Placing an obstacle between the bridge and the intersection would be fairly pointless, because once you hit it, it would be too late to stop before hitting the bridge.
(EDIT: in addition, a hanging warning sign would have to be suspended from something. My guess is any such structure would need to be made very sturdy and expensive, or else it would just get knocked down and increase the hazard to pedestrians and property when somebody hits the sign at significant speed.)
Placing the obstacle anywhere else would impede vehicles that aren't trying to go under the bridge.
Look at the video. Such a structure already exists. There is a yellow beam meant to protect the bridge right in front of it. It probably does most of the can openjng to spare the bridge
There's an intersection right before the bridge - the beam is already at the "warning" location to protect the bridge. Bridge strikes arent' cheap and train companies HATE closing bridges for any reason (including replacing them).
(EDIT: in addition, a hanging warning sign would have to be suspended from something. My guess is any such structure would need to be made very sturdy and expensive, or else it would just get knocked down and increase the hazard to pedestrians and property when somebody hits the sign at significant speed.)
Placing the obstacle anywhere else would impede vehicles that aren't trying to go under the bridge.