A grocery store plastic bag weighs about 5.5 grams. A gallon of gasoline weighs about 2.86 kilograms, or 520 times more. Minimizing your use of gasoline is far more important than worrying about a plastic bag.

If you worry that plastic recycling is sent to third-world countries and thence into the ocean—and you should worry—there's an easy solution. Don't recycle your plastic. Trash it; send it to a landfill. In the big picture, drilling for oil, making plastic, and then burying it again is its own sort of recycling. And in developed nations, landfilling plastic is environmentally benign. Landfills are lined and covered.