Lawrence D. Rosenblum

Lawrence D Rosenblum

Sensory Superpowers

Silent But Deadly

Accident data shows that the quietness of hybrid cars is a danger to pedestrians

Posted Oct 06, 2009

It’s a noisy world, and getting noisier. This is why there’s been such strong reaction to the news that a number of car companies and the U.S. Congress are discussing whether sounds should be added to quiet hybrid cars. But as a scientist studying the problem, I bring good news. We can have it both ways. Hybrid cars can stay quiet, and still provide enough sound to be safe for all of us.

The issue was first brought to public attention in 2005 by the National Federation of the Blind. The organization was concerned that the functional silence of hybrid cars (in their electric mode) poses a danger to visually impaired pedestrians — pedestrians reliant on sound to navigate intersections and parking lots.

Since then, the issue has been covered extensively in the press, and has spurred vigorous debate on the internet and elsewhere. But last week, a preliminary report from the National Highway Traffic Safety Administration (NHTSA) provided some sobering data. It turns out that during slow-speed maneuvers, hybrid cars are 50% more likely to collide with pedestrians than are internal combustion engine (ICE) cars. NHTSA suggests that these results are related to the sound level difference between hybrid and ICE cars moving at slow speeds.

These findings are consistent with research we’ve conducted in our lab. We've now conducted eight separate studies, using both recorded and live cars as stimuli. In all studies, we’ve found that listeners can determine the approach direction of slow moving combustion engine cars much sooner than they can hybrids, and often with greater accuracy. In fact, with the presence of normal background sounds (idling ICE cars), subjects are unable to make these judgments until the hybrid cars actually pass them.

NHTSA’s finding that hybrid cars are disproportionately more dangerous at slow speeds is no coincidence. It’s at slow speeds when hybrids are quietest. When moving slowly, many hybrids are often powered solely on their electric motor. And once moving faster than 20 mph, all cars produce enough tire and aerodynamic noise to be audible from a safe distance. Of course it’s at slow speeds that cars are closest to pedestrians, whether in parking lots or backing out of driveways. This is just where the greatest danger exists, according to the NHTSA report.

But another aspect of the NHTSA report is noteworthy. It’s likely that blindness has little to do with the incidence of hybrid-pedestrian collisions. While the NHTSA data does not include the visual status of the pedestrians, the same 50% increase in collisions occurred for bicyclists. While it’s true that some blind individuals do mountain bike (as I reported in an earlier blog entry), very few regularly ride in traffic. It’s safe to assume that most of the injured cyclists included in the NHTSA study were sighted.

This finding is consistent with a fact many of us have suspected all along: the quietness of slow moving hybrid cars is a danger to all of us—blind and sighted alike. Our auditory systems often work at an implicit level in warning of nearby dangers, allowing us to concentrate on more conscious tasks. Our ability to safely cross a parking lot while we talk to a friend, manage our children, or simply look for where we’ve parked, is aided by our implicit auditory warning system.

In fact, there’s evidence that our brains are exceedingly sensitive to approaching sounds. Research shows that when we hear a sound approach—vs. recede or remain stationary—brain regions associated with attention and motor action are quickly recruited. The auditory brain also possesses a disproportionately large number of cells sensitive to increasing sound loudness: one of the primary cues for perceiving approaching sounds. These brain findings jibe well with perceptual research showing that we consistently over-anticipate the location of approaching sounds. It’s likely that our auditory systems have been designed to use approaching sounds to avoid hazards. If there’s too little sound to effectively engage the system, as is the case with hybrids at low speeds, then any normal distraction becomes hazardous.

But our hyper-sensitivity to approaching sounds can also be part of the solution. It means that only a subtle enhancement of sound should be needed. Hybrids and electric cars won’t need to beep, chirp, or produce an alarm to be audible. Beeps and chirps are likely more distracting than they are perceptually useful. The enhancing sound, needed only at slow speeds, could be either the simulated sounds of a very quiet engine (think cooling fan), or of rolling tires. For purposes of both auditory utility and simple familiarity, the safest sounds are car sounds. And these sounds would be barely noticeable for most of us. Not much sound is needed for the auditory system to warn us about hazards, as long as it’s the right sound.

If you’ve not yet been surprised by the seemingly spontaneous appearance of a moving hybrid in a parking lot or driveway, you will be. Let’s hope that when this happens, neither you or the driver are talking to a friend or managing children. Better yet, let’s hope you’ll soon hear that hybrid make just enough quiet sound so that you’re not surprised at all.

Lawrence Rosenblum is a Professor of Psychology at the University of California, Riverside. He studies multimodal speech perception and general auditory perception. His book on our implicit perceptual skills, “See What I’m Saying: The Extraordinary Powers of Our Five Senses” ( will be published by Norton Press in March.


National Highway Traffic Safety Administration (2009). Incidence of Pedestrian and Bicyclist Crashes by Hybrid Electric Passenger Vehicles. Technical Report DOT HS 811 204.

Neuhoff, J. G. (2004). Auditory motion and localization. In J. G. Neuhoff (Ed.), Ecological Psychoacoustics (pp. 87–111). New York: Academic Press.

Robart, R. L., & Rosenblum, L. D. (2009). Are hybrid cars too quiet? Journal of the Acoustical Society of America, 125(4), 2744.

Seifritz, E., Neuhoff, J. G., Bilecen, D., Scheffler, D., Mustovic, H., Schächinger, H., Elefante, R., & Di Salle, F. (2002). Neural processing of auditory 'looming' in the human brain. Current Biology, 12, 2147–2151.

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