The proposed rule, released Thursday for public comment, is aimed at preventing runaway-car crashes like the one near San Diego in 2009 that killed an off-duty California Highway Patrol officer and his family. That event and an outpouring of complaints about unintended acceleration prompted Toyota to recall millions of vehicles in the last few years.

¡°America¡¯s drivers should feel confident that any time they get behind the wheel, they can easily maintain control of their vehicles ¡ª especially in the event of an emergency,¡± the transportation secretary, Ray LaHood, said in a statement. ¡°By updating our safety standards, we¡¯re helping give drivers peace of mind that their brakes will work even if the gas pedal is stuck down while the driver is trying to brake.¡±

Brake override systems, part of the software in a vehicle¡¯s on-board computer, cut the throttle when the brake pedal is pushed. The systems give the brakes precedence if a driver steps on both pedals simultaneously or if the accelerator is already depressed when the brake is applied.

Because most vehicles now have the systems, the National Highway Traffic Safety Administration said in its proposal that the cost of making them mandatory would be ¡°close to zero.¡±

The agency said the requirement would also have ¡°minimal¡± effect on vehicle design. But Jeremy Anwyl, vice chairman of the automotive information Web site, said override systems ¡°could impact everyday drivability and generate other sorts of customer complaints if not executed properly.¡±

Toyota made brake override systems standard across its full lineup after the recalls, and it also retrofitted some earlier models. General Motors has said all of its vehicles worldwide will have the feature by the end of this year, and the Alliance of Automobile Manufacturers, which represents most major carmakers, has publicly supported requiring brake override systems since May 2010.

¡°We are currently reviewing the N.H.T.S.A. notice of proposed rule-making in detail, but are proud that with the 2011 model year, Toyota was the first full-line automaker to make brake override systems standard across all model lines,¡± a Toyota spokesman, Brian R. Lyons, said in an e-mailed statement.

In some of the Toyota episodes, drivers said they had been unable to stop their cars when they pressed the brake pedal. Investigators believe the crash that killed the patrol officer, Mark Saylor, and his family occurred after the accelerator of the Lexus sedan they had borrowed from a dealership became caught under an unsecured, ill-fitting floor mat. A passenger told a 911 operator that the car had no brakes as it surged to 120 miles per hour.

Investigations of the Toyota episodes by the traffic safety agency and NASA found that in some cases, drivers were accidentally pressing the accelerator instead of the brake, which brake override systems would not address. The agencies found no evidence of any defects in the electronic throttle control systems.

The proposal cites ¡°sticky pedals¡± and floor mat interference, the problems that Toyota addressed in its recalls, as situations in which brake override systems could prevent crashes. In December, the safety agency proposed standardizing keyless, push-button ignition systems to help drivers turn a vehicle off if they were not able to control it.

The agency is accepting public comments on the proposal for 60 days, after which it is expected to release a final rule.

The requirement would take effect no sooner than Sept. 1, 2014. As proposed, it would exclude vehicles weighing more than 10,000 pounds. The agency said it would consider exempting vehicles with a manual transmission because a clutch pedal can be used to counter a stuck accelerator.