpretations/ncc-230120-001-571108-peterson-trailer auxiliary-door-light-110624-signed. To the extent that the lighting described here could be considered supplemental lighting, NHTSA would view it as supplementing required lights that flash.
11 Daimler Trucks North America, LLC, Denial of Petition for Decision of Inconsequential Noncompliance, 88 Fed. Reg. 17291 (Mar. 22, 2023).
12 Letter to David Coburn, Aug. 6, 1999, available at https://www.nhtsa.gov/interpretations/19886ztv.
13 Letter to Jeffrey Echt, incoming received Mar. 10, 1995, available at https://www.nhtsa.gov/interpretations/aiam552l. Automatically activating the hazard signal can also interfere with other required lighting, such as the turn signals. We have also addressed cases in which other vehicle systems were activated. See, e.g. Letter to Sen. Orrin G. Hatch, Aug. 5, 1999, available at https://www.nhtsa.gov/interpretations/20l80ztv (finding activating the hazard warning system when the horn is sounded would impair the hazard warning system).
14 See Letter to Stephen Powers, Jan. 19, 2021, available at https://www.nhtsa.gov/interpretations/571108-help
system-powers (limiting the automatic activation of the specified system to after a crash in part because it activated the turn signals up to three times faster than the required flash rate under FMVSS No. 108).
Daimler Trucks North America, LLC (DTNA), regarding certain Model Year 2020-2021 Freightliner Cascadia heavy trucks.15 These trucks contained a system that automatically activated the hazard signal during a brake assist application, when the brake system applies maximum braking force to assist the driver in bringing the truck to a complete stop.
In contrast to these scenarios, our 2016 letter to Mr. Latouf addressed a similar situation to the one described in your letter. There, we stated that under FMVSS No. 108 a system could, after finding a driver unresponsive and bringing the vehicle to a stop, automatically activate the hazard signal.16 We analogized the situation to the one after a crash and noted that being at a complete stop "is the prototypical situation in which the hazard lights are intended to be used, and it is one of the situations that other motorists have come to expect when they see the hazard signal."17 However, we also stated that "the purpose of the hazard warning is to indicate to approaching drivers that the vehicle is stopped or is proceeding at a slower rate than surrounding traffic."18 We also declined to state that such activations could occur only when stopped, finding that future approaches for automatic activation of the hazard signal would be handled on a case by-case basis.19
Based on our understanding of the specific circumstances described in your letter, the automatic activation of the hazard warning signal you describe is permissible under FMVSS No. 108. As an initial matter, because FMVSS No. 108 does not specify in what situations the hazard warning signal may or may not be activated, we decline to state categorically that the hazard warning signal can be automatically activated only after a vehicle has come to a complete stop. Indeed, in the letter to Mr. Latouf, we acknowledged that "proceeding at a slower rate than traffic" is a situation in which drivers typically activate the hazard warning system.
Where past agency interpretations have opined negatively about in-motion activations, it was because the use in question was not consistent with the purpose of the hazard warning system or it impaired the effectiveness of required lighting in violation of FMVSS No. 108.20 However, your system does not appear to activate the hazard warning system in a way that we have previously found unacceptable. Instead, we view the conditions described in your letter as largely similar to those described in our letter to Mr. Latouf.
This letter does not mean that NHTSA views FMVSS No. 108 as generally allowing the automatic use of the hazard warning signal while in motion or to indicate that a vehicle's ADAS
15 88 Fed. Reg. 17291. We believe the circumstances of the petition are distinguishable under FMVSS No. 108. First, Subaru's system does not adjust the flash rate. Second, it activates only in the rare event of a nonresponsive driver. Third, DTNA's system specifically activated during brake events. FMVSS No. 108 states that the stop lights communicate that a vehicle is stopping or diminishing speed by braking. Therefore, the instant circumstance presents much lower risk of confusion than activation during a brake assist.
16 Letter to Brian Latouf, Nov. 18, 2016, available at https://www.nhtsa.gov/intei:pretations/16-1289-gm-hazard
innovative-28-apr-16-rsy.
17 Id
18 Id See Supra, note 9, for clarification regarding this quote.
19 Id
20 This letter does not rescind prior NHTSA letters of interpretation on the topic of automatic activation of the hazard warning system.
or ADS systems are active. The presence of a nonresponsive driver creates a situation where vehicle operation and driving maneuvers are largely disabled and will be for an unknown duration, as is often true following a crash or if there is a health or mechanical issue. In such a circumstance, even if the vehicle is being decelerated to a stop, the hazard warning is not being used to communicate deceleration, activation of the stop lamps, or another action that may be the purview of a lamp required by FMVSS No. 108. Instead, the activation occurs only upon the detection of a separate hazard: a nonresponsive driver. Therefore, the risk of ambiguity or confusion of the signals is low.
Conclusion
Based on the information presented in your letter, the system described above would not violate FMVSS No. 108's requirements applicable to the vehicular hazard warning signal operating unit or vehicular hazard warning signal flasher. It would also not impair the effectiveness of required lighting. This interpretation is specific to the circumstances described above.
We note that Subaru indicates that when the driver is unable or unwilling to take control of the vehicle, the system will bring the vehicle to a stop. A vehicle system that stops a vehicle directly in a roadway might, depending on the circumstances, be considered to contain a safety-related defect, i.e., it may present an unreasonable risk of an accident occurring or of death and injury in an accident.21 Federal law requires the recall of a vehicle that contains a safety-related defect.22 We urge Subaru to fully consider the likely operation of the system it is contemplating and ensure that it will not present such a risk.
Finally, activation of the hazard signal is subject to State laws, which may control when a hazard signal may be used. Nothing in this interpretation should be read to preempt State law regarding when it is appropriate for a hazard warning system to be used. We are unable to advise you on those laws, but you should ensure your system complies with any and all State laws regarding when a hazard warning system may be activated.
If you have any further questions, please contact Eli Wachtel of my staff at interpretations.NHTSA@dot.gov.
Sincerely,
Peter Simshauser
Chief Counsel
Dated: 7/22/25
Ref: Standard No. 108
2149 u.s.c. §§ 30102, 30118.
22 49 U.S.C. § 30118.