Cars

Published on June 5th, 2016 | by Michael Barnard

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Will Electric Cars Make Traffic Quieter? Yes & No.

June 5th, 2016 by  


12_noisy_traffic_14There’s a lot of discussion and publication about electric cars being quieter. Some of it is focused on low-speed pedestrian and cyclist safety. Some of it is focused on reductions in traffic noise as it pertains to the health of people living near roads. But how much quieter will traffic really be due to electrification of transportation? The answer is interesting and complex, as almost everything related to noise and perceptions of noise is.

Basic traffic noise is a combination of engine noise, tire noise, road noise (different surfaces have different noise characteristics), and wind passage noise. The faster vehicles are traveling, the more road and tire noise dominate, and the more wind passage noise contributes.

Electric cars almost entirely eliminate engine noise, and the relatively high-pitched noise electric motors do emit doesn’t propagate as far. They currently tend to have lower coefficients of drag, reducing wind passage noise at higher speeds, although this is likely to diminish as an advantage as batteries get cheaper and more power and it’s easier just to push air out of the way for stylistic “advantages.” However, these contributions are enough lower than road and tire noise that the impacts will likely not be noticeable to most people most of the time. Electric car tires currently tend to be quieter than other tires but that too will likely diminish as an advantage, and tires overall have become much quieter in the past few decades. There is hope, however, so keep reading.

What’s important is the breakdown of contribution of different elements of noise from vehicles. Most publications and analyses focus on one aspect or another, but not comparisons. That said, there are a couple of sources available for consideration.

This graph from a mid-2000s publication out of Purdue and the University of Central Florida is the most coherent resource I have been able to find on the subject.

Screen Shot 2016-06-02 at 3.59.57 PM

For contrast, this Netherlands report on efforts related to diminishing traffic noise is also useful to look at.

Screen Shot 2016-06-02 at 3.59.16 PM

Note that, in one case, tire and road noise are separated, and in the other, they are merged. This is a result of different methodologies and approaches. There are lower and higher noise tires and lower and higher noise roads.

In general, these are broadband, dBA (weighted to human hearing) noise levels. As a result, they tend to be additive. However, the decibel scale is logarithmic, which means the apparently high level of 60 dBA at 50 kph in the first chart is only about half as loud apparently as the ~68 dBA for tire noise.

Further confusing things, humans can only perceive a difference in noise of 3–5 dBA. The variance between the tire noise contribution and the overall noise is under 3 decibels per the Purdue source. That means that, even at 110 kph (slow highway speed), eliminating the contribution of both engine and wind passage noise might not be a discernible difference to someone standing by the road.

However, there are four other factors at play which will make a difference to greater and lesser extents.

The discussion so far pertains to basic noise with well-tuned mufflers. It doesn’t account for the spiking noise of broken mufflers or intentionally noisy internal combustion vehicles such as Harley Davidsons, sports cars, and coal-rolling pickup trucks. These spike sounds can be very disruptive and much more annoying than basic traffic sounds. Electric vehicles don’t have mufflers, so there’s nothing to break there. And electric vehicles are rapidly taking over as performance vehicles due to their excellent torque characteristics. As electric vehicles begin to dominate, broken or intentionally noisy mufflers will substantially diminish. Noise pollution will be about as socially acceptable as diesel smoke emissions in the future, and about as necessary.

Squealing and chirping of tires will diminish. Electric motors are so much more controllable at all RPMs that traction control is much better. And torque jolts with shifting don’t exist in single-gear sports cars, so occasional chirps shifting to second or third disappear. Watch any “Tesla vs embarrassed sports car” drag race and you’ll notice the complete lack of smoke and tire spinning from the Teslas. They just roll up to the line and disappear into the distance without drama. This won’t help with painted pavement in parking garages of course, but that’s a relatively narrow problem.

aLJBbInternal combustion engine noise and wind passage noise have strong low-frequency sound components (as does tire/road noise). Low-frequency sound travels further than higher frequency sounds with less attenuation as lower frequencies are absorbed less by atmospheric humidity (the most relevant factor for traffic noise). Eliminating the engine noise contribution to low-frequency sounds and diminishing the wind passage sound might very well shift the balance of road noise to reduce the distance it propagates somewhat. This is speculation on my part, however, based on my knowledge of acoustics, and relatively irrelevant compared to road and tire contributions regardless.

