Traffic signal loops work on an inductive principle. When metal passes into an energy field, it changes the field and the detector does it's job.
The problem is most of the detectors are junk. Extreme weather changes will cause them to go out of tune.
The loops will get broken or grounded.
When you come to a light look for the slots that were cut. Pull your motor over the corner of the loop. It's more sensitive there.
A lot of citys are going to a camera detection instead of ground loops.
I worked 4 years at Memphis Traffic Signal