These traffic light sensors are NOT triggered (”tripped”) by the weight of a vehicle, but my the mass of steel (magnetic metal) which is located over it. Apparently the Smart Car does not have enough steel in it to “trip” some traffic lights.
Motorcycles have suffered from the same problem for decades.
If you are having a problem at a given signal light, contact your local highway department or public works department (as applicable) and report the specific lights which your motorcycle or “Smart Car” will not “trip”.
The sensitivity of the sensors IS adjustable and most lights can be set so a motorcycle can “trip” them.
Many localities are taking care of this issue by eliminating sensors under the pavement and switching to camera activated traffic lights. Personally, I like this type of traffic light, and even a person on a bicycle can “trip” them.
Also, there are some companies that market a small device to motorcyclists which is mounted on the underside and hangs down closer to the road than the bottom of the engine (it appears to me to be a poerful magnet). This device is supposed to “trip” the errant, less sensitive, sensors. I have no idea how they work because I have never used one. I do not like the idea of magnets hanging off the bottom of my motorcycles.
I think it isn’t the amount of steel per se (there’s more than enough in the engine) the the distance of the steel from the ground that’s causing the problem. If the metal is too high above the loop it won’t do anything.
Your instincts are correct, it's just a magnet. You can spend $20-30 on the "signal activator" or you can just go to the hardware store and get a large heavy-duty magnet for <$5.