What Are Truck Side Lights and Their Purpose?

When a truck or trailer cuts across your path at an angle—merging, turning, or backing into a dock—its headlights and taillights point away from you. Without side marker lamps, you’d have almost no way to judge how long the vehicle is or how fast it’s moving until it’s already in your lane. These lights exist to close that gap in visibility.

In the United States, side marker lamps aren’t optional accessories. Federal Motor Vehicle Safety Standard (FMVSS) 108 governs their design, color, and placement on everything from pickup trucks to 53-foot trailers. The standard’s stated purpose: reduce crashes by giving drivers better visual information about a vehicle’s length and position in darkness or reduced visibility.

What the Federal Rules Require

FMVSS 108, codified at 49 CFR 571.108, defines a side marker lamp as a lamp that shows to the side of the vehicle, mounted as near as practicable to the front and rear edges. The lamp’s job is to indicate the vehicle’s overall length. For longer vehicles, extra lamps may sit at intermediate points along the side.

The color rules are strict and easy to spot on the road: front side marker lamps glow amber, rear ones glow red, and intermediate lamps on longer vehicles are amber. Passenger cars manufactured after January 1, 1970, had to include side marker lamps under the standard’s requirements.

Trailers follow the same logic. Each trailer needs two amber side markers mounted as far forward as practicable and two red markers as far rearward as practicable. Trailers 30 feet or longer add amber intermediate side marker lamps and reflex reflectors near the midpoint. All side marker lamps must sit at least 15 inches above the road surface.

Side Markers vs. Clearance Lamps

A common mix-up: treating side marker lamps and clearance lamps as the same thing. They answer different questions. Clearance lamps—the ones atop the cab and at the outer edges of a trailer—mark the vehicle’s width and height. Side marker lamps mark its length when viewed from the side.

Both fall under FMVSS 108, and both use the same amber-front, red-rear color scheme on trailers. But if you’re replacing a broken lamp, the distinction matters: a clearance lamp is not a legal substitute for a side marker, and vice versa.

One more thing the rules make clear: you can’t add extra lighting that degrades the performance of the required equipment. Aftermarket lights that wash out or obscure your side markers could put you out of compliance.

How the Standards Are Tested

Manufacturers don’t just choose a bulb and call it a day. Side marker lamps are tested against SAE J592, the industry standard formally known as “Sidemarker Lamps for Use on Road Vehicles Less than 2032 mm in Overall Width.” The standard lays out test procedures, photometric requirements, and performance guidelines.

The 2016 revision superseded the March 2009 version. If you buy a lamp that claims SAE J592 compliance, it should meet the current spec—worth confirming if you’re replacing a unit on a commercial trailer that gets inspected.

When replacing lamps, matching the original specs matters for more than just passing an inspection. A lamp that’s too dim or aimed wrong defeats the whole safety purpose: helping other drivers judge your length and position.

Side markers on trucks and trailers have one job—make the vehicle’s footprint obvious from the side—but they do it through precise rules on color, position, and brightness. Knowing what those rules are helps you keep your rig legal and visible. If you’re in the market for replacements, our roundup of the best truck side lights on the market covers tested options that meet these standards.

FAQs

Do all trucks need side marker lights?

Yes, if the truck is driven on U.S. roads. FMVSS 108 requires side marker lamps on passenger cars, trucks, and trailers manufactured for highway use. The specific requirements—color, position, and whether intermediate lamps are needed—depend on the vehicle’s length and type.

Are amber side lights required on the front of a trailer?

Yes. Trailers must have two amber side marker lamps mounted as far forward as practicable on each side, and two red lamps as far rearward as practicable. Trailers 30 feet or longer must also have amber intermediate lamps and reflex reflectors near the midpoint.

Can you legally remove side marker lights from a truck?

No. Removing or disabling required side marker lamps puts the vehicle out of compliance with FMVSS 108. The standard applies to replacement equipment as well, so any new lamp must meet the same federal requirements as the original.