Why are they greasy? Do you mean wet? Did you clumsily employ that adjective just so you could post this GIF?
Take care on the roads tonight! We've had several crashes across the city in the last hour. The roads are a bit greasy 
after some rain this afternoon 
pic.twitter.com/ZVwhdcWup7
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Oil leaks from cars gets trapped on the roads, and the rain released the oil on to the surface of the roads. You need a big storm to wash it all away
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The first rains always bring both the grease and the morons onto the roads. Drive to the conditions, people!
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