I do not find this suggestion as necessary. Staff with access to the police car should not be blocking or closing roads under any circumstances.
The police cars are not needed to do our job, it is more of an added bonus for the hard working staff that have access to use the car.
I also agree that this would not be a good way to get messages across to players (use Real Ops as an example). I feel that typing in chat is more effective as it can be seen by anyone in the area and with the names being red in chat, they are hard to miss. A warning sign on the light bar would mean that a player would have to be fairly close to the police car, which means that it can be easily missed by said players.