Are Trailers Required to Have Mud Flaps?

Big semi-truck driving down a road with a blue skyline on a bright sunny day

Whenever I drive on interstate highways, I always notice the unique mudflaps that different semi-trucks with trailers have. I think it’s cool that so many of them have unique designs for a product that’s otherwise boring and safety-related. As I drove from coast to coast, I started wondering if trailers are required to have mud flaps, or if the truckers just like how they look.

All semi-trucks are required to have mud flaps if they are driven in the 50 States. The height, location, and size of the mud flaps vary from state to state, and there’s actually a lot of variability in the laws. It’s a state-level law that regulates mud flaps, not a federal one. However, the trailer part of the rig doesn’t need mud flaps as long as a trailer is attached. There are also no explicit laws about the material or design of these mud flaps.

In this guide, I’ll get into the laws pertaining to trailers and mud flaps. By the end of this quick guide, you will have an exact answer with plenty of examples.

What Is a Trailer?

If you’re wondering about mud flaps on a trailer, you’re picturing one of two things, either a tractor-trailer or a pull-behind trailer.

A tractor-trailer is another term for an 18-wheeler or semi-truck with a trailer — one of those massive vehicles that carry commercial cargo across the country. I’m going to assume that most people are asking about this vehicle as opposed to pull-behind trailers (this assumption will make more sense later in the guide).

Big rig long haul semi truck with empty flat bed semi trailer driving on a road in winter with other tractor trailer semi-trucks

If you’re thinking about a pull-behind trailer or utility trailer, you’re envisioning an open cart-like thing that gets hooked up to your truck and driven around. People will usually load a utility trailer with lumber, contractor equipment, or ATVs.

Utility trailers usually have a back gate that flips down, letting you drive vehicles right onto the trailer or walk into it to load the trailer.

Defining Mud Flaps

Mud flaps are those rubber squares you’ll see behind tires. You’ll see them a lot on big pickup trucks and Jeeps — especially ones that like to get dirty and go off-roading.

Off-road vehicle car with a side view close up of the large wheels with mud flaps visible behind each wheel ready for dirt road trails

These get installed directly on the wheel of the vehicle, and they’ll appear behind the tires. The location is very important because a mud flap doesn’t do anything if it’s not correctly installed.

They also don’t necessarily need to be made out of rubber. I’ve seen ones that use hard plastics or textured diamond plate as well. The more important fact is that the mud flap appears behind the tire and is at least as wide as the tire.

The Purpose of Mud Flaps

The distinctions I just made in the previous section will start to make sense now. Mud flaps are there to stop flying debris from the tire.

If a rock gets picked up by a tire, it can get flung behind the vehicle. That rock can either damage the undercarriage of the vehicle, the bodywork, or the car behind the vehicle. That means that a mud flap protects your own vehicle as well as every vehicle behind you.

A semi-truck ready to attach a trailer to it 18-wheeler with no trailer and the white mud flaps visible in a parking lot

Another purpose of mud flaps is to prevent too much splashing. If a truck drives over a puddle or through a section of mud, its big tires will splash liquid all over the cars behind them. This will impair the vision of the driver behind them and can lead to a dangerous situation.

I’ve been behind a lifted dually truck before that didn’t have mud flaps, and my windshield got cracked from a pebble that was thrown from its tire. With mud flaps, this doesn’t happen.

It’s All About Physics

This might leave you scratching your head. Why don’t you ever see mud flaps on motorcycles or little sedans? It’s all about the physics of how the tire works.

Bigger tires have more force behind them. They also have larger treads that are more likely to get ahold of a pebble or road debris. Physics will cause that debris to travel with the tire until the debris comes to the top of the wheel.

Close up of a semi truck wheel with a mud flap to prevent water spewing on the vehicle behind it on a wet road while it's raining

From there, all the force wants to shoot the debris backward at full speed — exactly how a trebuchet (or think of a catapult) works.

So, this means that larger tires have more use for mud flaps than smaller ones. Since larger vehicles have larger tires, the math works out. It makes sense why a trailer might be required to have mud flaps.

Are Semi-Trucks Required to Have Mud Flaps?

The answer to this question is “it depends”. The law was actually surprising to me — there is no federal regulation that deals with the size, location, or inclusion of mud flaps on semi-trucks. It was shocking to me because these are one of a handful of vehicles that are designed to travel across the country.

Instead, the rules change from state to state. The best way to find an answer for your specific state is to look up the National Truck Equipment Association (NETA), Federal Motor Carrier Safety Administration (FMCSA), or Department of Transportation (DoT) regulations for your state. Yes, there are a lot of different groups that deal with mud flap requirements on trailers.

