It's a question many drivers face

on their way to work every day: to swerve or not to swerve? Should you veer out of a giant pothole's way and play chicken with oncoming traffic, or stay the course and risk a flat tire, a bent rim or steering misalignment? Those of us who don't possess the driving finesse to align those craters between our wheels could end up paying hundreds of dollars per year in damages caused by potholes.

It's hard to believe those chasms in the pavement aren't the remnants of miniature explosions. Instead, they're carved by plain old water. The process usually starts with one little crack that fills with H2O. When water freezes, it expands, and when the water inside the crack expands, it pushes the asphalt outward and upward—a lot like when you forget a soda in the freezer and return to find a frozen volcano bursting from the bottle.

On pavement, "it's like a chain reaction," says Amid Bhasin, a civil engineer at the University of Texas, Austin. "It starts slowly at a weak spot, and once it starts falling apart, that creates larger cracks; then more water gets into it, and the cracks grow larger." That process is exacerbated by traffic, so it's not just bad for your car when you hit a pothole—it's bad for the road, too.

Potholes can also form when water gets beneath the road, says Bruce Hucker, a district operations engineer for the Illinois Department of Transportation: "When you step on a dry yard, you don't leave any indentation because it's hard and it fully supports your weight. But when it gets wet, your shoe sinks down in it." Similarly, when the ground beneath the road gets wet, it can sink down. As traffic drives over these unsupported spots, the pavement bends and flexes. Over time, it will crack. That provides the perfect opportunity for water to slip in and wreak havoc.

If it looks like those pits are widening lately and taking over the road week by week, you're probably right. February, March and April are the worst months for potholes because the roads are rapidly freezing, thawing and freezing again, tearing up the streets in an asphalt accordion motion.

The New York City Department of Transportation keeps a running tally of the potholes its workers fill, which sometimes tops 5000 per day. They've patched nearly 300,000 since last July. Filling potholes is a year-round occupation for the Illinois Department of Transportation as well, Hucker says, though the process of patching differs depending on the season.

During summer, the crews fill potholes with an asphalt "hot mix," which is kept warm in a truck until it is shoveled into the hole and tamped down. But in the winter, the hot mix doesn't work as well, Hucker says. That's because it cools too quickly for strong bonds to form between the new asphalt and the old. That could just lead to more pavement devastation. Instead, winter patching requires a special asphalt blend that's more sticky and pliable—and also more expensive.

No matter whether it's winter or summer, the road crews have to make sure there's no water in the hole before they fill it. That's because water doesn't mix with the oily asphalt and could weaken the seal, and also because trapped water could freeze again and ruin a whole lot of hard work. Hucker says a pothole fix might last several weeks to several years, but he suspects the summer fills last longer than the winter ones, simply because it's easier to get all the moisture out.

You can start your own pothole patrol by helping Popular Mechanics find the worst potholes in America. Email a picture of your favorite pothole—or one that is the bane of your daily commute—pmwebmaster@hearst.com.

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