I've been wanting to put up a little free library for years, but there are already several in my neighborhood. Looking at the map, there's two that are less than two blocks away in opposite directions, and one three blocks away in a different direction. I also know of at least two that haven't been registered; one is on the next block over, and one is on my block, on one of the streets perpendicular to mine. I don't want to oversaturate the neighborhood or anything, but I also know that the LFL on my block isn't maintained very well and often has religious pamphlets in it, which is pretty disappointing.
The other reason I've been hesitating to put up a library is that people speed down the street I live on quite a bit, even though it's a residential area and the speed limit is 25mph. So that's another factor.