Basically, the idea is that citizen volunteers are provided with some training and then are allowed to use radar speed "guns" to monitor traffic speeds through the neighborhood. Speeders then receive a letter advising them of the observed violation. There is no actual enforcement, but in the cities where it has been tried the presence of the volunteers together with the mailing of the letters is understood to send a strong message.
Read more about this here:
To my knowledge, nothing like this has been done in Bloomington. But, if this seems like something we'd like to pursue, then we might form a committee or bring it up for discussion at our next general neighborhood meeting (which will be scheduled for sometime this fall).