McCoy: they cant do this when the tarmac has the highest temperature cause the termal shock would disturbe its structure. Depends on the temp of the water. It only has to be a few degrees lower than the tarmac to allow it too cool. ;) There's no point to cool tarmac at night or early morning when the highest temps are during the day and the damage is done by vehicles etc then. Plus to cool the tarmac the water would have to be sprayed for longer than a few seconds.
Vincent: since roads are built with a camber to allow water to run away quickly to the sides. That's true. I'm sticking with the cleaning theory. That's the only reason they do it here.
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