Interpretation ID: NCC-230420-001 571.108 Angelina Twardawa Auxiliary Side Lamps Interpretation signed

April 16, 2025

U.S.Department of Transportation
National Highway Traffic Safety Administration
Office of the Chief Counsel
1200 New Jersey Avenue SE. Washington, DC 20590

Ms. Angelina Twardawa 4550 Gustine Ave
St. Louis, MO  63116
angelina@angiestrans.com 

Dear Ms. Twardawa: 

This responds to your letter, received December 5, 2022 in which you requested a letter of interpretation asking whether aftermarket auxiliary trifunctional side marker lights which illuminate red in the rear, amber in the side, and white in the front, and that attach to the rear of a 53-foot trailer on both sides are compliant with Federal Motor Vehicle Safety Standard (FMVSS) No. 108, Lamps, reflective devices, and associated equipment. We apologize for the delay in responding to your inquiry. Based on the information you provided in your letter, we have concluded that installing the device as described may conflict with FMVSS No. 108. However, an alternative color configuration should be permissible under that standard. 

Please note that our guidance below is based on our understanding of the specific information provided in your letter. The contents of this 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 that set performance requirements for new motor vehicles and new items of motor vehicle equipment. 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.
NHTSA also investigates safety-related defects. After first purchase of a motor vehicle or item of motor vehicle equipment other than for resale, section 30122 of the Safety Act requires that a manufacturer, distributor, dealer, rental company, or motor vehicle repair business not knowingly make inoperative any part of a device or element of design installed on or in a motor vehicle or motor vehicle equipment in compliance with an applicable motor vehicle safety standard. This letter represents NHTSA's opinion concerning whether your design, as you describe it, would be permitted under FMVSS No. 108 and section 30122.

In your letter, you state that the device is a trifunctional light that attaches to the side of the rear guard of a 53-foot trailer on both sides with an open-ended connector. The lights have a red light designated at the rear, amber light designated at the side, and a white light designated at the front. You note that the piece that attaches to the trailer is rubber and flexible so it will not break, and that the device is stationary. You state the purpose of the device is to assist drivers with backing up, lane changes, and other maneuvers by providing increased visibility of the trailer at night. You ask whether the described design is permitted (both with regards to the device overall and the described lighting configuration), and if it is how far the devices can extend. 

FMVSS No. 108 specifies requirements for original and replacement lamps, reflective devices, and associated equipment, including for trailers. For auxiliary lamps, the primary restriction imposed by FMVSS No. 108 is by S6.2.1, which states that "[n]o additional lamp, reflective device, or other motor vehicle equipment is permitted to be installed that impairs the effectiveness of lighting equipment required by this standard." Additionally, both original equipment and aftermarket lighting can run afoul of the "make inoperative" provision in 49 U.S.C. § 30122. These prohibitions bar installation by a manufacturer, distributor, dealer, rental company, or motor vehicle repair business of lamps that would impair the effectiveness of required lighting, but do not apply to the owner of a vehicle. We note that whether there is an impairment is determined in the first instance by the manufacturer of the vehicle (or the entity installing the aftermarket lighting) when it is certified as compliant with FMVSS No. 108. 

Typically, the impairment determination is made on a case-by-case basis and looks at four main characteristics of the auxiliary lamp to analyze whether it impairs the effectiveness of required lighting. These are the brightness (photometric intensity), color, location, and activation pattern of the lamp.1 This list is not exhaustive and other considerations may be relevant to the analysis. NHTSA has long stated that highway traffic safety is enhanced by the familiarity of drivers with established lighting schemes, which facilitates their ability to instantly recognize the meaning the lamps convey and respond accordingly.2 Therefore, we have long interpreted the impairment provision to prohibit auxiliary lamps that are colors which are likely to cause confusion to other road users.3 Additionally, auxiliary lamps must be located such that they would not interfere or be confused with the lamps required by our standards. For example, we have stated that two auxiliary 

1 This letter is limited to the information provided in your request. Your request did not state the activation pattern or intensity of the device, which can be relevant to determining if a device causes impairment. For the purposes of this letter, we assume without finding that these aspects of your device do not cause impairment. The following information is provided for your reference regarding these elements. Regarding activation pattern, 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 Paul Schaye (Sept. 9, 2019), available at htqJs://www.nhtsa.gov/interpretations/571108-ama-schaye-front-color-changing-light. Regarding intensity, NHTSA interprets the impairment provision to prohibit auxiliary lamps that are so bright as to obscure or distract from a vehicle's required lamps. For example, NHTSA has in the past stated that supplemental lighting can impair the effectiveness of the required lighting if it is so intense that it glares other road users or masks required signal lamps. See Letter to Rusty Riggin, Aug. 2, 2002, available at htqJs://www.nhtsa.gov/interpretations/24179ztv, (explaining that a supplemental rear cornering lamp could violate the impairment provision if it was so intense that it "create[d] distracting glare"). Letter to Mark Wallach (Oct. 17, 2006) available at htqJs://www.nhtsa.gov/interpretations/wallach3.
2 Letter to Robert Clarke (July 28, 2005), available at htqJs://www.nhtsa.gov/interpretations/gID0255l3.
3 As an example, for auxiliary lamps located on the front of vehicles, these colors include red, which could be confused for a taillamp or stop lamp, and green, which typically conveys the message that one may proceed forward and could therefore impair required lighting that indicates caution. Letter to Paul Schaye (Sept. 9, 2019), available at htqJs://www.nhtsa.gov/interpretations/571108-ama-schaye-front-color-changing-light See also, Letter to Kerry Legg (Oct. 19, 2006), available at htqJs://www.nhtsa.gov/interpretations/leggl.

