FMVSS 124 – Accelerator Control Systems NPRM

The National Highway Traffic Safety Administration (NHTSA) have published a NPRM in the Federal Register which proposes changes to FMVSS 124 – Accelerator Control Systems.

This NPRM addresses 2 areas;

1) Electronic Throttle Controls (ETC)

The current disconnection requirements were originally written for the mechanical throttle systems that were available at the time of publication. Since then, ETC have become more dominant (in fact, only a few vehicles have mechanical systems in 2012). NHTSA have published a number of interpretations about how the standard is applied to ETC. This NPRM will incorporate these interpretations into the standard.

  • Manufacturers shall select which criteria is used for ‘return to idle’; fuel delivery rate, air intake rate, electric power delivery or creep speed.
  • Normal Operation:
    • When tested, the throttle shall return to idle within the speed specified in S5.3 (<1 second for vehicles <4536 kg)
    • Idle position is determined by completing a baseline test of the chosen criteria.
  • Fail-safe Operation:
    • Option 1: In the event of a disconnection in the accelerator control system, including electronic components, the throttle shall return to within 50% tolerance of idle within the time specified. The idle baseline is determined. Load is placed on the engine and an electronic component / connection is severed. It is verified if the vehicle returned to within 50% of the idle baseline condition.
    • Option 2: Maximum creep speed allowed is 50 km/h. The vehicle must continuously decelerate to a speed less than 50 km/h from any initial speed when any component is disconnected. The time required to coast to 50 km/h shall not exceed the time taken if the vehicle was in neutral position with no disconnected components.

2) Brake Throttle Override (BTO)

To address concerns of reported cases of unintended acceleration due to a number of factors (both driver error and vehicle based reasons like pedal interference), vehicles with a ETC will require a BTO system;

  • Equipment requirement:
    • Software and / or hardware that have the capability to identify and react to conflicts in the brake and accelerator pedal inputs.
    • When a conflict is identified, the BTO must substantially reduce the propulsive force to the driving wheels.
    • The BTO must stay engaged while the brake pedal is still applied and the accelerator pedal is in conflict.
    • A BTO override is permitted provided the driver is alerted and it returns to the on position at ignition cycle.
  • Performance Requirements:
    • Accelerate vehicle to selected speeds between 30 and 160 km/h.
    • With the accelerator pedal applied (25-100% of pedal travel), apply brakes and measure stopping distance.
    • Repeat 6 times.
    • Confirm that the stopping distance is within 5% of the FMVSS 135 requirements.
    • Only 1 of the 6 runs must be within the 5%, but all must identify the conflict.
Lead Time:
  • Disconnection requirements: 1st September after publication of the final rule – 01/09/2013 likely.
  • BTO requirements: 1st September 2 years after publication of the final rule – 01/09/2014 likely.

Comments are due in the docket (NHTSA-2012-0038) by 15th June 2012.


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