Friday, 29 May 2015

A car crash so quiet that we didn't know it had happened

On Monday morning there was quite a serious car crash just 70 m from our home. We didn't know about it until later in the day when it was reported on the local news. This may seem like an unlikely story, and you may wonder how it could be possible that we would miss such a thing so close to our home, but this actually makes a good example of how successfully Dutch residential areas are insulated from the effects of major roads, even when they pass nearby.

The crashed car, by the closest bus-stop to our home. Photo: RTV Drenthe.
The huge road on which the crash took place. The car in the photo above is in the same position as the bus in the photo below.
Our home is on the left of the Google Maps measurement tool, the crashed car was on its right.

The road on which this crash occurred is the ring-road of Assen. This has a 70 km/h speed limit (43 mph), this being the highest speed limit that you will generally find is allowed on a road within a city in the Netherlands. The relatively low speed for what is not far off an urban motorway does of course help to keep noise levels down, but you'll also note that the photos above show a noise barrier alongside this road. The barrier is extremely effective at reducing noise, and also makes it next to impossible to see the road. This is why it was possible that we didn't even know the crash had happened until later in the day.

Cyclists are often unaffected by crashing cars
The cycle-route skips past the traffic
and goes nowhere near the road
The cycle-path which we use to ride into the city is visible on the image from Google Maps above. It passes almost exactly under where the crash occurred so cyclists who were riding here even as the crash occurred may well not have noticed it happening, and will not have been put in danger.

By using this tunnel, cyclists not only cycle in a safe and almost entirely noise free environment but also skip past a set of traffic lights.

The tunnel and noise barrier between them reduce danger for cyclists, reduce noise and also reduce how often cyclists have to stop on their journeys. These factors add together to make cycling more pleasant and relaxing and also to offer the prospect of faster and shorter journeys if you travel by bike.

Measuring noise levels - how effective is the barrier?
It's quite straightforward to measure the effect of a noise barrier if we have the right equipment. The photo on the right shows my SPL meter measuring the sound level in our front garden, a mere 50 metres from the road. The glimpse of blue in the centre top of the photo is as much as we can see of the noise barrier, which is otherwise completely invisible due to trees. My meter barely ever registers any noise here. Set on the lowest possible range of 60 dBA, the needle very rarely moves from the -10 dB setting due to traffic noise, indicating a level which barely ever rises above 50 dBA. Birds in trees nearby make a subjectively louder sound than traffic noise. 50 dBA is the sort of level sometimes described as being equivalent to an "average home interior" or "moderate rainfall"

My SPL meter barely moves
on this side of the barrier
By comparison, on the other side of the barrier I have to adjust my meter's range so that it is not overloaded by the sound. When standing there, the meter often shows figures of 82 dBA or higher as I am passed by cars, and can peak higher yet for buses, trucks and motorbikes, so it would appear that our front garden is at least 30 dB quieter than the road-side.

The inverse-square law does not apply
Readers with a physics or maths background will probably have thought of the inverse-square law by now. According to the inverse-square law, you would expect that a ten times difference in distance from the source of a sound would bring a 20 dB difference in sound level, which is a good part of what I have measured. However, you have to be careful with that assumption in this case. The inverse-square law applies only for measurements in a free field and with a point source.

While noise from aircraft travelling overhead is attenuated due to the inverse-square law, but when considering noise from motor vehicles on roads we usually have neither a free field (airborne sound bounces from the road more than it propagates through it) nor a point source. A busy road with many cars on it (such as during rush-hour when I made my measurements) approximates more closely to a line-source than it does to a point.

Noise from roads is so pervasive in large part because it is not attenuated by distance to the extent that you might hope it would be.

Besides, if you need further evidence of the effectiveness of the barriers, I should point out that my meter still measures only slightly over 50 dBA even if I stand very close to the quiet side of the barrier, when my distance from the noise source (cars on the road) is only about double that of standing on the noise side.

Barriers also reduce air pollution
It has been shown that air pollution from motor vehicles is also reduced in the local immediate downwind area behind noise barriers. This is due to the pollution being lifted to a higher level where it is blown away more quickly. We breath cleaner air as a result of the noise barrier.

And help to encourage neighbourlyness
Children play on our street because it's
quiet and traffic free, This is normal
in Dutch residential streets.
This is all part of creating a pleasant residential environment. Research has found that streets which are quiet (both in terms of through traffic and noise) lead to better relationships with your neighbours.

Quietness is not unusual in the Netherlands
On our study tours we demonstrate the surprising quietness of Dutch residential areas and also how even motorways with vehicles travelling at 130 km/h (80 mph) now cause comparatively little noise nuisance.

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