These things were light, small and thin. Even if most of the weight had been a bomb that went off they wouldn't have done much more than blacken the paint on those bridges.
It was easy to tell what these things were just by looking at them. Its inner workings were open to see.
Dumb (not you).
batteries, exposed wires and placed on infrastructure. could it be a timing device that would detonate something placed elsewhere? i don't know jack about explosives, but i think that the gimmick was reprehensible and that one cannot be cavalier about stuff randomly placed. i mean for God's sake, you can't leave a piece of luggage unattended at an airport, why would it make sense not to be suspicious of electrical devices placed on infrastructure.