They are, but that usually comes down to their design. If you've got a wide street with no curves and stoplights every couple thousand feet, but it's completely lined with businesses and apartments, then tossing down a 25mph sign isn't going to get people to drive 25mph. The street needs to be designed so that people don't feel safe driving 40mph on it.

Narrowing a street is usually the easiest way to do it. If you're worried you don't have enough room to get around that car parked on the side of the street, you're going to slow down.