a unpleasure event with the city of government

We drove on the road in the day, the traffic signal light fallen down on our running car without any warning and destroyed the roof and wind shield. The police filed a report and told us this is absolute fault of city government and we can get reimbursement from them. However, when we contacted the city, they refused any reimbursement and claimed the city has immunity to any demage caused by public devices, they can not pay the reimbursement with public fund. We have asked a lawyer to deal with them for us, however the lawyer gave it up finally since the city is very difficult to deal with and it might take years and get nothing, as he commented.
Could anyone give any suggestion what we should do from here? It is really unfair to be treated like this!