RIVERSIDE, CALIF.—People love their hybrid automobiles because, above all, they sip rather than guzzle now pricey gasoline. But as the owner of a standard minivan, I also covet a less touted benefit of electric engines: they are delightfully quiet.

So when I heard a graduate student say his group’s latest research suggests that hybrid gas-electric vehicles are too quiet, my ears perked up. He described his colleagues’ vision to equip these cars with sound-emitting devices to warn pedestrians. Were they planning to load a Toyota Prius with annoying beeps or the roar of a Maserati?

Certainly there seems to be some justification. In experiments led by perceptual psychologist Lawrence D. Rosenblum of the University of California, Riverside, blindfolded subjects who listened to recordings of cars approaching at five miles per hour could locate the familiar hum of a Honda Accord’s internal-combustion engine 36 feet away. But they failed to identify a Prius, running in electric mode, until it came within 11 feet—affording them less than two seconds to react before the vehicle reached their position. And that was in the absence of traffic noise or other distractions.

In a second trial, Rosenblum added some realistic background noise to the recordings. My own appreciation of the safety issue stepped up dramatically when I experienced this scenario for myself: the Prius glided past me, undetected, time and again. By the rules of the game, I was hybrid roadkill 40 times over—as were the majority of Rosenblum’s formal subjects. In contrast, I correctly determined the approach direction of the Accord in all 40 attempts, from an average of 22 feet away.

Whether these laboratory results translate into the real world is unclear, from a scientific point of view. No concrete evidence proves that hybrid cars are involved in more pedestrian accidents than their noisier counterparts. And recent studies from Western Michigan University indicate that hybrids and conventional vehicles are equally safe when traveling more than about 20 miles per hour, because tire and wind noise generate most of the audible cues at those speeds. Hybrids also tested safe when leaving a stoplight; all Prius models in the study engaged their internal-combustion engines when accelerating from a standstill.

But many groups are not awaiting scientific certainty—or a death toll—to take action. Last November the Society of Automotive Engineers created a special committee to examine whether hybrid cars should be made more audible for the sake of pedestrians, particularly the blind. In April congressional lawmakers introduced a bill designed to tackle the same question, and the National Highway Traffic Safety Administration held a public hearing on the topic in June.

Adding noise to hybrids may be inevitable, but at least it won’t have to be loud, Rosenblum says. That good news stems from the human brain’s extreme sensitivity to approaching sounds relative to those that are fixed or moving away. Because they are more likely to pose a threat, approaching sounds most readily stimulate regions of the brain associated with motor action, he explains. Also, a large number of brain cells are specially attuned to sounds that get louder—which usually means they are drawing near.

So a warning can be subtle—as long as it’s the right kind of sound. Chirps, beeps and alarms are more distracting than useful, say Rosenblum and Everett Meyer of Enhanced Vehicle Acoustics in Santa Clara, Calif.; the best sounds for alerting pedestrians would be carlike—akin to the soft purr of an engine or the slow roll of tires across pavement.

Even with sound-emitting safety measures in the works, Robert S. Wall Emerson of Western Michigan University predicts a future of more tranquil transportation. Several high-end (and not so high end) gas-powered motor vehicles are already quieter than hybrids, he says. Ironically, in Wall Emerson’s most recent studies, hybrid SUVs turned up noisier than many internal-combustion vehicles. Pedestrian safety, he points out, is not a hybrid issue. It’s a quiet car issue.

Note: This article was originally printed with the title, "Didn't Hear it Coming".