Originally Posted by
MattFoley
It's because the water interacts with all the car oil and junk that gets trapped in the pavement when it's dry, and turns the road into an oil slick. With enough moisture, the oil and dirt wash away, which is why traction improves, but light moisture just brings it all to the surface.
Yup. I was a motorcycle instructor for 5 years and we always taught riders to wait out the start of the rain to let all that oil and grease run off.