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We are always camping in areas we have not been to before. We usually use Google maps (satellite) to try and guess about the appropriateness of a camp before going there.

From experience we know that a couple hundred feet (~100 meters) of forest between a camp site and busy highway, means lots of road noise in the camping experience. A couple miles (3KM) of forest between camp and busy roads, almost always results in only hearing forest sounds.

Are there any scientific data on how much forest (or other terrain) will dissipate the majority of traffic noise?

Rephrase Question; What is the minimum amount of forest I need between me and the highway so I won't hear the highway?

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    no scientific data, just experience: i grew up in a village near a german autobahn. In clear nights you could hear cars in the distance, which was still 1 or 2km away. However, it was not disturbing, but really put an emphasis on the silence when there was no car. Forests, depending on tree types and density, are very good at silencing noise. i would even consider 100m of trees enough. However, i was also camping near a stream of water, a mere 100m away from a busy street, no trees: i heard no cars due to the sound of flowing water. – Peter1807 Jul 10 '17 at 12:59
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    Also, the ability to transmit sound through the air depends on stuff like wind direction as well. I would recommend that next time you go camping, you walk into the woods and take a break every 50m to listen for passing cars. Maybe you will be able to give a proper answer to your question for us yourself :) – Peter1807 Jul 10 '17 at 13:02
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    Depends on the highway, its surface, the type and size of vehicles using it, their speed, their tyres, whether any noise reduction scheme is in place, the density of the forest, the topography, the weather, the wind – Separatrix Jul 10 '17 at 13:19
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    Related question on Engineering: How much noise does a road noise barrier block? – gerrit Jul 11 '17 at 15:09
  • On cold clear frosty still nights, I can hear a train 10 kilometres away. So weather comes into it as well. – Criggie Jul 11 '17 at 22:27
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The answer is it depends. Highway noise is typically 70-80 dB at 15 m. To a first order approximation, sound waves obey the inverse square law where there is a 6 dB drop every doubling of the distance. This means a sound source that has a level of 72 dB at 15 m would have a level of 0 dB at 60 km. While absolute threshold is typically described as 0 dB, highway noise probably becomes imperceptible from the forest background noise at 30 dB, or about 3.8 km from the highway under ideal conditions.

The first order approximation is not very good. This book chapter seems to touch on the key factors meteorological conditions (e.g., temperature and humidity), terrain (e.g., hills and what is on the ground) and obstructions that will influence the propagation of the sound. Some of these things cause sound to get attenuated quicker while others can cause it to propagate further (e.g., a forest can act as a wave guide). If the vegetation is thick enough, it can provide substantial attenuation (10 dB per 61 m). Further, the inverse square law is for point sources and, as the linked chapter points out, highway noise is potentially better modeled as multiple point sources 9or maybe even a line source if there is lots of traffic). As I said, the answer depends.

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    The inverse square law holds for point sources. A typical highway will rather resemble a line source, in which case the noise is inversely proportional to the distance itself (not distance squared). This makes a drop of only 3 dB for every doubling the distance. – Emil Jeřábek Jul 11 '17 at 17:19
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    @EmilJeřábek I think this depends on the amount of traffic and probably the direction from the highway you are measuring. – StrongBad Jul 11 '17 at 17:27
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    Sure, but your answer doesn’t suggest that the amount of traffic and/or direction from the highway affects whether or not the inverse-square law is applicable. It just states that it is. For a straight, high-traffic highway traveling perpendicular to the shortest distance between you and it, it isn’t. Which means that in that circumstance, your answer is wrong, and would be better if it recognized that limitation (or, better, accounted for it). – KRyan Jul 11 '17 at 20:34
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    @KRyan I start with a first order approximation assuming the inverse square law. I then say the first order approximation is not very good and provide a link that covers many of limitations including the fact that a highway can be modeled as multiple point sources (which is more realistic than a line source). I do not think that the number of cars will influence the propagation as much as the key factors I listed. – StrongBad Jul 11 '17 at 21:44
  • Was going to +1 just for your username/icon, then I noticed the answer was good too. ;) Seriously though: I understand what you say in comments here, but perhaps you should just emphasize that part of your answer a bit. At the very least, Emil/KRyan might be appeased if you added ", and traffic patterns" after the spot where you already have "conditions (...), terrain (...) and obstructions)" – Aaron Nov 27 '17 at 16:44
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This depends enormously on landscape, weather, and infrastructure.

I grew up in The Netherlands several hundred meter from a motorway, but with good noise barriers, we heard almost nothing. My friend lives less than 100 metre from a noise-barriered motorway and it's nearly inaudible. Now I live several km from a motorway in England, without noise barriers. Depending on the weather, I either hear nothing at all or it's so annoying I can not stand being in my garden, and hear it even with the windows closed. I've been considering to buy a sound pressure level meter to compare my impression to something measurable and try to relate it to my circumstances. The quietest weather tends to be dense fog, when traffic is either crawling or stationary, both of which are sufficiently quiet several kms away.

If there's a hill between you and the road, or if the road is underground, you will hear (nearly) nothing even very close. But if the road is in a valley and you're up on the slope, you might hear it many kms away. I've been 10 km from roads and still found some heavy powerful lorries going uphill disturbing, but that on a mostly open landscape with at most small sub-Arctic mountain birch forest type trees.

In some countries there are interactive noise maps, such as this one for England. I found the value of this quite limited, as my personal experience is worse than this map claims for my area.

It also depends on the road, of course. Some roads have traffic all night, others become completely quiet.

Personally, my strategy if I'm near a road, is to seek out a sound I find pleasant rather than annoying: specifically, a stream or river. Those may drown out the road sound such that I can pretend to be in the wilderness rather than right next to a road.

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I work in an office that's inside a forest, and about 800 m away from a busy motorway (2x2 lanes, 120 km/h speed limit), with noise barriers along the motorway. I go for a walk in the forest every day. At 800 m, the motorway is inaudible. At ~400 m, it starts to become audible if the wind is from the motorway toward me. At ~300 m, it's audible in any weather.

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It is not just the distance, but depends on the barriers. The barrier may be highway installations, landscapes, forest etc.

Refer the picture below. The place behind the barrier has less noise than the one on the top of the hill, even though it is further away in distance

The place behind the barrier has less noise than the one on the top of the hill, even though it is further away in distance Image source: nonoise.org

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Adding to what @strongbad had said, you also have other complex issues to deal with as well... For example, if it is hot outside, you can be as little as 50-100 yards from the road (or less) and not hear anything from the road because the heat carries the sound waves upwards. Meteorology plays a significant factor in the science of sound. That's why a few battles during the Civil War were won by those armies. The enemy never heard their cannon fire.

Edit: administrators, please feel free to delete this answer. My SE app failed to load part of strongbad's answer, so I missed half of what they said. My apologies.

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