Tire and road noise have been strong focus areas for decades in terms of engineering reductions to traffic noise, for the simple reason that they are the largest contributors. As a result, there are much-less-noisy road surfaces mandated in many areas and tires have reached a bit of a plateau of quietness on the average vehicle. Basically, almost all traffic has become much quieter with the exception of intentionally noisy or otherwise broken vehicles in the past decades.

By some estimates, improvements in materials, construction, contours and, above all, tread patterns, have made today’s passenger cars less than 20% as noisy as those produced 30 years ago. 

Commercial trucks have improved even more dramatically, producing less than 10% of the noise of their 1980s predecessors.

The references to 10% and 20% trigger the question as to whether that is absolute-sound-pressure level or perceived-sound-pressure level. It’s likely absolute, and given the logarithmic nature of decibel scales and how people perceive noise, that likely means that tires sound about half as noisy, not one-tenth as noisy.

So, what does this net out to? Well, basic traffic noise from highways won’t get perceptibly quieter due to electric cars. Urban road traffic noise below 40 kph likely will, as engine noise starts to dominate at lower speeds. And we can all look forward to fewer and fewer noisy motorcycles and trucks, and no economy cars with broken mufflers.

Here are some related references for those wanting to go deeper:






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About the Author

works with startups, existing businesses and investors to identify opportunities for significant bottom line growth in the transforming low-carbon economy. He regularly publishes analyses of low-carbon technology and policy in sites including Newsweek, Slate, Forbes, Huffington Post, Quartz, CleanTechnica and RenewEconomy, with some of his work included in textbooks. Third-party articles on his analyses and interviews have been published in dozens of news sites globally and have reached #1 on Reddit Science. Much of his work originates on Quora.com, where Mike has been a Top Writer annually since 2012. He's available for consultation, speaking engagements and Board positions.



  • Brent Jatko

    You might be able to hear drivers cussing each other out in traffic jams….

  • Ronald Brakels

    A lot of traffic noise comes from internal combustion engine cars idling at traffic lights, and EVs will eliminate that. Except for stereo noise.

    • JamesWimberley

      Yes. I should have added that the Rio flat is near a bus stop followed by an intersection. So the buses and other vehicles have to stop, then accelerate.

  • Carl Raymond S

    In Central Business Districts, where there are lots of people to hear the noise, the large noisy vehicles are predominately diesel buses. Electrify the buses alone and the reduction in noise levels will be dramatic. Complete the job by slapping a hefty congestion tax on anything with an exhaust pipe entering the CBD – peace in our time.

  • eveee

    Its not highway tire and wind noise that dominates our everyday experience.

    Sound pressure levels are proportional to inverse square law with distance. Because of this, our neighbors or the vehicles close to us when we walk, bike, or travel are the loudest by far. Most frequent loud noises are the garbage trucks and other delivery vehicles, motorcycles, performance cars, and ones with modified exhausts. These are a frequent experience in urban and suburban settings. Added to the fact that cities are more dense, and thus people are closer to sources, the din of low speed auto traffic is deafening.
    The noise from high speed auto traffic is usually deadened by sound walls and most do not live close, but an occasional loud car, motorcycle, or truck pierces the silence of even the deepest night.

    http://hyperphysics.phy-astr.gsu.edu/hbase/acoustic/invsqs.html

    In terms of our everyday most frequent experience, electric vehicles will definitely lower noise especially at home and at city intersections while walking, biking, or driving.

