This week’s rainstorm apparently reminded a Road Warrior reader of something she’s been wondering about:


What residue is on the roadways that seems to be causing the asphalt to literal foam (like soap) when the first rains are happening?

I drive from Calistoga to Santa Rosa every day, and these last two spring/summer storms are leaving white waves of bubbles on all the shoulders. Is there something different in the “newer” asphalt material they are using? Is is more dangerous or slippery? I just don’t ever recall seeing that before.
Thank you, Connie

Answer from Rob Silva, roads division manager of the Sonoma County Transportation and Public Works Department:

The resurface of Petrified Forest Road was a rubberized chip seal, a larger rock was placed on the bottom coat with a smaller ¼” chip as the top surface coat. What happens with a traditional asphalt surface in hot weather is the heat draws oil to the surface. In the fall when we have our first rains you always hear the CHP advise traffic to slow down as the pavement surface is especially slippery through the first couple of rains. This is because road oil that has come to the surface in the summer heat is sitting on top, mixing with rainwater. Chip seal oils will do the same. I’ve seen this more often with chip seal surfaces than pavement -– an asphalt paved surface is smooth where a chipped surface has a more coarse surface, which allows more oxygen to mix with oils and dirt down between the rock, creating the foamy look when traffic rides on it and mixes it.

I have not been out on that route since it’s rained, but I’m sure this is nothing more than road oils and dust.


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