lamps located next to the three-lamp trailer identification lamp cluster would detract from the purpose of the cluster.4    '

For lighting relevant to your inquiry, FMVSS No. 108 requires that trailers have two amber side marker lamps as far to the front as practicable and two red side marker lamps as far to the rear as practicable. Trailers 30 feet or longer must also have amber intermediate side marker lamps and reflex reflectors on each side located at or near the midpoint between the front and rear side marker lamps.5, 6 

Discussion 

We now turn to your inquiry. Your lamp is not required equipment, and you correctly observe that it is auxiliary or supplemental lighting.7 Therefore, we turn to the question of impairment and look to the characteristics discussed. The factors which appear most relevant to your inquiry are the location and color of the lamps, which we analyze together. Due to the color and location of the lamp as described in your letter, it is likely to impair the effectiveness of the side marker lamps required to be installed on trailers by FMVSS No. 108, and, if installed by you or another entity subject to the "make inoperative" prohibition as aftermarket equipment, could make inoperative lighting required by FMVSS No. 108.8 However, an alternative color scheme would be permissible under FMVSS No. 108. 

Regarding side marker lamps, FMVSS No. 108 requires that intermediate side marker lamps be amber, and rear side marker lamps be red. Your design notes that your device will be mounted at the rear and that the side of the device will have amber lights. Therefore, in driving conditions
with poor visibility, such as at night or in heavy weather, your device may impair the effectiveness of the required intermediate side marker lamps by confusing nearby drivers about whether it indicates the rear or middle of the trailer, which could lead to unsafe driving decisions. 

4 Letter to Robert Clarke (July 28, 2005), available at htt;ps://www.nhtsa.gov/inter_pretations/gf00255l3.
5 We also note that S6.l.3.l of FMVSS No. 108 requires that each lamp, reflective device, and item of associated equipment must be securely mounted on a rigid part of the vehicle. The purpose,of this requirement is to ensure that lamps and reflectors do not sway in the wind on rigid hinges or flexible mud flaps when the vehicle is in motion. Letter from Frank Burndt to Dietmar K. Haenchen, (date unavailable), available at https://www.nhtsa.gov/inter_pretations/aiam3320. S6.l.3.1 is organized in S6.1 ofFMVSS No. 108, which contains requirements applicable to required lamps, reflective devices, and associated equipment. Although this provision does not explicitly apply to auxiliary lamps, an auxiliary device that is not securely mounted to a rigid part of the vehicle may impair the effectiveness ofrequired lighting because the motion of the light could distract drivers.  We have assumed for the purposes of this letter that your device is securely mounted to a rigid part of the vehicle and encourage you to ensure that this is the case.
6 49 CFR 571.108 Table 1-b. 49 CFR 393.11, which you referenced in communication with my staff, which requires commercial motor vehicles operated by motor carriers to be equipped with such lighting, contains similar specifications for color and placement of side marker lamps.
7 Your request describes your device as a "side marker lamp." Side marker lamps are a type of required equipment under FMVSS No. 108. Because your device is not required lighting, to avoid confusion this letter does not refer to your device as a side marker lamp.
8 Section 30122 applies to any "manufacturer, distributor, dealer, rental company, or motor vehicle repair business." We assume, for the purposes of this letter, that you are subject to this provision. 

Our opinion is based on the location and color of the lights as described in your letter. An alternative color scheme, with a red light facing the side in place of the amber light, is unlikely to have the same risk of impairing the effectiveness of the side marker lamps because it would
correspond to the color required for the required side marker lamps installed on each side as far to the rear as practicable, close to where you plan to install the device. We also encourage you to consider designs which mitigate any risk that a forward-facing white light could be perceived as a headlamp by other road users. 

Finally, you inquired about how far out the devices may extend from the trailer. NHTSA's regulations do not contain requirements specific to this question. Vehicle width is regulated by the Federal Highway Administration, who's regulation states that no State shall impose a width limitation of more or less than 102 inches (except Hawaii) for vehicles operating on the National Network.9 Five items are excluded regardless of how far they extend beyond the exterior of vehicles; rear view mirrors, turn signal lamps, handholds for cab entry/egress, splash and spray suppressant devices, and load induced tire bulge. Other excluded devices are: all non-property carrying devices or components at the front of a semitrailer or trailer, devices that do not extend more than 3 inches beyond each side or the rear of a vehicle, devices needed for loading or unloading that do not extend more than 24 inches beyond the rear of the vehicle, and aerodynamic devices that do not extend more than 5 feet beyond the rear of a vehicle, provided they have neither the strength, rigidity nor mass to damage a vehicle that strikes a trailer so equipped from the rear and provided also that they do not obscure tail lamps, turn signals, marker lamps, identification lamps, or any other required safety features, such as hazardous materials placards or conspicuity markings.10 

We also note that, because it is an item of motor vehicle equipment subject to the requirements of the Safety Act, it must be designed free from defects regarding motor vehicle safety. Additionally, truck tractors and trailers operated commercially in interstate commerce are subject to the regulations of the Federal Motor Carrier Safety Administration (FMCSA). Finally, States may have laws applicable to certain motor vehicle lighting. We are unable to advise you on those laws, but you should ensure your system complies with any and all applicable State laws.

We hope you find this information helpful. If you have further questions, please contact Eli Wachtel of my staff at (202) 366-2992.

Sincerely,

Peter Simshauser Chief Counsel

Dated: 4/16/25
Ref:  Standard No. 108
 

9 23 CFR 658.15.
1°FHWA Vehicle Size and Weight Q&A, available at https://ops.fhwa.dot.gov/freight/sw/fags/qa.cfin?categ01y=8. For
more excluded devices, see 23 CFR 658.16 and Appendix D to 23 CFR 658.