Easy, there is only one way they will listen.  "slip and fall" and file a police report.  Demand that the police document this, it is typically called an IR (incident report).  In the IR request that the DPW be issued a copy.  Make sure in the IR that it states that you are requesting that the DPW get a copy.  Most cops are friends with the DPW and if it is not in the IR they will "forget".