Question:  My City has been responding to calls lately about bikes not getting detected at traffic signals.  We use the Type D loop at the front of all our Stop Bar detection, which is supposed to pick up bicycles.  Do we need to connect that front loop to its own Detector Cable? 

Answer:  The typical detector loop has a Type D loop right at the stop bar (the bike loop), and three Type A loops connected in series-parallel extending away from the intersection.  As you noted, this style of detection should be able to pick up bicycles.  One possible problem is the number of windings in the Type D loop. 

Caltrans Standard Plan ES-5A (http://www.dot.ca.gov/hq/esc/oe/project_plans/highway_plans/stdplans_US-customary-units_06/viewable_pdf/rspes-05a.pdf) has a detail in the lower right corner that shows how the loops connect together.  ES-5B has some notes in the lower left corner (see excerpt below) that calls for 5 windings of the loop wire when the loop is in a series with three other loops.  My guess is your loops were installed with the normal 3 windings.  The extra two windings give the loop the extra sensitivity to detect bicycles.    





On the other hand, this loop was not designed to detect expensive graphite-frame bicycles.  The best way to detect them would be with a separate DLC for the front loop.   
Thanks to Allan Morris, EE, TY Lin International, who supplied the technical input for this posting.