The good news that makes things simpler is that every single state requires semi-trucks to have mud flaps. The differences lie within the size and height of these mudflaps.

If you want to be safe, you can install mud flaps that meet the requirements of the strictest state. That way, you’ll pass inspection in all 50 states since certain ones are more lenient.

Black big rig semi-truck on an overpass interstate highway road over the Williamette River in Portland, Oregon

In four states (TX, AZ, DE, MO), your mud flaps can’t be more than 8 inches from the ground. It’s okay for the flaps to be less than that, meaning that they’re even closer to the ground. It’s against those states’ laws for a mud flap to be 9 inches or more from the ground.

Comparatively, other states like Maryland just care about the total length of the mud flap. There, they expect tractor-trailers to have mud flaps that are as long as the tires are wide (so 12-inch-long mud flaps on a 12-inch-wide tire).

This varying degree of leniency is why I think it’s best to just stick to the state laws of the most strict state when it comes to mud flaps.

What About Mud Flaps on the Tractor Tires?

The tractor (semi-truck) is the front piece of a tractor-trailer. It’s where the driver sits and where the engine is.

You’ll notice that tractors have 10 tires, but are they supposed to all have mud flaps? According to the Department of Transportation, these tires only need mud flaps if the vehicle doesn’t have a trailer attached to it.

Red big rig long haul tractor-trailer semi-truck transporting cargo side view driving down the road

If you just see the tractor driving on the road with no mudflaps and no cargo container behind it, that’s a violation. If it does have a cargo container, the tractor section does not need any mudflaps.

I think the reasoning comes from the distance between the tractor and any vehicles behind the tractor-trailer. A typical trailer is about 50 feet long. If the front tire of a tractor kicks up a rock, the motion will be absorbed and mitigated within the first 50 feet before it hits your car.

Do Pull-Behind Utility Trailers Need Mud Flaps?

If you came to this guide wondering about pull-behind utility trailers, this section is for you. It’s a good question since these trailers can be loaded with hundreds of pounds of product and the tires are largely unguarded. With all that extra weight on the trailer, driving through a puddle can cause a much larger splash than if a regular vehicle drove through.

As someone who owned a utility trailer for a long time, you don’t need any mud flaps. Some options on the market don’t even have wheel shrouds around the tire — they’re completely exposed.

A Toyota Corolla sedan with a pull-behind trailer and roof cargo rack near the sea and mountains at dawn sunset

The logic behind this has to do with the size of the tires. As I mentioned earlier, vehicles with big tires kick back more rocks, make bigger splashes, and are more dangerous for drivers behind them. With tractor-trailers, the tires can be multiple feet high.

With pull-behind trailers, the tires are within a foot or two. They’re significantly smaller than tractor-trailer tires and typically smaller than standard car tires.

With that being said, there’s not a huge risk associated with uncovered utility trailer tires. You don’t need to include mud flaps on them. From my research, there aren’t any states that require mud flaps on utility trailers on their roads (but you should double-check in your state to be sure).

Conclusion

Now you know more about tractor-trailers (semi-trucks with trailers) and why mud flaps are so important for them. Without them, vehicles behind the trailers can get debris and water spewing at them creating a hazard for someone behind a semi-truck.

If you want to know more about the rules of the road, check out the rest of my blog. I also have plenty of how-to guides and DIY content if you’re interested. As always, see what car products I highly recommend if you want to make your life a little easier.

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Ernest Martynyuk

An automotive enthusiast who's been tinkering with vehicles since I was 15-years old. Repairing automotive electronics has been my main job for over a decade now and have a passion for everything technical regarding cars.

2 thoughts on “Are Trailers Required to Have Mud Flaps?”

  1. Hey Ernest, thanks for the piece! The description is quite detailed. The trucking industry has so much regulation, that time-to-time I even lose motivation and think of quitting it. I am the owner of a small transportation company – we are helping people to move to another house. My colleagues accepted the policy of reducing carbon footprint and eliminating paper documentation from our business. But then we realized, that e-signature legislation is extremely different for each state and decided to hold this opportunity for a couple of years.

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    • Hey there, thanks for checking out the article. Between the influx of fuel prices and strict regulations especially when it comes to log books, it’s certainly a challenge to keep it going smoothly with a transportation company. I have a cousin who was in the car hauling business. He had a three-car trailer and experienced similar difficulties operating the business.

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