Electric Vehicles & Loud Pipes

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Based on sales trends and environmental necessity, electric vehicles (EVs) are the future. But one concern of increasing EV use is the potential effects on pedestrian safety. Several studies and common sense suggest that a near-silent EV poses a risk to pedestrians who are used to judging traffic by sound. This has led to requirements that EVs generate some form of pedestrian warning noise when operating at low speeds where wind noise is negligible. In the US, under the Pedestrian Safety Enhancement Act of 2010, all hybrid and EVs traveling under 18.6 mph (30 km/h) will be required to generate a vehicle-in-motion noise by the year 2020.

The basis for the US “quiet car rule” was a 2009 National Highway Traffic Safety Administration (NHTSA) report that found a 40% increase in pedestrian accident rates for hybrid EVs over conventional vehicles—mostly due to low-speed accidents. However, hybrids were still relatively rare at that time and the sample included only 77 pedestrian accidents involving hybrids.

A larger sixteen-state follow-up NHTSA study in 2011 found the accident rate was 35% higher for hybrids than conventional vehicles. Again, the increase was due to low-speed accidents. The increase in the absolute accident rate between conventional cars (0.57%) and hybrids (0.77%) was only 0.2%. However, over many cars and many years, that is a lot of preventable accidents. This seems like pretty damning evidence. Except it’s not.

The primary issue with these studies is that they don’t rule out an important factor—geographical setting. The studies collected data from entire states and sorted data by a variety of factors, but did not differentiate whether the accident occurred in a rural or urban setting. Hybrids (especially plug-in hybrids) and EVs are commonly purchased by drivers who do mostly suburban and urban driving where the mileage benefits are the greatest, trips are shorter, clean cars are eligible for HOV lanes, and political affinities tend to be more environmentally focused. Guess what else is more common in urban areas—pedestrians! So if quiet clean cars are involved in low-speed accidents with pedestrians at a higher rate than conventional cars, it’s hard to say that the data isn’t just measuring that clean cars are being used mostly in cities.

This is a common research problem. When a seemingly logical hypothesis is supported by data, we tend to accept the results without skepticism.  It’s a well-known issue called confirmation bias. So, if the data is less conclusive than the NHTSA studies suggest, then another larger study that also accounts for urban use is warranted. However, many hybrid and EVs are already using pedestrian alert systems, so newer data may not resolve the quiet car safety question. Another approach is needed.

One possibility is to look at a similar long-running debate in the world of motorcycles, commonly known as the “loud pipes save lives” defense, to see if it sheds light on the EV issue.

Do Loud Pipes Save Lives?

The loudness of motorcycles varies—plush touring models are generally quieter than sport models. However, some motorcycle manufacturers are known for being louder than others. For example, Harley-Davidson motorcycles have a reputation for being loud right off the factory floor and are easily modified with straight exhaust pipes to be even louder. Conversely, similar Honda and BMW models are known for being comparatively quiet.

An analysis of the almost 10,000 motorcycle accidents in New York (a large state with accessible data) in the years 2014 and 2015 is shown in Figure 1. The graph shows the ten most common motorcycle brands in New York with the addition of an electric motorcycle brand (to make a point later). These brands accounts for over 85% of all motorcycles in the state. The graph shows a few interesting statistics. First, while Harley-Davidson motorcycles get into the most accidents, this is only because they are the most common. When the number of Harley accidents is divided by the number of Harleys registered in the state, we see that Harley-Davidson has a below-average rate of accidents compared to the other popular manufacturers. It’s also interesting that there is such variation. Some motorcycle brands seem to get into accidents at twice the rate of others. Considering that Harleys have the second lowest rate of accidents, riders who claim that loud pipes save lives seem to have statistics to back up their assertion.

Figure 1: Comparison of total accidents and accident rates for common brands of motorcycles in NY

Unfortunately, it’s never quite that simple. If loud pipes do save lives, then there are two problems with the graph. First, the “safest” motorcycle brand is BMW which is known for being rather quiet compared to Harleys. Second, although there were less than 100 Zero brand electric motorcycles registered in NY, it would appear that a near silent motorcycle is still “safer” than four other major brands of motorcycles which are clearly louder. So, perhaps there are other factors that are more important than loudness.

