CARB Compliance for Trailers: Side Skirts, Undertray Devices, and the Law

If a trailer will ever run in California, its aerodynamic equipment isn't optional trim — it's a compliance requirement under state law, separate from anything the EPA requires federally. This is one of the more commonly misunderstood specs on a used-trailer listing, so it's worth understanding what the rule actually says, who it applies to, and how to tell a compliant device from one that just looks like it.

The Law: California's Tractor-Trailer GHG Regulation

The requirement comes from CARB's Tractor-Trailer Greenhouse Gas (TTGHG) Regulation, adopted in 2008 and phased in starting in 2010. It applies to owners and drivers of 53-foot-or-longer dry van and refrigerated box-type trailers operated on a highway within California, regardless of where the trailer or the fleet that owns it is based — if the trailer runs into or through the state, the rule can reach it.

The regulation requires two things: SmartWay-verified (or CARB-approved) aerodynamic devices on the trailer, and SmartWay-verified low rolling resistance (LRR) tires on both the tractor and the trailer. The aerodynamic devices have to deliver a minimum fuel-savings benefit — 5% for dry vans, 4% for refrigerated vans — measured by EPA's SmartWay verification testing.

Who Needs to Comply, and When

The original compliance deadline was January 1, 2013 for most fleets, though large fleets running 21 or more trailers could file a phase-in compliance plan with CARB by July 1, 2011 to spread compliance out through 2015–2016. Those dates are long past now, which means any 53-foot dry van or reefer that's supposed to be running in California today should already have compliant equipment — the open question for a buyer is usually whether a specific used trailer actually does, not whether the rule still applies.

There are real exemptions, though, and they're easy to miss. A trailer operated only by a registered local-haul tractor (one that stays within a limited radius and mileage under CARB's local-haul definition) is exempt from the aerodynamic device requirement — but not from the low rolling resistance tire requirement. Short-term exemptions also exist for narrow situations like relocating a storage trailer or a one-time ownership transfer. Flatbeds, tankers, dry bulk, and other non-box trailers fall outside the aerodynamic device requirement entirely, since the rule targets box-type dry van and refrigerated trailers specifically.

How Compliance Is Verified

A device only counts toward compliance if it's actually on CARB's executive order list or EPA's SmartWay verified list — an aftermarket skirt or fairing that looks the part but was never submitted for testing doesn't satisfy the regulation, no matter how much drag it actually removes. Verified devices carry a manufacturer's label or plate referencing their SmartWay ID or CARB executive order number, which is what an inspector checks against.

Enforcement happens at border crossings, weigh stations, truck stops, fleet yards, and random roadside inspections. CARB and CHP can and do check for the required equipment, and a trailer found out of compliance can be cited and, in some cases, held out of service until it's fixed.

Side Skirts vs. Undertray (UT1-Type) Devices

EPA's SmartWay program treats aerodynamic devices as two separate categories, and a trailer generally gets credit for one or the other — not both stacked together for extra savings.

Side skirts (also called trailer skirts or side fairings) mount along the underside of the trailer between the landing gear and the rear wheels, and work by blocking and smoothing the turbulent air that would otherwise churn around the wheels and undercarriage. They're the most common device on the road today, largely because they're cheap, simple, and have been on the market the longest.

Undertray systems — the category SmartTruck's UT1 belongs to — mount to the belly of the trailer instead of the sides, and work by channeling airflow along a shaped underside rather than blocking it. Because they don't wrap the wheel area the way skirts do, manufacturers often market them as easier and faster to install, lighter, and better for keeping airflow moving around the brakes and tires. Functionally, though, they're evaluated against the same SmartWay fuel-savings tiers as skirts — Elite (9%+), then descending tiers from roughly 5–8.9%, 4–4.9%, and 1–3.9% — and a device from either category can satisfy the trailer's compliance requirement as long as it clears the applicable minimum.

For a buyer, the practical takeaway is the same either way: what matters for compliance is that the specific device installed is the one that was actually verified, not just that the trailer has "a skirt" or "an undertray."

What This Means When You're Buying or Selling

If you're buying a used 53-foot dry van or reefer for use in California, don't assume compliance from a photo — ask the seller which specific device is installed and look for the manufacturer's SmartWay or CARB executive order plate on the device itself. If you're selling into that market, having that documentation ready (or noting it clearly in the listing) is one of the fastest ways to answer a serious buyer's first question.

Read our guide to trailer specs, including doors, suspension, and cargo control →

This Isn't Legal Advice

This article is a general summary for buyers and sellers, not legal or regulatory advice, and CARB's rules, exemptions, and enforcement practices can change. If you're unsure whether a specific trailer, device, or use case is compliant, confirm directly with CARB or a qualified transportation compliance professional before making a decision based on anything here.