Never make a U-turn:

Question

Never make a U-turn where a No U-turn sign is posted, at or on a railroad crossing, on a divided highway by crossing a dividing section, curb, strip of land, or two sets of double yellow lines, when you cannot see clearly for 200 feet in each direction, on a one-way street, in front of a fire station, never use a fire station driveway to turn around, in business districts (the part of a city or town where most offices and businesses are).

Accepted Answer

at or on a railroad crossing.

Answer

in residential districts.

Answer

where there isn't a "No U-turn" sign posted.