    • GearsOfWoe

      Totally agree. When traffic is moving smoothly, it is probably pink, maybe white noise that is far less intrusive than the short loud bursts of engines. While waiting at a pedestrian crossing, I was pleasantly surprised by how quiet the nearby cars were. At least until the start/stop system fired up the engines. Weirdly, I find that more jarring than an ordinary idling engine.

      • CU

        But it is a good warning sound that something near you starts to move. Think about all the blind persons next to you.

        • All too true.

        • eveee

          I think there are some of those legislated.

      • eveee

        At intersections where pedestrians cross, an EV world would be eerily quiet. I could enjoy that. The average intersection is not a place for calm conversation.

  • Hank1946

    They are quiet enough that you will be able to hear each and every vulgar word yelled at you by the moron in the car behind you while standing still and moving down the road! Or even the one in your passenger seat.

    • Brent Jatko

      You might be able to hear drivers cussing each other out in traffic jams….as I typed above before scrolling down to read your comment

  • neroden

    Meanwhile, the US government is trying to require that electric cars create fake traffic noise in order to make more noise pollution.

    • Yeah, personally I think that they should be requiring all vehicles to be as quiet as electric cars at slow speeds. Noise pollution is still pollution.

      • Wayne Williamson

        many new ice cars are very quite. You can still hear the tire/road noise just like an electric. Agree with you, it’s just silly to make more noise….

    • Snarkiness notwithstanding, I’d have to assume that pedestrian safety is the impetus behind the proposed EV minimum sound requirements. I’d bet, or at least hope, you were taught as I was in driver training decades ago that, “…after a bouncing ball comes a running child…”

      Perhaps a free market approach would be more palatable, where PIP (personal injury protection) insurance for a super quiet EV would be prohibitively more than such insurance for an EV equipped with a low speed warning.

      This *may* all be moot, depending upon how quickly advanced collision avoidance / automated braking technologies diffuse into the marketplace. An EV that automatically avoids collisions with inattentive people of whatever age would, i’m guessing, not have to announce it’s presence.

      • Freddy D

        Some sound from the vehicle enables the pedestrian to better detect the vehicle.

      • John Terrence

        >> An EV that automatically avoids collisions with inattentive people of
        whatever age would, i’m guessing, not have to announce it’s presence.

        It’s hard to see how this doesn’t increase risk versus announcing it – which makes it hard to be favored by regulators.

        But i’m curious if it’s possible to add some sort of more positional speakers to the car, and track pedestrians, and just emit warning sounds towards them, as needed. Or at least make the sound closer to the ground, as not to reach higher floors in houses.

  • JamesWimberley

    My wife has a flat in Rio on a main road. The high noise level is mainly down to the large number of diesel buses. I am sceptical of Mike’s claim that commercial vehicles are ceasing to be a noise problem.

    • m2cts

      Diesel busses need to be replaced -at least in the US – after about 12 years. Not by law, but because they wear out. That means that fleet operators have to refresh their fleets constantly, and that creates openings for alt fuel and electric busses. Change is happening.

      • JamesWimberley

        The Rio buses look fairly new to me, maybe 10 years old on average, certainly all are far more recent than 1980. The mayor, Eduardo Paes, struts the stage at green city conferences, but SFIK there is no bus switch yet planned.

  • phineasjw

    In my neighborhood all I hear is VRRRRRROOOOOOOOMMMM, as my next door neighbor starts his Toyota pickup, or VROOOOOOM as car after car accelerates down my small side-street.

    So, EVs probably won’t be much quieter on a highway, but they’ll be silent in local neighborhoods.

    • Omega Centauri

      A point I was going to make. Noise in neighborhoods where people live, is much more engine noise than tire/wind noise. The periodic noise when the traffic light switches to green and everyone steps on the pedal, that will be nearly gone. Also your neighbor warming up his car/truck/motorcycle in his driveway.

      Of course highway noise can be quite noticeable a half mile away, and that won’t be greatly reduced.

      • eveee

        Not to mention the occasional smells like gasoline, oil, and catalytic converter startup exhaust.

