A minor point, but I'm fairly sure that there are metal detecing loops under the ground, not weight detectors. Often times, at least around here, they have replaced or added new loops, and you can actually see where they are at and stop with your motor over the loop.
There are one or two lights that I've run into that I just can't get to change when I'm in the left turn lane, in those cases I jump over a lane and go straight when the light is green, and loop back around. But yeah, if you can't get the light to change then you will just have to go through it somehow.
Fortunatly Oregon is really bicycle friendly, so on a lot of left turn lanes there are bicycle spots, marked with little bicycle logos, that will detect the presence of a bicycle, or a motorcycle if I happen to be riding through there.