Automakers were not exactly helpless to respond. The ill-fated Solargen Electric Motor Car Company was already selling electric-conversions of the AMC Concord which were "remarkably silent," as the Syracuse Post-Standard put it, "the only noticeable sound being an electrical 'whine' intentionally engineered into the design to warn pedestrians during acceleration of up to 18 mph." The following year, the equally ill-fated Amectran Exar-1 boasted "a noise generator that emits a harmonic tone at speeds under 30 mph." In Pennsylvania, when a Gettysburg-area power company used modified electric Dodge Omnis, Chevrolet Citations, and Volkswagen Rabbits, "a noise generator was added to alert pedestrians."
Despite these efforts, and years of complaints by the National Federation of the Blind—Honda was aware enough of the problem to file a 1994 patent for an EV noise-generator—automakers could not or would not hear the problem creeping up behind them. The complaints became harder to ignore when, presaging the NHTSA collision findings the next year, studies in 2008 from UC-Riverside and from Western Michigan University showed electric vehicles are hard to hear at low speeds.
The response of the industry was clumsy. Many, including Honda and high-end manufacturer Tesla Motors, doggedly continued to manufacture hybrid and electric cars that ignored the issue. One motive for Tesla becomes apparent when you read their 2011 SEC filings: The safety feature "could negatively impact consumer interest in our vehicles." Nissan Leafs made a half-hearted effort by installing a grating boop-beep sound—but featured a mute button, something the new law wouldn't allow. Toyota and Hyundai have been more proactive: This 2010 Japanese video shows Toyota tinkering with the Jetsons-style sound that is now standard on 2012 Priuses.
The most likely sound of the future, though, may be the sound of the past. Advocates for the blind have long asked for sounds that mimic other cars, and a recent NHTSA study (pdf) shows that simulated conventional engine noises can effectively warn pedestrians at lower overall volumes than conventional vehicles. Audi's new R8 eTron sound, for instance, emits a familiar growl. Whichever standard emerges, by 2017 new hybrids and electric cars will need federally mandated noisemakers installed—and, in a little-reported catch in the law's language, by then the NHTSA may already be moving on extending the law to all cars that run too quietly, no matter what kind of engine they use.
But that still leaves a fleet of over 1 million quiet cars already on the road. This includes the new "deep space quiet" Focus Electric: Ford says it won't install warning units on the ones now shipping out. "We just don't want to be too hasty," one executive informed the Autotrader website.
Considering that they've now had about a century to solve this problem, perhaps they can avoid haste with a tried-and-true solution: Might we suggest the sound of sleigh bells?