Hybrid caution is approaching

Do hybrid cars have to be louder to be safe for pedestrians?

by Sarah Simpson

RIVERSIDE, CA - People love their hybrid cars mainly because they don't guzzle a lot of gas, which has become so expensive these days. But I, with my regular minivan, gravitate to another feature of electric motors that gets the least publicity: it's just a pleasure how quiet they are.

That is why I was very interested in the words of a student who said that the latest study conducted in his research group showed that hybrid vehicles that combine gasoline and electricity are too quiet. He said his colleagues would like these cars to be equipped with devices that would make a sound to warn pedestrians. Do they plan for the Toyota Prius to emit annoying beeps or the thunderous roar of a sports car from the maker of Maserati?

There is no doubt that there is some justification for this. The expert in perceptual psychology, Lawrence D. Rosenbaum of the University of California, Riverside, conducted experiments with blindfolded people who listened to recordings of cars approaching at a speed of 8 km/h. The subjects were able to identify the familiar roar of the internal combustion engine of the Honda Accord from a distance of 11 meters. But they failed to hear a Prius hybrid, which was driving in electric mode, until it was only 3.5 meters away. They therefore have less than two seconds left to react before the vehicle reaches the place where they were standing. And this without the noise of traffic on the road and other distractions.

In another experiment, Rosenbaum added some realistic background noise to the recordings. My awareness of the safety issue increased greatly when I tried this experiment myself: the Prius passed me, time after time, without my noticing it. According to Rosenbaum's rules of the game, the hybrid car ran over me 40 times, as well as most of the official test subjects. On the other hand, I successfully identified the direction from which the Accord car emerged in all 40 attempts, from an average distance of 7 meters.

From a scientific point of view, it is not clear whether these laboratory results are realized in reality. There is no solid evidence that hybrid cars are involved in more pedestrian accidents than their noisy counterparts. Recent studies conducted at Western Michigan University show that at speeds above 32 km/h, hybrid and regular vehicles are equally safe, because the tires and the wind produce most of the sounds that reach our ears at these speeds. The tests also showed that the hybrids are safe even after stopping: all Prius models that participated in the study activated their internal combustion engines when accelerating from rest.

However, many groups do not wait for scientific certainty - or a blood price - to spring into action. In November 2007, the American Society of Automotive Engineers established a special committee to examine whether it is necessary to make the hybrid cars louder for the benefit of pedestrians, especially the blind. In April 2008, lawmakers in Congress passed a bill designed to address the same problem, and the US National Highway Traffic Safety Administration held a public hearing on the issue in June.

Adding noise to hybrids may be an inevitable step, but at least that noise doesn't have to be loud, Rosenbaum says. The good news is that the human brain is very sensitive to approaching sounds compared to sounds from a fixed distance or receding. Approaching sounds are a more likely candidate for threat, so they easily stimulate areas of the brain associated with motor activity, he explains. Also, a large number of brain cells are specifically tuned to sounds that get louder, because such an increase usually indicates that the source of the sound is getting closer.

The warning can therefore be subtle, as long as it is the right kind of sound. Chirps, beeps and alarms are more distracting than helpful, say Rosenbaum and Everett Meyer of Enhanced Vehicle Acoustics in Santa Clara, California. The best sounds to warn pedestrians are sounds that sound like a car: similar to the soft rumble of an engine or the slow rolling of tires on the pavement.

Even with the development of noise-emitting safety measures, Robert S. Wall Emerson of Western Michigan University predicts a future of quieter transportation. According to him, already some of the most modern fuel-driven vehicles (and even the least modern ones) are quieter than hybrid vehicles. Ironically, in Wall Emerson's most recent studies, hybrid SUVs were noisier than many internal combustion engine vehicles. Pedestrian safety, he says, isn't just about hybrid cars. It is related to quiet cars in general.

Sound experiment

By the end of the summer of 2008, the company Enhanced Vehicle Acoustics from Santa Clara, California, hopes to present an add-on that can be installed in hybrid vehicles to break their silence: speakers placed behind the tires will produce a sound warning in the direction of pedestrians. Although the sound is still being tested, the inventors say it is not a chirping, beeping or whistling sound but a quieter sound than an internal combustion engine that will change its frequency when the car accelerates or decelerates, and will stop when the car goes faster than 32 km/h. According to them, the price for the consumer in the USA will be 395 dollars, and the installation will take about half an hour.