DOT & FMVSS 108 Truck Lighting Guide: Marker Lights, Clearance Lights & Supplemental Lighting

If you operate a semi-truck, trailer, box truck, fleet vehicle, or other commercial vehicle, you've probably heard terms like DOT compliant lighting, FMVSS 108, marker lights, clearance lights, identification lights, and supplemental lighting.

But what do they actually mean?

Commercial vehicle lighting isn't just about putting more lights on a truck. Different lamps serve different purposes, and federal requirements establish where certain lights belong, what colors they use, and what information they communicate to other drivers.

This RoadLites™ guide breaks down the basics of truck and trailer lighting under FMVSS No. 108, explains the difference between required and supplemental lighting, and helps make sense of some of the terminology you'll encounter when choosing LED lights for a commercial vehicle.

What Is FMVSS No. 108?

Federal Motor Vehicle Safety Standard No. 108 (FMVSS 108) is the federal safety standard covering lamps, reflective devices, and associated equipment on motor vehicles.

It establishes requirements for numerous types of vehicle lighting and reflective equipment, including equipment used on commercial trucks and trailers.

Depending on the vehicle, these can include:

• side marker lamps;

• clearance lamps;

• identification lamps;

• stop lamps;

• tail lamps;

• turn signal lamps;

• backup lamps;

• license plate lamps;

• reflex reflectors; and

• conspicuity treatments.

The exact requirements depend on factors including the type, size, and configuration of the vehicle.

FMVSS 108 is important because vehicle lighting is a communication system.

A driver approaching a commercial vehicle at night should be able to recognize information about the vehicle's presence, direction, width, length, and movement from its established lighting pattern.

What Does “DOT Compliant” Mean for Truck Lights?

This phrase is frequently misunderstood.

You'll often see vehicle lights advertised as “DOT approved” or “DOT compliant.”

Those phrases should not automatically be treated as meaning that the U.S. Department of Transportation or NHTSA individually tested and approved a particular lamp.

The federal vehicle-safety system generally relies on manufacturers to determine whether regulated equipment complies with the applicable Federal Motor Vehicle Safety Standards and to certify covered equipment accordingly.

That distinction matters when shopping for commercial vehicle lighting.

Instead of relying solely on a “DOT” claim in a product description, it is important to understand:

• what function the lamp is intended to perform;

• whether it is required or supplemental equipment;

• what standards apply to that function;

• where the lamp will be installed;

• what color it displays; and

• whether the installation could interfere with required vehicle lighting.

In other words, the application matters just as much as the lamp itself.

What Are Truck and Trailer Marker Lights?

Side marker lamps help other road users understand the presence and length of a vehicle when viewing it from the side.

On trailers, federal lighting requirements use established colors to communicate position.

Generally, required side marker lighting toward the front is amber, while required side marker lighting toward the rear is red.

Longer trailers also require intermediate side marker lamps.

That color pattern isn't arbitrary.

It helps a driver looking at the side of a commercial vehicle understand which portion of the vehicle they are seeing.

This is one reason why RoadLites emphasizes choosing lighting based on location, direction, color, and intended function, rather than simply choosing a light because it looks good on the vehicle.

Marker Lights vs. Clearance Lights: What's the Difference?

The terms marker light and clearance light are sometimes used interchangeably in everyday trucking conversations, but they describe different lighting functions.

Side Marker Lights

Side marker lamps primarily communicate the presence and length of the vehicle when viewed from the side.

They help another road user identify where the vehicle begins and ends.

Clearance Lights

Clearance lamps are associated with communicating the overall width of a large vehicle.

Their mounting locations and function differ from ordinary side marker lamps.

Identification Lights

Identification lamps are another distinct category.

On applicable vehicles, these are the familiar group of three lamps used to help identify a wide vehicle.

So while all three types of lighting contribute to visibility and recognition, they are not automatically interchangeable.

A lamp intended to perform a required clearance-light function needs to satisfy the requirements applicable to that function.

What Color Should Truck and Trailer Marker Lights Be?

For conventional required side marker lighting, an easy rule to remember is:

Amber toward the front. Red toward the rear.

But commercial vehicle lighting can become more complicated once intermediate lamps, combination lamps, supplemental lamps, turn signals, clearance lamps, and multi-directional lighting are involved.

Color is more than a styling choice.

It communicates information.

NHTSA has repeatedly emphasized the importance of established vehicle-lighting patterns because drivers learn to associate particular colors and positions with particular meanings.

That's why the proper question isn't simply:

“Can I put this color light on my truck?”

A better question is:

“What function is this light performing, where is it mounted, which direction does it face, and how does it interact with the required lighting around it?”

