What Causes Traffic Noise?
August 23, 2009 8:25 AM   Subscribe

Traffic noise. What percentage of it is caused by: 1) The internal combustion engine and associated systems (as opposed to an all electric vehicle) 2) Air turbulence of the vehicle moving through space, 3) Other mechanical issues.
posted by rhcclark to Travel & Transportation (13 answers total)
I suspect that a significant amount is tire/pavement noise.
posted by HuronBob at 8:44 AM on August 23, 2009

I think it depends on the gradient and condition of the road, the types of vehicles using it, and the speed they're travelling at. I lived right next to a relatively busy stretch of straight road, where most of the traffic was going downhill, quite fast (about 80 kph) and a lot of trucks used the road. I hardly heard any engine sounds at all, but there was the constant "woooosh" as the vehicles soared past my bedroom window. It was enough to wake me up almost every morning :/

Later, I lived on ANOTHER hilly road, also traversed by many trucks, but there were traffic lights at the bottom of the hill. So, I got woken up by air brakes. Grrrr!
posted by Diag at 8:53 AM on August 23, 2009

For traffic moving at highway speeds, HuronBob is right.

There's a park near me with a multi-use trail that parallels a busy highway. Parts of that trail are below the level of the road. As soon as you reach the section where you cannot see the road surface itself, the reduction in noise level is astonishing. You can still see the vehicles, but the tire/road roar is blocked. What's left is engine noise and a bit of aerodynamic noise, and it's much quieter than the road noise.

For city traffic that isn't moving fast and is stopping and accelerating, it seems to be mostly engine noise that you hear, especially for large vehicles (trucks and buses). If you're very close to the road, you might get some other mechanical noise on occasion, e.g. a particularly loud power steering pump. But usually the mechanical noise is not loud enough to be heard.

From the park experience, and also from waiting for the train every day at a station right by another major highway, the only other thing I can add is that vehicles with particularly loud exhausts are able to be heard over all the other din.
posted by FishBike at 9:13 AM on August 23, 2009

I read an article years ago that compared American to European (German?) asphalt. Apparently the German asphalt was much quieter and lasted a lot longer but was expensive and so wasn't used in the US because of the whole lowest-bidder process.

And here's an ad for asphalt-rubber. So the technology's out there- we just don't want to pay for it.
posted by small_ruminant at 9:51 AM on August 23, 2009

maybe it's not an ad, but a study. I have gotten very cynical these days.
posted by small_ruminant at 9:55 AM on August 23, 2009

There are a couple sections of highways in San Diego where the surface changes to asphalt from the standard interstate concrete that we normally have, and even from inside the car, the reduction in noise at those parts is significant. But when I used to live near a downgrade in the freeway, trucks downshifting was the most annoying traffic sound for me.
posted by LionIndex at 11:21 AM on August 23, 2009

They've been surfacing the concrete freeways in Arizona with rubberized asphalt, and the noise difference is absolutely dramatic. Most cars these days are very quiet, so the engine noise isn't a significant contributor. They put it down on the rebuilt stretch of I-10 here in Tucson, and it's the quietest driving experience I've ever had.
posted by azpenguin at 11:33 AM on August 23, 2009

Seconding Huron Bob (thirding?), when you walk by a busy freeway, as I do from time to time, you notice the noise of tires on pavement more than engine noise or any other noises. The friction factor must be measurable, since it's certainly audible, I would think.
posted by Lynsey at 12:28 PM on August 23, 2009

The design of the tread of the tire makes a huge difference as well. When designing a tire you have to make all sorts of design trade-offs: how long it lasts vs. how well it grips a dry surface vs. how well it pumps away water when wet vs. how loud it is vs. how long it lasts. You could for example design a tire that has incredible dry performance but is terrible in the rain and only lasts 15k miles. Since that's not an option for most mass consumer tires, what you end up with is a big compromise on all fronts, including noise.
posted by Rhomboid at 3:53 PM on August 23, 2009

There is a similar "quiet" pavement that azpenguin mentions in Bellevue, Washington. It significantly dampens the amount of noise made on the highway. I would imagine that friction between pavement and highways, especially with semis, is a large contributor to the traffic noise.
posted by carpyful at 5:01 PM on August 23, 2009

Tyre on road. The noise of the near by road doubles when its raining, interestingly enough.
posted by lundman at 5:06 PM on August 23, 2009

It's worth mentioning that my mechanical engineering professor spent a good deal of time studying the noise created by air interacting with the radiator and liquid cooling system (both wind and forced air from the fan). Definitely not as loud as pavement/tire noise, but you can often hear it clearly when outside of a idling/parked car.
posted by kenbennedy at 10:29 AM on August 24, 2009

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