NHTSA Interpretation on Outline Marker Lights: What FMVSS 108 Means for Supplemental Truck & Trailer Lighting

Commercial trucks and trailers depend on a familiar system of marker lamps, clearance lamps, identification lamps and reflective devices to communicate a vehicle’s size, position and direction to other road users.

But what happens when new lighting technology is designed to supplement that required lighting and make the outline of a large commercial vehicle more visible?

RoadLites™ asked the National Highway Traffic Safety Administration (NHTSA) that question directly.

On April 16, 2025, NHTSA’s Office of the Chief Counsel issued RoadLites a formal letter of interpretation addressing a multi-directional auxiliary lighting design for 53-foot trailers and explaining how Federal Motor Vehicle Safety Standard No. 108 (FMVSS 108) applies to this type of supplemental truck and trailer lighting.

The response provides useful guidance not only for RoadLites, but also for fleets, trailer owners, manufacturers and installers trying to understand the difference between required vehicle lighting and supplemental lighting.

What Is FMVSS No. 108?

FMVSS No. 108 is the federal motor vehicle safety standard governing lamps, reflective devices and associated lighting equipment on new motor vehicles and certain replacement equipment.

It establishes requirements for lighting including side marker lamps, clearance lamps, identification lamps, stop lamps, turn signals and other required vehicle lighting.

For trailers, FMVSS 108 establishes specific requirements for the color and placement of side marker lamps.

For example, trailers are required to have amber side marker lamps toward the front and red side marker lamps toward the rear. Trailers 30 feet or longer also require amber intermediate side marker lamps and reflectors at or near the midpoint of the trailer.

These established colors and positions help other road users quickly interpret a vehicle’s orientation and dimensions at night and during reduced visibility.

Required Lighting vs. Supplemental Lighting

This distinction is critical.

Not every light installed on a truck or trailer is a required FMVSS 108 lamp.

Lighting that is added beyond the equipment required by the standard may instead be considered auxiliary or supplemental lighting.

In its interpretation to RoadLites, NHTSA concluded that the lighting device RoadLites described was not required equipment. It was auxiliary or supplemental lighting.

That does not mean supplemental lighting can simply be installed anywhere, in any color or in any configuration.

One of the central principles of FMVSS 108 is that additional lighting must not impair the effectiveness of lighting equipment required by the standard.

That concept — impairment of required lighting — became the central issue in NHTSA’s analysis of the RoadLites design.

What NHTSA Evaluates With Supplemental Vehicle Lighting

NHTSA explained that determining whether an auxiliary lamp impairs required lighting is generally a case-by-case analysis.

Among the primary characteristics considered are:

• brightness or photometric intensity;

• color;

• location; and

• activation pattern.

Other characteristics may also be relevant.

The purpose is not simply to determine whether an additional light is visible. The question is whether that light could interfere with, obscure, distract from or create confusion about the meaning of the vehicle’s required lighting.

That distinction matters significantly for outline and perimeter lighting.

What RoadLites Asked NHTSA

RoadLites presented NHTSA with a multi-directional lighting concept designed for installation near the rear of a 53-foot trailer.

The device described to NHTSA incorporated separate light directions: red toward the rear, amber toward the side and white toward the front.

The purpose was to improve nighttime visibility of the trailer and assist with situations such as backing, lane changes and other maneuvers.

RoadLites asked NHTSA whether the overall design and color configuration would be permitted under FMVSS 108 and also asked about how far the devices could extend from the trailer.

NHTSA Guidance Helped Refine the Lighting Configuration

NHTSA confirmed that the RoadLites design was being evaluated as supplemental lighting, rather than as a replacement for the marker and clearance lamps already required on a trailer.

As part of its review, the agency looked closely at how the colors of the light correspond with where the device is mounted on the trailer.

Because FMVSS 108 uses different marker-light colors to help drivers understand the position and orientation of a commercial vehicle, NHTSA recommended a different color arrangement for a rear-mounted application.

That guidance gave RoadLites a clearer path forward and helped reinforce an important design principle: supplemental lighting should work with the established lighting pattern on a truck or trailer.

A Clearer Path for Rear-Mounted Outline Lighting

For a light mounted near the rear of the trailer, NHTSA explained that a red side-facing light is more consistent with the lighting already associated with the rear portion of a trailer.

The agency also encouraged careful consideration of any forward-facing white light so that the overall configuration remains clear and recognizable to other road users.

For RoadLites, this was valuable guidance.

It showed that supplemental outline lighting can be evaluated not simply by whether an additional light is present, but by how its color, position and function work together.