        The door slams will be the same, but all the engine noise is there.

        Good point focusing on neighbors, because they are the closest and loudest. For those living near a hot rodded vehicle or motorcycle, its a daily inconvenience. Worse, sometimes nightly and at all odd hours ruining rest and relaxation or sleep.

        • Brent Jatko

          Our condo complex in Houston once featured a resident who bought a 1970s Dodge Demon with a modified 340 engine and little or no exhaust silencing. He has since been evicted for an unrelated legal issue.

      • Freddy D

        Have you ever heard a red light turn green in Rome? It’s a symphony and blue cloud of motorcycles!

    • CU

      And buses in city traffic – even a hybrid bus makes all the difference.

  • jonesey

    The problem with this analysis, which is acknowledged above, is that it assumes modern, functioning ICE cars, which are pretty quiet already. Just as with exhaust, it is the 2% of vehicles that are extremely noisy or release extreme amounts of exhaust that are more than 95% of the problem. Unfortunately, those are big trucks, old vehicles, and modified vehicles, and those will probably be among the last to be fixed.

    • Freddy D

      True points. For example Harleys are a style statement, including the classic potato-potato sound.

      • jonesey

        As some people have noted below, the article above really applies only to highway noise. The graph only goes down to 50 kph (30 mph). Vehicle noise that invades homes and workplaces will typically come from vehicles operating at below that speed. At low speeds, engine idling noise is the dominant factor. Obnoxious engine noises are typically in the low-frequency range, which means they penetrate walls and windows — think of the neighbor uselessly and harmfully “warming up” his vehicle in the driveway next to your bedroom window, or the bus or garbage truck diesel engine noisily driving by. Those noises will be gone with EVs, which is what this or another article should analyze.

        • Bob_Wallace

          Google has started making its autonomous car ‘horn smart’.

          Softer ‘toot-toot’ if it appears another driver needs to be made aware that they’re starting to back up too close to the car, for example. And full horn blast if danger is imminent.

          As cars become able to reliably track warm bodies they can use the same strategy. No need to make any noise if the curbs don’t contain people are animals who are moving into the danger zone.

          • Freddy D

            The new cars already bristle with sensors to warn the driver of what’s getting too close, particularly when in reverse.

        • Freddy D

          Yes! Good points. Freeway noise will be essentially unchanged based on this data but stop and go urban traffic will be much quieter. Think about the radical difference in sound level between:
          Diesel bus
          Hybrid bus
          Trolley bus ( overhead line feed), which is fully electric and nearly silent.

          • Matt

            If you live within a mile of a interstate that is not flat, you know that converting trucks to electric would impact the noise. Their engine braking (coming down hill) and rev’ing as they go up hill; it carries a long way.
            Freeway and city street are two totally different items to study, treating them the same gives no useful information. Also large buses/truck have different noise/speed profiles from cars. In a city core (where you will be standing close to traffic) 100% EV would have a big impact on noise.

          • Freddy D

            Good points on hills and freeways. The cool thing about the charts in the article is that you can pretty much predict noise reduction for all the use cases.

          • JonathanMaddox

            Trucks wouldn’t need to go all-BEV for electrification to have a huge impact on this type of noise and on total fuel consumption on such routes. Hybrid is necessary and sufficient: regenerative braking FTW!

          • Bob_Wallace

            I would think PHEV trucks would make a huge impact on noise. There would be no engines revving up and straining to start the truck moving.

            PHEV trucks and pickup with a 30 mile electric range could make a huge impact on fuel use while still leaving them able to deal with being heavily loaded or needing to move outside their normal zone from time to time.
            Just think of all those pickups that do nothing much more than make a commute run five days a week and a local outing on the weekend. They still could be used to haul the bass boat or take the longer trip into the mountains.

  • Ivor O’Connor

    Good article.

    • nitpicker357

      Second that. Thank your for an excellent article. I wish this were stickied somewhere.

      • Freddy D

        How do we pin it to the new forums over in ev obsession?

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