I see folks reporting problems triggering traffic lights, which isn't a big deal for me (there's only three of them in the whole county!) but I do go in and out of an airport that has an automatic gate opener system...I don't know for certain but I suspect there's an inductance loop (coil of wire) in the soil and the metal of a vehicle passing over causes a change in the electrical characteristic of the coil called 'inductance' which in turn causes a frequency change in a controller's circuit that is then detected by a ratio detector or a discriminator circuit. The Vespa doesn't seem to have enough metal to trigger it. I know weight isn't involved because the system works even when the ground is frozen rock hard.
I looked all over for powerful magnets to try suggestions I found in MV, but found only mildly strong ones. I then went to a place that takes in almost anything and keeps it for anyone else that may be in need.... poor folks, those burned out of their homes, etc.
I got two old junk microwave ovens (no cost) and took out the ring magnets from the magnetron - gave me a total of four magnets. They are so powerful that you get a blood blister if they snap together and pinch your fingers. I piled the four together and let them snap on to the bottom of the Vespa. Will let you now how this works when I try it out tomorrow.