One obvious explanation is that rider demographics and style of motorcycle are the primary factors affecting safety. Riders of motorcycles with lower than average accident rates (left side of the graph) are more likely to be older and riding expensive cruisers and touring motorcycles, while the brands with higher than average accident rates have more young riders who prefer sport motorcycles. This isn’t surprising. A 2012 Insurance Institute for Highway Safety report noted that sport motorcycles account for less than 10% of all registered motorcycles in the US, but more than 25% of all fatalities. In the end, rider behavior—mostly speed—is the most important safety factor.

Further deflating the argument for loud pipes is the fact that 40% of motorcyclist deaths in 2016 were single-vehicle crashes. We can assume that loud pipes would have no benefit in those cases. Likewise, according to NHTSA’s National Center for Statistics and Analysis, roughly one-third of motorcycle rider fatalities are associated with rider excessive blood alcohol content.

Another interesting comparison is between motorcycle accidents and bicycle accidents. Bicycles are both quieter and less visible than motorcycles. Coupled with much looser rider restrictions (no licensing requirements or age limitations), bicycles should be far more dangerous. Yet, bicycles appear to be safer than motorcycles. The actual accident rate is hard to determine because no one tracks the total number of bicycles being used. However, New York City has been tracking bicycle ridership and estimates that about 1.6 million people in NYC bicycle at least once a year, 1 million bike once a month, and about 450,000 people bike daily. By comparison, there were 362,227 motorcycles registered in the entire state of New York in 2016. Depending on which bicycle ridership numbers you use, bicycles are either slightly safer or multiple times safer. Again, considering the number of irresponsible bicyclists out there, the only plausible reason that the motorcycle accident fatality rate is up to three times higher than for bicyclists is speed, not loudness.

The unsatisfying lesson here is that the best way to travel safely is to travel slowly. A loud motorcycle is probably harming the rider’s hearing more than any potential safety benefit from the loud exhaust.

Better Options

Returning to the EV quiet car rule, if engine noise were that useful of a safety feature, pedestrians should rarely be surprised by combustion engine vehicles—and yet they frequently are. According to data from the US National Center for Injury Prevention and Control and the NHTSA, there were 185,775 hospital-treated pedestrian injuries and 5,987 pedestrian deaths from motor vehicles in 2016. Making EVs as loud as combustion engines does two things. First, it merely maintains the rather poor status quo of pedestrian-vehicle accidents. Second, it artificially maintains noise pollution in cities, one of the most intractable of urban problems that EV adoption is supposed to solve.

Lawmakers are looking at safety from the wrong direction. We want to protect pedestrians, particularly vision-limited pedestrians, but the best answer is not the technological equivalent of a cow bell on EVs. Although more expensive, available automatic emergency braking systems that intercede when a pedestrian collision may occur would be far more effective at preventing pedestrian accidents. This technology is likely to become required safety equipment in the near future anyway—much like air bags or electronic stability control did in the past.

Motorcyclists and lawmakers shouldn’t feel bad for believing the loud pipes theory. It’s seems logical, but closer inspection shows that noise generation is an ineffective solution to the real dangers of fast moving vehicles. The fear of silent electric vehicles is reminiscent of the laws enacted at the dawn of the automotive age. Accustomed to slow moving horses, fear of automobiles led lawmakers to require that a person walk in front of an automobile waving a red flag when passing through a town. Such laws were eventually abandoned for more practical regulation.

As quiet EVs become more common, pedestrians will again become accustomed to the technological change. In the meantime, it might seem prudent to use a standardized low-speed pedestrian alert system until behaviors adjust and automatic braking becomes universal—a kind of belt and suspenders safety approach. However, the US quiet car rule doesn’t require a standardized sound, so pedestrians may now have to recognize a variety of sounds as potential warnings. Only time will tell if this is effective.

Unfortunately, adopted technologies can sometimes become cultural norms that are hard to replace even when better options arise (like the century-old QWERTY keyboard). So even if pedestrian alert systems prove to be ineffective, it may be difficult to get rid of them. Perhaps Tesla, which already includes automatic emergency braking as a standard safety feature, has been wise to resist the pedestrian alert noise.

Consumers and city residents will need to let manufacturers and lawmakers know that they prefer automatic braking systems to EV cow bells. Hopefully, as automated safety features become standard, the lack of safety value in noisy vehicles will become more evident. Then our future cities will be cleaner, safer, and more peaceful.