Science and technology.
When the rubber hits the road.
Turning old tyres into new roads can help cut noise pollution.
AROUND one heart attack in 50 in rich European countries is caused by chronic exposure to loud traffic, according to the World Health Organisation (WHO). The ill-effects of noise pollution in such countries are second only to those from dirty air, says the WHO. Long-term exposure can cause hormonal imbalances as well as mental-health problems.
Roadside barriers can help dampen the racket, but they are expensive-up to $600,000 per kilometre-and they often serve as magnets for graffiti. Besides, they work less well on windy days and are impractical along city streets. Happily, there is another option.
By adding rubber "crumbs", reclaimed from shredded tyres, to the bitumen and crushed stone used to make asphalt, engineers are designing quieter streets. First used experimentally in the 1960s, this rubberised, softer asphalt cuts traffic noise by around 25%. Even better, it also lasts longer than the normal sort.
Not surprisingly, rubberised asphalt is catching on. Enough tyres are recycled in America each year to produce 20,000 lane-miles (32,000 lane-kilometres) of the stuff, enough to re-pave about 0.5% of America's roads, according to Liberty Tyre Recycling, a Pittsburgh firm that handles around a third of America's recycled tyres. Rubber roads are also popular in China, Brazil, Spain and Germany. Their popularity could spread further, since it is now possible to make rubberised asphalt less expensively than the traditional sort.
That is because rubber can partially replace bitumen, the binding agent used to hold the crushed stones together in ordinary asphalt. Bitumen is derived from oil, which means its price has risen over the past decade alongside that of crude oil. Discarded tyres, by contrast, are cheap and are likely to get cheaper. In rich countries, around one tyre is thrown away per person per year. They are piling up especially quickly in Europe, where dumping them into landfills was banned in 2006.
Rubberised asphalt keeps the noise down in a couple of ways. Pores between the stones in standard asphalt must be small, because if the gaps are too big the bitumen binding cannot do its job properly. Adding rubber thickens the bitumen. That allows bigger pores, which help to trap and disperse sound waves. The rubberised bitumen itself is flexible and slightly springy, which enables it to absorb more unwanted sonic energy.
Shredded tyres are not the last word in exotic road toppings. A substance called PERS, or poro-elastic road surfacing, is being developed with a mix of private and public money in the European Union. It is made from a blend of crushed rock, rubber and polyurethane, a synthetic plastic that replaces bitumen as the binding agent and allows even bigger pores in the road surface. PERS is not cheap, costing around five times as much as rubberised asphalt. But you get what you pay for: tests suggest it can cut road noise in half. In some particularly noisy areas, reckons Luc Goubert, who is co-ordinating the PERS project at the Belgian Road Research Centre in Brussels, the resulting boost to property values-and, therefore, land taxes-could help cover the cost.