Just saw this post and have to throw in my two cents. It is indeed a 500 Mhz P25 system. It has been in place for a number of years and run/maintained by the Mercer County Office of Emergency Management. All of the PDs, Hospitals, Colleges, and Lifecomm / Central / Sheriffs were on it. In the past few years it was decided that it would be reallocated for a much wider use and include all emergency responders. This would be Fire, EMS, Police, and Public Works*. The existing system was not robust enough to handle that much use, so it was upgraded. In fact the actually switch over to the "new" system happened a few weeks ago. It is currently being tested by OEM and Cassidian, the manufacturer. The information in the database here is somewhat correct, but the new controllers have different site IDs. Testing is mainly being done on TG 10, but I've seen 20, 21, and 40 pop up. 500.9750 is the current control channel even though the database doesn't have it listed as one. There will actually be 3 sites. The Ch52 tower (NJN) in Lawrence, the Highstown water tower, and the Ewing PD tower (This will also be a backup system controller).
I can only speak for the fire service, but once ready, it will be used for response and command chatter. All on scene operations will take place on conventional UHF frequencies. This is very similar to Burlington County. The differences: Central will be able to monitor the fireground channels (monitor, not transmit back) and the channels will be assigned geographically, not like the standard Ops1, Ops2, and so on in Burlington.
The actual transition will happen in two phases.
Phase one: Response will move the P25, on scene operations will stay on the VHF channels.
Phase two: On scene ops will move the UHF conventional channels.
Paging will still be done on the VHF Fire-1 of 154.4300.
It you're really itching for more information, here is a link to the presentation that was given to all of the fire service officers from the fire academy's website.
*The County DOT will not be on the P25 system as they already switched to their own MotoTRBO system. I don't believe it is in the database, nor do I care to look for it.