What Is Supplemental Truck and Trailer Lighting?

Not every light installed on a commercial vehicle is required equipment.

Additional lights can fall into the category of supplemental or auxiliary lighting.

Supplemental lighting can be used for purposes such as improving vehicle visibility, defining portions of a vehicle, providing work illumination, or supplementing existing lighting functions.

The important federal principle is that additional lighting must not impair the effectiveness of required lighting equipment.

That gives fleets and vehicle builders room for additional lighting while preserving the familiar lighting language drivers depend on.

Can You Add Extra LED Lights to a Semi-Truck or Trailer?

Potentially, yes — but installation matters.

NHTSA has addressed supplemental vehicle lighting many times and has explained that additional lighting may be permissible when it does not impair the effectiveness of lighting required by FMVSS 108.

When evaluating auxiliary lighting, important considerations can include:

Color. Could the color confuse another driver about the vehicle's orientation or the meaning of a required signal?

Location. Is the light positioned in a place where it could be mistaken for required lighting?

Brightness. Is it so bright that it masks, overwhelms, or distracts from required lamps?

Activation. Is its operating pattern consistent with the function being communicated?

That means two installations using a similar lamp can potentially raise different considerations depending on where and how the lamp is installed.

What Does “Impair the Effectiveness” of Required Lighting Mean?

This phrase is central to understanding supplemental lighting under FMVSS 108.

Imagine looking at a trailer at night.

Its required lamps communicate where the trailer begins, where it ends, how wide it is, and whether it is stopping or turning.

Now imagine adding another lamp that is so bright, positioned so unusually, or displaying a color so inconsistent with its location that another driver has difficulty interpreting the required lights.

That is the type of concern the impairment principle is intended to address.

The goal of properly designed supplemental lighting should therefore be simple:

Add useful visibility without taking away from the information already being communicated by the vehicle's required lighting.

RoadLites Asked NHTSA Directly About Supplemental Outline Lighting

RoadLites didn't want to guess about how this framework applies to multi-directional outline lighting.

We asked NHTSA directly.

RoadLites submitted a request to NHTSA's Office of the Chief Counsel concerning a multi-directional supplemental lighting concept for commercial trailers.

NHTSA subsequently issued RoadLites a formal letter of interpretation explaining how FMVSS 108 applies to the configuration presented.

The agency evaluated the RoadLites concept as auxiliary or supplemental lighting and provided guidance concerning how color and mounting location interact with the required marker-light system.

That guidance helped RoadLites refine its approach to rear-mounted outline lighting and reinforced an important principle behind our product development:

Supplemental visibility should complement the established lighting system on the vehicle.

We've published a detailed explanation of that correspondence in our guide, “NHTSA Interpretation on Outline Marker Lights: What FMVSS 108 Means for Supplemental Truck & Trailer Lighting.”

What Are Outline Lights?

RoadLites uses the term outline lighting to describe supplemental lighting intended to help visually define portions of a commercial vehicle's shape, boundaries, or orientation.

Some RoadLites designs use multiple light-emitting faces within a single housing.

Depending on the product and mounting orientation, this can allow the light to be seen from more than one viewing direction.

This is different from simply replacing a conventional side marker lamp.

The purpose is to provide supplemental visual information about the vehicle's outline while working alongside the required lighting already installed on the truck or trailer.

Are Outline Lights the Same as Required Marker Lights?

Not necessarily.

This distinction is important when selecting a RoadLites product.

A supplemental outline light should not automatically be treated as a substitute for a required FMVSS 108 side marker, clearance, identification, or other required lamp.

Required lighting has defined functions and applicable performance requirements.

Supplemental outline lighting is intended to work alongside the required lighting system, unless a particular product is specifically designed and properly certified for a required lighting function.

When shopping for RoadLites products, check the stated function and intended application of the individual light rather than assuming every marker-style or outline light performs the same regulatory function.

Why Multi-Directional Lighting Is Different

Traditional vehicle lamps are generally designed around defined lighting functions and viewing directions.

Multi-directional lighting introduces another consideration.

One housing may present separate illuminated surfaces toward different viewing angles.

For example, depending on its design and installation, a lamp might be visible from the side as well as a front- or rear-quarter angle.

That creates an opportunity to improve the visual definition of a large vehicle, but it also makes color, placement, intensity, viewing direction, and interaction with required lamps especially important.

RoadLites believes this is an area where commercial vehicle lighting technology can continue to evolve.

Why More Visibility Isn't Simply About More Brightness

It's tempting to assume that the brightest possible light automatically creates the safest vehicle.

Vehicle lighting is more sophisticated than that.

