Paul Collins / Slate

Forget gas mileage: The most striking aspect of the new Ford Focus Electric is what it doesn't have. “Battery-powered cars are intrinsically quiet, the motor sound falling between a whir and a whisper,” marvels a New York Times review of the car. “But the Focus is deep-space silent.”

And that, it turns out, is a problem. Thanks to the Pedestrian Safety Act of 2010, by this summer the National Highway Traffic and Safety Administration is required to initiate a rulemaking process for minimal vehicle noise — not how quiet, but how loud a car must be. That's because NHTSA studies in 2009 and 2011 confirmed what many long suspected: Hybrids and electric cars are too quiet for some people to hear them coming.

Though the NHTSA found little statistically significant difference in collisions over 35 mph — when wind and tire noise negate the difference in engine noise — at lower speeds, hybrids and electric vehicles are 37 percent more likely to hit walkers and 66 percent more likely to collide with cyclists than traditional gas-powered cars.

The victims didn't hear it coming — but did automakers? As counterintuitive as adding car noise may seem, we've long had comparable safety laws in place: For instance, we add foul-smelling Butanethiol to natural gas so that it doesn't sneak up on us in our homes.

But there's an even more apt comparison: sleigh bells. Back in the age of real horsepower, the jingling of bells had little to do with winter cheer, and plenty to do with not getting trampled to death. As early as 1797, Baltimore slapped $1 dollar fines on anyone who didn't make their sleighs noisy enough. Other cities followed suit, and even the future Motor City had a tough sleigh-bell ordinance that could land silent sleigh drivers in jail.

Despite 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 NHTSC 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 its 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 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?