said by moonpuppy 
:I will add on to this.
Not every system works in every area the same way.
While 800Mhz is nice, it is not a long distance thing unless you have relatively flat terrain.
Another issue with "trunking" radios is that they are heavily dependant on computers. Each one acts almost like a cell phone when it is on the network. A dispatcher can even change someones radio over to another channel so that everyone working the same incident can be away from everyone else. Computer goes down, problems come up.
A national system, in my opinion, is not that feasible. Trying to find one set of frequencies that work well everywhere is going to be hard. Cell phones still have spotty coverage and in areas where there isn't much, some public agencies still use VHF-lo band stuff.
The way the network I have worked on work is there is actually an intelligent man in the middle that sends the same signal on another band in the different area. It uses ip and private fiber to bring it to that point if it has no relay.
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