NHTSA Proposes Amending Federal Brake Standards for Autonomous Vehicles

On June 26, the National Highway Traffic Safety Administration published another proposed rulemaking designed to promote the deployment of autonomous vehicles on public roads.  The proposal would amend Federal Motor Vehicle Safety Standard (FMVSS) No. 135, “Light vehicle brake systems,” by removing requirements for hand- and foot-operated brake controls in vehicles that operate without a human driver, while retaining stopping distance performance requirements.  This rulemaking represents a meaningful—if still incremental—step in modernizing the federal regulatory framework for autonomous vehicles, particularly those that are manufactured without the standard equipment designed for human drivers.

Background

FMVSS No. 135 is a key safety standard that governs the performance of braking systems.  As with various other FMVSS, the braking standard presents challenges for autonomous vehicles.  FMVSS No. 135 was originally published in 1995, under the assumption that each vehicle would have a human driver.  Accordingly, the standard requires service brakes to be activated by a foot pedal, and the parking brake to be controlled by either a hand or foot control.  For ADS-equipped vehicles designed never to be operated by a human driver, these requirements present a regulatory barrier—and, in some cases, a potential safety risk if a passenger were to inadvertently use a brake pedal that overrides the ADS.

Recent Rulemaking Activity on FMVSS and Autonomous Vehicles

This proposal is the latest in a series of NHTSA rulemakings to address barriers in existing safety standards for autonomous vehicles.  In 2022, NHTSA finalized updates to 13 crashworthiness FMVSS, including the addition of a definition for “manually operated driving controls” that now serves as the linchpin for the current generation of modernization efforts.

Earlier this year, NHTSA proposed amendments to FMVSS No. 102 (transmission shift position sequence), FMVSS Nos. 103 and 104 (windshield defrosting, defogging, washing, and wiping systems), and FMVSS No. 110 (tire placards), each carving out exceptions for vehicles without manually operated driving controls.  The new FMVSS No. 135 proposal follows the same pattern—and addresses one of the more comprehensive standards in the FMVSS catalog.

The New Braking Proposal

The proposed rulemaking would make three categories of changes to FMVSS No. 135—some focused on ADS-equipped vehicles, others addressing more general issues with the existing standard:

• Brake controls and test specifications to account for ADS-operated vehicles. The proposal would amend the standard to provide that the requirements for a foot-operated service brake pedal and a hand- or foot-operated parking brake control apply only to vehicles with manually operated driving controls.  For ADS-equipped vehicles without such controls, the service brake and parking brake would instead be required to be activated by on-board systems—precluding reliance on external command signals.  The proposal would also add a definition of “service brake control” tailored to vehicles without a brake pedal (such as a linear actuator that translates electronic signals into mechanical braking input) and would adjust test procedures to use manufacturer specifications for brake control input in such vehicles in lieu of pedal force specifications.  Importantly, NHTSA is not proposing any changes to stopping distance requirements; all vehicles subject to FMVSS No. 135 would still need to demonstrate the same braking performance, regardless of how the brakes are activated.
• Telltales and indicators. The proposal would rename the “brake system warning indicator” to “brake system warning telltale,” consistent with terminology in FMVSS No. 101.  More substantively, it would require that the telltale be visible to occupants in all designated seating positions in vehicles without manually operated driving controls—rather than requiring it to be visible only from the driver’s seat.  NHTSA tentatively concluded that passengers in ADS-operated vehicles need access to this safety-critical information to make informed decisions about whether and how to use the vehicle.  The proposal would also except ADS-operated vehicles from the requirement to activate the brake warning telltale when the parking brake is applied, on the theory that the ADS monitors parking brake status as part of normal operations.
• Parking brake requirements. The proposal would remove the existing requirement that the parking brake be of a “friction type.”  NHTSA concluded that this requirement—carried over from a predecessor standard dating to an era when parking brakes needed to double as emergency brakes—is unnecessarily restrictive.  Other mechanisms, such as mechanical drivetrain locks, can satisfy the functional purpose of the parking brake.  The proposal would retain the existing requirement that the parking brake system use solely mechanical means to retain engagement (thus precluding fully electric systems that could fail upon loss of power) and continue to require that the system hold the vehicle stationary for five minutes on a 20% grade.

In addition to these changes, the proposal would make a number of housekeeping amendments: removing testing provisions that predate the adoption of anti-lock brake systems, correcting longstanding typographical errors in brake temperature and pedal force specifications, and making various plain-language edits for clarity.

What May Be Next

The FMVSS No. 135 proposal is more substantial than the earlier rulemakings targeting FMVSS Nos. 102, 103, 104, and 110, but it does not resolve all open questions about braking in autonomous vehicles.  In a footnote that likely followed substantial internal discussion, NHTSA indicated that it “is taking no position at this time as to how a passenger should be able to direct an ADS-operated vehicle to stop, or how the ADS should respond to such direction.”  The agency indicated it will address this issue through its defect authority in the near term and through future ADS performance standards over the longer term.

The FMVSS No. 135 proposal is also distinct from—and does not substitute for—a future standard specifically governing ADS braking decisions.  NHTSA acknowledged that its proposal does not address “whether the vehicle’s brakes are actuated appropriately given a specific driving scenario.”

And still more rulemakings may be on the way to adjust existing FMVSS to account for automated vehicles.  One FMVSS update included in the Department of Transportation’s Spring 2025 Unified Agenda—an amendment to FMVSS No. 108 covering lamps and reflective devices—has yet to be proposed.  Other standards, such as FMVSS No. 111 (rear view mirrors and backup cameras) and FMVSS No. 101 (illuminated indicators and hand-operated controls), also remain candidates for future modernization.

Immediate Actions and Takeaways

• The public comment period closes on July 27. Stakeholders with interests in autonomous vehicle design, deployment, or safety should submit comments and ensure their perspectives are in the administrative record.
• Manufacturers seeking to deploy ADS-equipped vehicles without manually operated driving controls should note that, until a final rule takes effect, the existing requirements of FMVSS No. 135 continue to apply. The NHTSA exemption process remains available in the interim; NHTSA has previously announced steps to expand the availability and streamline the process for seeking exemptions.
• The proposal’s removal of the friction-type parking brake requirement has implications beyond the ADS context. Conventional vehicle and equipment manufacturers that have sought greater design flexibility in parking brake systems may benefit if the change is finalized.
• NHTSA has yet to propose substantive performance standards for automated driving systems. In press releases and public statements, the agency has indicated those standards are under consideration.
• As NHTSA notes, its regulatory authority over autonomous vehicles goes beyond the FMVSS. Many investigations and recalls arise from safety-related defects rather than FMVSS noncompliance, and NHTSA has signaled it will use its defect authority actively as autonomous vehicles become more prevalent.
• State and local regulatory frameworks remain a parallel concern. Even as NHTSA modernizes federal standards, states retain considerable authority to regulate autonomous vehicle operations, and some states continue to impose significant restrictions.  Meanwhile, Congress continues to consider federal legislation focusing on autonomous vehicles.

This post is as of the posting date stated above. Sidley Austin LLP assumes no duty to update this post or post about any subsequent developments having a bearing on this post.