FMVSS 127: Automatic emergency braking explained.

What FMVSS 127 says it is for

The eCFR heads this section "Standard No. 127; Automatic emergency braking systems for light vehicles." Under 49 CFR part 571, the Federal Motor Vehicle Safety Standards are the numbered standards that set safety performance requirements for motor vehicles and equipment. The text below is quoted from 571.127 as current through October 29, 2026.

This standard establishes performance requirements for automatic emergency braking (AEB) systems for light vehicles.

The purpose of this standard is to reduce the number of deaths and injuries that result from crashes in which drivers do not apply the brakes or fail to apply sufficient braking power to avoid or mitigate a crash.

What it applies to

The application section of the standard says:

This standard applies to passenger cars and to multipurpose passenger vehicles, trucks, and buses with a gross vehicle weight rating (GVWR) of 4,536 kilograms (10,000 pounds) or less.

Applicability is decided by the text of the standard as written, including any exclusions it lists, and by the date-specific rules in it. This page does not decide whether a particular vehicle or item is covered.

How the standard is organized

The eCFR text of 571.127 has these top-level sections:

• S1: Scope
• S2: Purpose
• S3: Application
• S4: Definitions
• S5: Requirements
• S6: Test conditions
• S7: Testing when approaching a lead vehicle
• S8: Testing when approaching a pedestrian
• S9: False AEB activation
• S10: Subject vehicle brake application procedure

In all, the text has 97 numbered headings. Use the finder above to search them; for example, a search for "test" lists the headings that mention tests.

Key requirements, quoted from the text

These are excerpts from the requirements sections of 571.127, in the order the standard presents them. Long paragraphs are shortened where marked; the eCFR has the complete text, tables and figures, which this page omits.

S5 Requirements. (a) Except as provided in S5(b), vehicles manufactured on or after September 1, 2029 must meet the requirements of this standard. (b) The requirements of S5(a) do not apply to small-volume manufacturers, final-stage manufacturers, and alterers until one year after the dates specified in S5(a).

S5.1.1 Forward collision warning. A vehicle is required to have a forward collision warning system, as defined in S4 that provides an auditory and visual signal to the driver of an impending collision with a lead vehicle. The system must operate under the conditions specified in S6 when traveling at any forward speed that is greater than 10 km/h (6.2 mph) and less than 145 km/h (90.1 mph). (a) Auditory signal. (1) The auditory signal must have a high fundamental frequency of at least 800 Hz. (2) The auditory signal must have a tempo in the range of 6-12 pulses per second and a duty cycle in the range of 0.25-0.95. [excerpt continues in the eCFR]

S5.2.1 Forward collision warning. A vehicle is required to have a forward collision warning system, as defined in S4, that provides an auditory and visual signal to the driver of an impending collision with a pedestrian. The system must operate under the conditions specified in S6 when the vehicle is traveling at any forward speed that is greater than 10 km/h (6.2 mph) and less than 73 km/h (45.3 mph). The forward collision warning system must meet the auditory signal and visual signal requirements specified in S5.1.1.

S5.3 False activation. The vehicle must not automatically apply braking that results in peak additional deceleration that exceeds what manual braking would produce by 0.25 g or greater, when tested using the procedures in S9 under the conditions specified in S6.

S6.1.1 Temperature. The ambient temperature is any temperature between 0 °C and 40 °C.

S6.2.1 Test Track surface and construction. The tests are conducted on a dry, uniform, solid-paved surface. Surfaces with debris, irregularities, or undulations, such as loose pavement, large cracks, or dips may not be used.

S6.3.1 Malfunction notification. Testing is not conducted while the AEB malfunction telltale specified in S5.4 is illuminated.

S7.1 Setup. (a) The testing area is set up in accordance with figure 2 to this section. (b) Testing is conducted during daylight. (c) For reference, table 1 to S7.1 specifies the subject vehicle speed (VSV), lead vehicle speed (VLV), headway, and lead vehicle deceleration for each test that may be conducted. (d) The intended travel path of the vehicle is a straight line toward the lead vehicle from the location corresponding to a headway of L0.

S7.2 Headway calculation. For each test run conducted under S7.3 and S7.4, the headway (L0), in meters, providing 5.0 seconds time to collision (TTC) is calculated. L0 is determined with the following equation where VSV is the speed of the subject vehicle in m/s and VLV is the speed of the lead vehicle in m/s:

S7.3.1 Test parameters. (a) For testing with no subject vehicle manual brake application, the subject vehicle test speed is any speed between 10 km/h and 80 km/h, and the lead vehicle speed is 0 km/h. (b) For testing with manual brake application of the subject vehicle, the subject vehicle test speed is any speed between 70 km/h and 100 km/h, and the lead vehicle speed is 0 km/h.

