A bicycle loop detector is a traffic control device. Read more here: http://www.techtransfer.berkeley.edu/newsletter/07-2/bike_detection.php
One of the pitfalls of the loop detectors is that bikes need to know how to place themselves to actuate it. In the event that one of these is installed, I will be sure to post more information here about its use.
Here are some photos on flickr: http://www.flickr.com/photos/voidoid21/1408419197/