Why you ask? Well, several years ago when we lived here the last time, the speed limit on the road was 30. There were never any accidents (except the one caused by ice, but that was a single car and a tree). It is not a residential street. The street it merges with is virtually identical in nature (as far as proximity to residents, walking paths and bike paths), and is a 30 MPH zone. There is no known reason for it to be a 20 mile zone, other than fund raising purposes, that anyone knows of.
I have been pulled over myself on that road, and talked myself out of the ticket by arguing that we had just moved back to the area, and when we left it had been a 30 MPH zone. Oh did I mention the two posted speed sign's that are hidden by trees? Unless you are a pedestrian walking on the other side of the road from the path and on the narrow shoulder, you cannot see them (I also used this in my argument).
There are weekly complaints about this road in the local newspaper, but to my knowledge there has never been a satisfactory answer to the why-ness of the 20 MPH limit. The only responses are the posted speed is 20 MPH, therefore you should obey this limit.
Like I said, it is a fund-raiser road.