The interest owed on the bridge is massive,and unless paid off for in tolls,will be put on every council tax bill from, I think,2038.
As for Castle Street,the simple solution is get rid of all the traffic lights andhave all traffic filtering left only to use Garison Street roundabout to the east,and Daltry Street roundabout to the west to the other way.
Then build two or three new pedestrian bridges,and you have no traffic lights and freeflow Castle Street at very little costs and no long building delays.The cut and cover will take years,and all the traffic would have to use Hull city centre.That was gridlocked during the round the world return for half a day.Think about that for four years!