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Question

If tyres wear on roads and roads wear with traffic, where does all this stuff go?

Answer

Chris - But first, just a ball park estimate for you two, what weight of rubber do you think gets worn out on the road every year around the world?

Dave - It must be a kilo or two per car per year. So, in the UK, that's sort of 20 million kilos or about 20,000 tons or so off the top of my head.

Chris - Well, when this question came in, I was just playing with the numbers. So, I looked up how many cars do we think there are on the roads and in 2011, they announce they think, alongside the 7 billionth person, there were 1 billion cars on the world's roads. So, if you assume that each of those has got 4 wheels, so there are 4 billion tyres. So, how much tread is on a tyre? So I said, okay, if you imagine that a tyre is about 3 metres in circumference and let's make a really ball park estimate.

Let's pretend it's 10 centimetres wide, so pretty narrow, but some are going to be bigger, some are going to be smaller, all the numbers will come out in the wash. Let's assume the tread is a centimetre deep to make the maths easy. That means that the thickness of tread on your average tire must be 3,000 cubic centimetres because I'm multiplying 3 metres, 300 centimetres by 10. So that in other words, it's 3 litres of rubber on a tyre.

So, on all the cars in the world, that must be 4 billion tyres times 3 litres, that's 12 billion litres of rubber. If we assume that a rubber tyre wears out in a year, which in some cases, with a bit of hard driving it's going to, then you could be rubbing 12 million cubic metres of rubber onto the roads around the world every year, which is quite a lot, isn't it? So, then I looked up the density of rubber.

It's 1200 kilograms per meter cubed which means that there are 14.4 million tonnes of rubber being rubbed off on roads all the way around the world every year. That's extraordinary, isn't it?...