stock here, reader submitted this summary
Microwaves superheat the metal and insta-boil wet innards of living things (trees, people). Asphalt is spared because it does not react nearly as well because no metal in which to accept heat-generating eddy currents. Grounded metal (signposts, parking meters, guardrail supports, etc) having wide path to ground do not respond nearly as promptly nor thoroughly as insulated metals, such as vehicles in rubber car tires. Metal housing wiring that is “grounded” superheats and melts because at high-frequency the thin (not wide) path to ground presents high impedance and allows eddy currents to form causing heat. The microwave weaponry likely operates at 10 GHz or higher.