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20 May, 2005

Greasy road

Why is it that a road surface during/after light rain feels 'greasy' underfoot and is drastically more slippery than during/after heavy rainfall?  Further rain onto this 'waxy' surface somehow improves traction, which isn't intuitive.


When the weather is warm and dry, the oil from the asphalt tends to separate. With a light rain, it is brought to the surface, making the road slippery. Presumably, with a heavy rain, there is enough laminar flow on the road surface to wash away much of the oil.

Make sense?

Posted by Jon at May 20, 2005 05:45 PM

I like the theory, but I'm not sure it's the whole story, as:

  • The weather isn't warm and dry at present there are still frosts at night, and daytime temperatures are currently <15°C.
  • I could imagine this happening on new tarmac, but that laid 15-20 years ago?
  • Stone/concrete paving slabs are affected too.
I did wonder whether it was biotic; perhaps rotting vegetation in the crevices in the road surface, swelling to smooth those irregularities when wetted, then washed away by laminar flow, but this seems to happen well away from trees and on streets without gardens.

Another complication is that the same phenomenon can occur the very next day, before there's been time for significant replenishment of oil or vegetation.

Posted by NRT at May 21, 2005 11:41 AM

I personally think it has to do with the pollution coming from the cars' exhaust pipe. The emited gas are ful of molecules that land on the bitumen and are revealed greasy when mixed with water. Roads are very slipery especilly when it has rain for a while and the first rain comes.

Posted by hash at May 21, 2005 01:57 PM
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