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Hybrids and electric cars don't produce much powertrain noise running in electric-only mode.
The smooth, near-silent ride is a perk for drivers and passengers, but some regulators believe it could constitute a safety issue for pedestrians.
That led Congress to pass a law in 2010 ordering the National Highway Traffic Safety Administration (NHTSA) to issue regulations requiring "quiet cars" to make more noise.
The deadline for those rules was January 3, 2014, but the agency is only just now beginning to finalize them.
Those rules are finally coming close to being enacted, Secretary of Transportation Anthony Foxx said at an event celebrating the 25th anniversary of the passage of the Americans with Disabilities Act.
The rules will be finalized by November, NHTSA Administrator Mark Rosekind said in a subsequent interview with The Detroit News.
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Rosekind said the delays were due to the need to conduct additional research following proposed rules announced in 2013.
However, he did not say how the final rules will differ from that previous proposal.
The rules proposed in January 2013 would apply to all electric cars, hybrids, and other vehicles that don't emit powertrain noises.
ALSO SEE: Laws To Add Noise To Silent Electric Cars: Latest Updates (Jan 2014)
Regulators believe minimal noise levels are necessary to alert pedestrians--particularly those who are visually impaired--to the presence of a vehicle.
Last July, the NHTSA said it didn't expect to finalize its rules until April 2015--a deadline it also missed.
Around the same time two industry trade groups--the Alliance of Automobile Manufacturers and the Association of Global Automakers--asked regulators to waive a phase-in period proposed for 2016, and require full compliance by September 2018.
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Since the NHTSA missed the original January 2014 deadline, the carmakers argued that they wouldn't have enough to time to add noisemakers before 2018.
They also claimed the proposed regulations would require sounds that were too loud--exceeding the noise of high-performance sports cars, even.
MORE: Will Europe Too Write Rules To Make Quiet Electric Cars Noisier? (Nov 2013)
The 2013 proposal called for sounds that could be heard over background noises while a vehicle is traveling at low speeds--up to 18.6 mph, the NHTSA has said.
It would also allow carmakers significant leeway in choosing sounds, although certain minimal requirements will have to be met.
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Each vehicle of the same make and model would also reportedly have to emit the same sound or sounds.
The NHTSA expects compliance to cost $23 million in the first year, and about $35 per car.
It believes the potential safety benefits will make the addition of noisemakers worthwhile.
Agency estimates claim the odds of a hybrid being involved in a collision with a pedestrian are 19 percent higher than a non-hybrid car, and 38 percent higher in the case of a car-bicycle collision.