Originally Posted by
anastrophe
Bridges are a better option than most others because if they are actual engineered bridges such as traffic underpasses, they are grounded. If you are under the bridge, but not in the water, you're unlikely to get hit as the bridge will conduct the electricity away. Flash flooding is a legitimate concern though.
(That's according to NOAA. They could be wrong of course. I've not tested this myself.)
It's not the structure or the possibility of getting hit by lightning under the bridge that you should be concerned about. It's the flash flooding.