A supplemental lamp that overwhelms a turn signal, stop lamp, marker lamp, or another required signal can actually make the vehicle's lighting pattern harder to interpret.

Effective commercial vehicle visibility is about clarity, not simply maximum brightness.

RoadLites focuses on lighting that can help a vehicle be seen while maintaining a clear relationship with the required lamps and signals around it.

What About State Truck-Lighting Laws?

Federal requirements are only part of the picture.

NHTSA has specifically noted that states may have their own laws applicable to vehicle lighting.

Commercial vehicles may also be subject to additional federal motor-carrier requirements depending on the vehicle and operation.

For fleets operating across multiple states, this is particularly important.

A lighting configuration should therefore be evaluated not only for the product itself, but also for the vehicle, installation, application, and jurisdictions where it will operate.

What Should Fleets Consider When Adding Supplemental LED Lighting?

Before adding supplemental lights to a truck or trailer, consider the installation as a complete system.

Ask:

• What is the intended function of this light?

• Is it replacing required equipment or supplementing it?

• Where will it be mounted?

• Which direction does it face?

• What color does it display?

• How bright is it relative to surrounding required lamps?

• Is it steady-burning or does it perform another signaling function?

• Could it obscure or be confused with required lighting?

• Does the installation comply with applicable state requirements?

• Are the vehicle's original required lamps still properly installed, visible, and functioning?

Those questions are much more useful than simply asking whether an LED light is “DOT.”

How RoadLites Approaches Commercial Vehicle Lighting

RoadLites is focused specifically on commercial vehicle visibility.

We work with lighting applications for semi-trucks, trailers, flatbeds, box trucks, emergency vehicles, fleets, and other commercial vehicles.

Our approach combines modern LED lighting with real-world trucking applications.

We also believe commercial-vehicle lighting companies should understand the regulatory environment surrounding the products they sell.

That's why RoadLites has gone directly to NHTSA for guidance rather than relying solely on assumptions about how newer supplemental lighting concepts fit within the existing federal framework.

For us, visibility is not simply about adding more LEDs.

It's about helping create a commercial vehicle that is easier to see, recognize, and understand.

Frequently Asked Questions About DOT and FMVSS 108 Truck Lighting

Are truck marker lights required by DOT?

Certain marker lamps are required on applicable commercial vehicles under federal vehicle-lighting requirements. The exact equipment depends on the vehicle's type, dimensions, and configuration.

What color are trailer marker lights?

Required side marker lighting generally follows the familiar pattern of amber toward the front and red toward the rear, with additional requirements applying to longer trailers and other configurations.

Are clearance lights and marker lights the same thing?

No. They perform different regulatory functions. Side marker lamps primarily help indicate vehicle presence and length from the side, while clearance lamps help indicate overall vehicle width.

Can I add extra marker lights to my trailer?

Supplemental lighting may be permissible, but the installation should not impair the effectiveness of required lighting. Color, location, brightness, activation pattern, and applicable state laws should all be considered.

Are LED lights allowed on commercial trucks?

LED technology itself is widely used in commercial vehicle lighting. Compliance depends on the function, performance, application, installation, and requirements applicable to the particular lamp — not simply whether the light source is LED.

Does NHTSA approve truck lights?

NHTSA does not generally operate a system in which it individually pre-approves every truck light before sale. Manufacturers are responsible for certification of equipment subject to applicable Federal Motor Vehicle Safety Standards.

What is supplemental vehicle lighting?

Supplemental or auxiliary lighting is additional lighting beyond the equipment required by the applicable federal standard. NHTSA generally evaluates such lighting based on whether it impairs the effectiveness of required lighting.

What is an outline light?

RoadLites uses “outline lighting” to describe supplemental lighting intended to improve the visual definition of portions of a commercial vehicle's boundaries or orientation. It should not automatically be considered a replacement for required marker or clearance lamps.

Building Better Commercial Vehicle Visibility

Truck and trailer lighting has one fundamental job: communicate clearly.

Required lamps establish a standardized visual language that helps road users recognize a vehicle's dimensions, orientation, and actions.

Supplemental lighting provides an opportunity to build on that foundation.

For RoadLites, the future of commercial vehicle lighting isn't about replacing the established system. It's about finding better ways to complement it.

That means combining durable LED technology, thoughtful placement, appropriate colors, real-world trucking experience, and a serious understanding of the regulations surrounding commercial vehicle lighting.

And as lighting technology evolves, RoadLites intends to keep pushing that conversation forward.

This guide is provided for general educational purposes and is not legal advice or a vehicle-specific compliance determination. Requirements can vary based on vehicle type, configuration, installation, use, and jurisdiction. Consult the applicable federal and state requirements for your specific application.