As cities continue to grow, there are more people crammed in with more traffic and noise. While we might get used to it, it has a range of effects on our health and happiness.
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Although, in the worst case, continued noise exposure over 85 decibels can cause hearing loss, there are health implications at lower levels. The WHO recommends that the average noise level should not exceed 55db.
Above that, noise triggers a stress response and long-term exposure is associated with negative health outcomes including heart disease, hypertension, stress and faster cognitive decline.
A study in San Francisco found that exposure to traffic noise made people less likely to speak to others, assist strangers and remain patient. They become more agitated, disagreeable and argumentative.
Cities vary greatly in how noisy they are. The dubious record for the worst is Karachi's rush hour where noise levels have been measured at over 140db. At 360 times higher than the WHO guidelines, that is insanely loud, about as loud as the deck of an aircraft carrier.
According to one study, noise levels in Cairo average 90dB, making it 11 times over the guidelines and putting it into the range of permanent hearing loss. It never drops below 70dB, which is like spending all day inside a factory.
A survey in Kolkata, India, found that, at one of the city's busiest crossings, there's an average of 18,857 honks in 24 hours, or one honk every five seconds. And, since a car horn can be as loud as 110db, that's a lot of noise.
The overwhelming majority of noise in cities is caused by motor vehicles. Without cars, a city centre can be as quiet as 40db - about as quiet as a library.
The noise from cars comes from two major sources. One is the car's engine, exhaust, transmission and brakes. The other is rolling noise caused by the tyres' friction with the road surface.
While engines are the most obvious source, noise from the tyres becomes significant at about 30km/h. Above 55km/h, road noise becomes louder than the engine itself.
It is possible to reduce tyre noise significantly by using low-noise tyres, but manufacturers have lobbied against this for years.
Another approach is porous asphalt surfaces that can reduce rolling noise by up to 6db. Although it is more expensive than regular asphalt, it's cheaper than installing noise barriers and soundproofing such as hedges.
However, since motor vehicles are the largest source of noise pollution, the best solution is simply to reduce traffic volumes. And that, of course, has other benefits as well.
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