That is exactly the approach RoadLites continues to take when developing lighting intended to improve the visibility and visual outline of commercial trucks and trailers.

The takeaway is simple: supplemental lighting should complement the vehicle’s required lighting system and make the truck or trailer easier to understand at a glance

Why This Matters for Outline Marker and Perimeter Lighting

Traditional vehicle-lighting categories were developed around defined functions: side marker lamps, clearance lamps, identification lamps, stop lamps, turn signals and other specific devices.

Modern multi-directional outline lighting can be different.

A single housing may contain separate emitting faces intended to be visible from different viewing angles — for example, from the side, rear-quarter or front-quarter of a large commercial vehicle.

RoadLites' goal with this type of lighting is supplemental visibility: helping make the boundaries, orientation and configuration of a truck or trailer easier to perceive, particularly at night or in reduced visibility.

That does not mean supplemental outline lighting replaces the lamps required by FMVSS 108.

It does not.

Required lighting must remain installed, visible and effective.

Instead, supplemental outline lighting must be designed and installed in a manner that respects the established lighting language already used on American roads.

Supplemental Does Not Mean “Unregulated”

A common misunderstanding is that an auxiliary light is automatically outside the federal vehicle-lighting framework simply because FMVSS 108 does not require that particular lamp.

That is not the case.

FMVSS 108 restricts additional lighting that impairs the effectiveness of required lighting. Federal law also includes a “make inoperative” prohibition applicable to certain commercial entities modifying vehicles after first sale.

NHTSA's interpretation to RoadLites also noted that state vehicle-lighting laws may apply.

As a result, fleets, installers and vehicle owners should consider the complete installation — not merely the specifications of an individual lamp.

Color, placement, intensity, activation pattern and interaction with required lighting can all matter.

Does NHTSA “Approve” Aftermarket Truck Lights?

This is another important distinction.

NHTSA does not operate a product-approval program where the agency tests a lamp and issues a general government approval for the product.

Federal motor vehicle safety law relies on manufacturer certification for products subject to applicable federal motor vehicle safety standards.

A NHTSA letter of interpretation is different.

It explains the agency's interpretation of existing federal requirements based on the specific facts presented in a request.

RoadLites therefore does not describe this letter as “NHTSA approval” of every RoadLites product or installation.

Instead, the April 2025 interpretation gives RoadLites direct regulatory guidance about how NHTSA analyzes the particular supplemental lighting design that was presented to the agency.

What About the Physical Extension From the Trailer?

RoadLites also asked how far the devices could extend from a trailer.

NHTSA explained that its regulations do not establish the specific vehicle-width requirement at issue. Vehicle width on the National Network is regulated under Federal Highway Administration requirements.

The agency's response discussed the federal 102-inch vehicle-width framework and several categories of devices that may be excluded from width measurements under particular conditions.

That means dimensional compliance should be evaluated separately from the FMVSS 108 lighting analysis.

Where RoadLites Goes From Here

The interpretation helped clarify how the existing FMVSS 108 framework applies to the supplemental lighting design RoadLites presented.

It also highlights a broader question facing innovative commercial-vehicle lighting:

How should multi-directional outline and perimeter lighting be evaluated when the technology does not fit neatly into a conventional single-function lamp category?

RoadLites continues to examine that question.

Our objective is not to replace required marker, clearance or identification lamps. It is to explore how properly designed supplemental lighting can improve the visual definition of large trucks and trailers while preserving the established color, location and signaling conventions relied upon by other road users.

That work includes evaluating applicable FMVSS 108 requirements, photometric performance, installation configurations and the regulatory framework surrounding supplemental commercial-vehicle lighting.

The RoadLites Approach to Commercial Vehicle Visibility

RoadLites develops and distributes LED visibility lighting for trucks, trailers and other commercial vehicles.

Our work with supplemental outline lighting is based on a straightforward principle: additional visibility should complement required vehicle lighting, not compete with it.

The NHTSA interpretation gives RoadLites an important foundation for continuing that work while helping fleets and commercial-vehicle operators better understand the regulatory distinction between required and supplemental lighting.

As commercial-vehicle lighting technology continues to evolve, RoadLites intends to remain part of the conversation surrounding visibility, vehicle conspicuity and the future of supplemental truck and trailer lighting.

This article is provided for general educational purposes and does not constitute legal advice. Vehicle owners, fleets, manufacturers and installers should evaluate applicable federal requirements and the laws of the states in which vehicles are registered or operated.