The rectangular cuts you see in the ground do not detect weight (It would be really, really, really expensive to put weigh scales in at an intersection). They are induction sensors, and will detect a large amount of metal over them, like a wheel or body of a car. The MINI has no problem being detected.
The optical sensors at the top of the lights sense a certain frequency flashing that is carried by emergency vehicles. If you could somehow flash your lights at the right rate and fast enough then yes it would let you through but chances are you cant, and with one flash, if it activated then it is malfunctioning.
Anyway chances are it sensed your mini but is on a delay, or waited for a break in traffic in the other direction, and just happened to change when the truck came along behind you.