Stopped Commercial Motor Vehicles

• 392.22 Emergency signals; stopped commercial motor vehicles

(b) Placement of warning devices —

(1) General rule. … whenever a commercial motor vehicle is stopped upon the traveled portion or the shoulder of a highway for any cause other than necessary traffic stops, the driver shall, as soon as possible, but in any event within 10 minutes, place the warning devices required by § 393.95 of this subchapter…

Commercial motor vehicles typically carry warning triangles which are used to comply with this section. The regulations also set forth the exact placement of the devices. But, how do you handle compliance when the vehicle is autonomous, and there is no driver. Autonomous truck technology developer Aurora petitioned the Federal Motor Carrier Safety Administration (FMCSA) for a waiver exempting autonomous vehicles from this regulation. Aurora was seeking permission to use an alternative warning method such as a cab mounted warning beacon. FMCSA denied the waiver request saying that the “broad exemption is not supported by the data presented and lacks necessary monitoring controls to ensure highway safety.”

Aurora has filed a lawsuit in U.S. District Court appealing FMCSA’s denial. Aurora contends that alternative warning methods may actually be safer:

• Safe, accessible roadside warning systems have already been implemented outside of trucking. Emergency and construction vehicles use high-visibility flashing lights to alert other drivers when stopped on the roadside. After prototypes, data-backed research, and engagement with safety experts, Aurora proposed a similar solution for trucking. Why are we denying this safety tool for America’s truckers?
• Today’s roadside warning system is outdated. The current warning system to indicate when a truck is stopped on the side of the road – hand placement of warning triangles – is not only over half a century old, it has never been updated and is not backed by any data or research showing that it improves safety.

FMCSA Announces Study of Warning Devices

In early January 2025, FMCSA published a notice that they will begin a study of warning devices for stopped commercial motor vehicles. FMCSA feels this action is warranted because of compliance issues regarding automated driving system vehicles. At the same time, alternative types of warning devices developed by the industry, including those intended to increase driver safety during device deployment, have resulted in multiple applications for exemption from the corresponding safety regulations.

The question of whether the use of such devices improves traffic safety and, if so, how and to what extent is unresolved. Past attempts by the Federal Highway Administration (FHWA) and other researchers to answer those questions yielded generally inconclusive or inconsistent results. FMCSA itself has never conducted experimental research on the impact of using warning devices.

FMCSA feels there is a need to evaluate the effectiveness of warning devices under current regulations. In addition, advanced research instruments unavailable or not in use at the time of all past research on this topic are now in common use and would permit far more sophisticated analyses of the effects of warning devices on driver behavior. This includes sensors that can precisely measure and record the location of vehicles (e.g., differential GPS), eye-tracking devices that allow the researcher to determine the precise moment when a driver first glanced at a disabled CMV, and instrumented vehicles that record accurate, high-frequency data related to drivers’ interactions with a vehicle’s controls.

FMCSA plans to implement these modern tools in a controlled experiment at a closed-course, state-of-the-art driving research facility that will allow the most comprehensive examination of the effects of warning devices to date. The results of the study may support future rulemaking related to warning devices and provide baseline data necessary to inform Agency decisions on exemption applications for alternative warning device products.

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