We had a similar situation in Royston where the train line cut the town in half. It took over ten years to get BR to accept responsibility and many more years to get an acceptable solution out of them :-
The key was forcing an admission of liability - in your case it sounds like the highways department for making the bridge unsafe for pedestrians in the first place.
Once liability has been admitted they are obliged to make good as they would be held responsible for any resultant death or injury.
Don't know if this will be much help, but if your legal system recognises vicarious liability, any pedestrian who is injured by a motor vehicle whilst crossing the bridge could sue the roads department, the council and/or the state on grounds of negligence and failing in their duty of care.
OK, I'm stretching it a bit on that duty of care; BUT if the person who is injured is a public employee..........