In most cities a vehicle is detected by an inductive loop under the pavement.
An 1800+ pound (mostly steel) smart should have no trouble being detected.
But, I've heard the sensitivity is adjustable and maybe the light which is giving you trouble needs a tweak??
HowStuffWorks "How does a traffic light ...
I've never had any trouble tripping lights in my smart. Long ago I had a moped, and learned to trip them with it by pulling up slow and steady over the loop.
I don't think I'd want any super magnets suspended under my car picking up road debris.