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It's a good place to plant a tree or two, and provides an excellent sightline around the edge of the curve. But a lot of effort went into scooping out the center, when the area could just as easily been flat or even slightly crowned. Why, the way they are now, water could run right into them.

And that's the idea. Water from rain or flooding doesn't soak into asphalt or cement, and picks up speed as it goes downhill. The highway bowls provide a place for the runoff to settle.

But there's another reason these areas are carefully manicured by highway engineers. Roadbeds have all sorts of noxious chemicals dripped on them -- gasoline, oil, radiator fluid, Icee dribblings -- and rushing rainwater washes it off and carries the chemicals downhill.

The bowl areas provide a place for the chemicals to settle out before being absorbed by groundwater. This is more of a problem on the Mainland with salt from winter seedings, but it's a design feature of most modern highways. According to the Department of Transportation's Marilyn Kali, the H-3 and Kamehameha interchange pictured here is pretty typical.

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