S7.4.1 Test parameters. (a) For testing with no subject vehicle manual brake application, the subject vehicle test speed is any speed between 40 km/h and 80 km/h, and the lead vehicle speed is 20 km/h. (b) For testing with manual brake application of the subject vehicle, the subject vehicle test speed is any speed between 70 km/h and 100 km/h, and the lead vehicle speed is 20 km/h.

S7.5.1 Test parameters. (a) The subject vehicle test speed is 50 km/h or 80 km/h, and the lead vehicle speed is identical to the subject vehicle test speed. (b) [Reserved]

S8.1.1 General. (a) For reference, table 2 to S8.1.1 specifies the pedestrian test mannequin direction of travel, overlap, obstruction condition and speed (VP), the subject vehicle speed (VSV), and the lighting condition for each test that may be conducted. (b) The intended travel path of the vehicle is a straight line originating at the location corresponding to a headway of L0. (c) If the road surface is marked with a single or double lane line, the intended travel path is parallel to and 1.8 m from the inside of the closest line. If the road surface is marked with two lane lines bordering the lane, the intended travel path is centered between the two lines. [excerpt continues in the eCFR]

S8.2 Headway calculation. For each test run conducted under S8.3, S8.4, and S8.5, the headway (L0), in meters, providing 4.0 seconds time to collision (TTC) is calculated. L0 is determined with the following equation where VSV is the speed of the subject vehicle in m/s and VP-y is the component of speed of the pedestrian test mannequin in m/s in the direction of the intended travel path:

S8.3.1 Test parameters and setup (unobstructed from right). (a) The testing area is set up in accordance with figure 3 to this section. (b) Testing is conducted in the daylight or darkness conditions, except that testing with the pedestrian at the 25 percent overlap is only conducted in daylight conditions. (c) Testing is conducted using the adult pedestrian test mannequin. (d) The movement of the pedestrian test mannequin is perpendicular to the subject vehicle's intended travel path.

S8.4.1 Test parameters and setup. (a) The testing area is set up in accordance with figure 6 to this section. (b) Testing is conducted in the daylight or darkness conditions. (c) Testing is conducted using the adult pedestrian test mannequin. (d) The pedestrian mannequin is set up at the 25 percent right overlap position facing away from the approaching vehicle.

S8.5.1 Test parameters and setup. (a) The testing area is set up in accordance with figure 7 to this section. (b) Testing is conducted in the daylight or darkness conditions. (c) Testing is conducted using the adult pedestrian test mannequin. (d) The movement of the pedestrian test mannequin is parallel to and in the same direction as the subject vehicle.

S9.1 Headway calculation. For each test run to be conducted under S9.2 and S9.3, the headway (L0, L2.1, L1.1), in meters, providing 5.0 seconds, 2.1 seconds, and 1.1 seconds time to collision (TTC) is calculated. L0, L2.1, and L1.1 are determined with the following equation where VSV is the speed of the subject vehicle in m/s:

S9.2.1 Test parameters and setup. (a) The testing area is set up in accordance with figure 8 to this section. (b) The steel trench plate is secured flat on the test surface so that its longest side is parallel to the vehicle's intended travel path and horizontally centered on the vehicle's intended travel path. (c) The subject vehicle test speed is 80 km/h. (d) Testing is conducted with manual brake application and without manual brake application.

S9.3.1 Test parameters and setup. (a) The testing area is set up in accordance with figure 9 to this section. (b) Two vehicle test devices are secured in a stationary position parallel to one another with a lateral distance of 4.5 m ±0.1 m between the vehicles' closest front wheels. The centerline between the two vehicles is parallel to the intended travel path. (c) The subject vehicle test speed is 80 km/h. (d) Testing is conducted with manual brake application and without manual brake application.

S10.3 Displacement feedback procedure. For displacement feedback, the commanded brake pedal position is the brake pedal position that results in a mean deceleration of 0.4 g in the absence of AEB system activation. (a) The mean deceleration is the deceleration over the time from the brake pedal achieving the commanded position to 250 ms before the vehicle comes to a stop. (b) The pedal displacement controller displaces the brake pedal at a rate of 254 mm/s ±25.4 mm/s to the commanded brake pedal position. (c) The pedal displacement controller may overshoot the commanded position by any amount up to 20 percent. If such an overshoot occurs, it is corrected within 250 ms from when the commanded position is first achieved. [excerpt continues in the eCFR]

S10.4 Hybrid brake pedal feedback procedure. For hybrid brake pedal feedback, the commanded brake pedal application is the brake pedal position and a subsequent commanded brake pedal force that results in a mean deceleration of 0.4 g in the absence of AEB system activation. (a) The mean deceleration is the deceleration over the time from the brake pedal achieving the commanded position to 250 ms before the vehicle comes to a stop. (b) The hybrid controller displaces the brake pedal at a rate of 254 mm/s ±25.4 mm/s to the commanded pedal position. (c) The hybrid controller may overshoot the commanded position by any amount up to 20 percent. If such an overshoot occurs, it is corrected within 250 ms from then the commanded position is first achieved. [excerpt continues in the eCFR]

