How Does Rain Create More Potholes?

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A rainy winter has made potholes a huge problem in the San Francisco Bay Area.  (Justin Sullivan/Getty Images)

When you get enough rain to end a drought, there's gonna be collateral damage.

We're talking about our roads, where some drivers have noticed the extra potholes popping up on our highways and local streets.

"I ran over a pothole on Van Ness," Muni transit operator Matt Smith says. "It damaged the inner boot of my car and gave me a flat front tire. The pothole was massive. It was massive! It’s no reason why that pothole should not have been fixed."

Bay Curious got this question from Anne Schaefer-Salinas and her son, Declan: Why does bad weather cause potholes?

We spoke to Caltrans and got a lesson in how potholes are formed.


Imagine for a moment life as a section of roadway. Passing cars are constantly pressing down on you, followed by periods of relaxation when there's no car. All day, every day, year after year. All this repeated pressing and releasing creates tiny imperfections in the pavement that turn into cracks. When you add water into the mix, those cracks get bigger, faster.

"Whenever it rains and the rainwater accumulates on the road, tires from the road actually squeeze the water into the pavement," Caltrans spokesman Bob Haus says. "So that repeated squeezing into the asphalt and into the concrete causes the cracking."

Water gets through those cracks and weakens the soil under the road. That leads to even more cracking and eventually a pothole.

The more traffic, the more potholes. And the Bay Area has no shortage of traffic.

"There's just a lot of traffic on Bay Area roads," Haus says. "We all know that. We feel the congestion every day. So there's just a lot of wear and tear on our highways."

I’m sure a lot of you are wondering why crews aren't out there fixing all these potholes. One thing to note: Road workers have to wait for dry conditions to make permanent fixes. So the rain not only makes more potholes, it makes it harder to fix them.

Got a question you want to see the Bay Curious team take on? Submit it!