U-turns are sometimes legal, sometimes illegal. What's that you say? You'd like to be able to tell the difference? This must be your lucky day.

You may legally make a U-turn

1. across a double yellow line, as long as it's safe and there are no signs prohibiting it. No doing that across a triple yellow line though. Those are pretty rare (non-existent), but now you know.
2. in a residential area, as long as no oncoming traffic is within 200 feet or there's a traffic signal protecting you from oncoming traffic.
3. at most intersections, as long as there's not a "No U-turn" sign. Make sure you still wait for that green light, though. If you'll recall, that means, "go."
4. on a divided highway when there's an opening that's specifically provided for vehicles to move into in order to make either a U-turn or a left turn. Be cautious, however, as the road may be unstable, for we all know that a highway divided against itself cannot stand.

Do not—we repeat, do not—make a U-turn

1. near or on railroad tracks. Gee, wonder why that is.
2. on a divided highway when no opening is provided. You're driving a car, not a lawnmower, and certainly not a wrecking ball.
3. if you don't have a clear line of vision both in front of and behind you. This could be because of bad weather, or because you forgot to remove your windshield sunshade.
4. anywhere a "No U-turn" sign is posted. Unless there's a second "No" at the beginning, which would make it a double negative.
5. if there are cars coming that might hit you. They might want to hit you, too. They look mad.
6. when you're driving down a one-way street. If the meaning of "one-way street" just finally hit you, give yourself a pat on the back. A very, very hard pat on the back. Don't be afraid to knock yourself over.
7. in front of a fire station. If there's a fire somewhere, and you're in their way, they won't hesitate to run you right over with their big truck. They might even sic their Dalmatian on you.
8. in a corporate area. You don't want to be making unpermitted U-turns in front of hidden alleyways and busy parking lots. That's a recipe for disaster.