S10.5 Force feedback procedure. For force feedback, the commanded brake pedal application is the brake pedal force that results in a mean deceleration of 0.4 g in the absence of AEB system activation. (a) The mean deceleration is the deceleration over the time from when the commanded brake pedal force is first achieved to 250 ms before the vehicle comes to a stop. (b) The force controller achieves the commanded brake pedal force within 250 ms. The application rate is unrestricted. (c) The force controller may overshoot the commanded force by any amount up to 20 percent. If such an overshoot occurs, it is corrected within 250 ms from when the commanded force is first achieved. [excerpt continues in the eCFR]

Test conditions and procedures as stated

These excerpts come from headings of 571.127 that cover test conditions, procedures or similar subjects.

S10.3 Displacement feedback procedure. For displacement feedback, the commanded brake pedal position is the brake pedal position that results in a mean deceleration of 0.4 g in the absence of AEB system activation. (a) The mean deceleration is the deceleration over the time from the brake pedal achieving the commanded position to 250 ms before the vehicle comes to a stop. (b) The pedal displacement controller displaces the brake pedal at a rate of 254 mm/s ±25.4 mm/s to the commanded brake pedal position. (c) The pedal displacement controller may overshoot the commanded position by any amount up to 20 percent. If such an overshoot occurs, it is corrected within 250 ms from when the commanded position is first achieved. (d) The achieved brake pedal position is any position within 10 percent of the commanded position from 250 ms after the commanded brake pedal position is first achieved to the end of the test.

Defined terms

The standard defines 16 terms in its definitions section. Definitions are quoted from the eCFR.

• Adaptive cruise control system is an automatic speed control system that allows the equipped vehicle to follow a lead vehicle at a pre-selected gap by controlling the engine, power train, and service brakes.
• Ambient illumination is the illumination as measured at the test surface, not including any illumination provided by the subject vehicle.
• Automatic emergency braking (AEB) system is a system that detects an imminent collision with vehicles, objects, and road users in or near the path of a vehicle and automatically controls the vehicle's service brakes to avoid or mitigate the collision.
• Brake pedal application onset is when 11 N of force has been applied to the brake pedal.
• Forward collision warning is an auditory and visual warning provided to the vehicle operator by the AEB system that is designed to induce immediate forward crash avoidance response by the vehicle operator.
• Forward collision warning onset is the first moment in time when a forward collision warning is provided.
• Headway is the distance between the subject vehicle's frontmost plane normal to its centerline and as applicable: the vehicle test device's rearmost plane normal to its centerline; a parallel contact plane (to the subject vehicle's frontmost plane) on the pedestrian test mannequin; and the leading edge of the steel trench plate.
• Lead vehicle is a vehicle test device facing the same direction and preceding a subject vehicle within the same travel lane.
• Lead vehicle braking onset is the point at which the lead vehicle achieves a deceleration of 0.05 g due to brake application.
• Pedestrian test mannequin is a device used during AEB testing, when approaching pedestrians, meeting the specifications of subpart B of 49 CFR part 596.
• Small-volume manufacturer means an original vehicle manufacturer that produces or assembles fewer than 5,000 vehicles annually for sale in the United States.
• Steel trench plate is a rectangular steel plate often used in road construction to temporarily cover sections of pavement unsafe to drive over directly.
• Subject vehicle is the vehicle under examination for compliance with this standard.
• Travel path is the path projected onto the road surface of a point located at the intersection of the subject vehicle's frontmost vertical plane and longitudinal vertical center plane, as the subject vehicle travels forward.
• Subject vehicle braking onset is the point at which the subject vehicle achieves a deceleration of 0.15 g due to the automatic control of the service brakes.
• Vehicle test device is a device meeting the specifications set forth in subpart C of 49 CFR part 596.

Limits of this page

This page quotes the text of 49 CFR 571.127 as published in the eCFR, current through October 29, 2026. It omits tables, figures, cross-referenced test procedures in other parts, and the preambles and rulemaking history that explain the rule. It does not tell you whether a given vehicle, part or product complies, how to run a test, or what a manufacturer must certify. Compliance questions go to the manufacturer's engineers, qualified counsel or NHTSA. The eCFR is an editorial compilation, not the official legal edition of the Federal Register.

Sources

Sources opened and read November 2, 2026.

• eCFR: 49 CFR 571.127, Standard No. 127; Automatic emergency braking systems for light vehicles. (current through October 29, 2026) (read November 2, 2026)
• eCFR: 49 CFR part 571, Federal Motor Vehicle Safety Standards (current through October 29, 2026) (read November 2, 2026)