Good luck

Had it happen once started looking into it, and some of the things you have to prove are,
1 all roads in the area are rated on an importance level, each level needs a check every X weeks months etc dependant on importance. As long as it was checked within this criteria then they not liable.
2 If it has been reported by X amount of people and dependent on the priority of the road repaired within X time.
There were other get outs for em cant remember all of em
But even things like just cos it big and dangerous that means it big and you should have been able to see it and you avoid it

Hope you can get summit out of em

Some have so must be possible

Steve