Stations are calculated at distances where interference is not likely to occur.
Think about this,
1st any federal agency can be granted permission from the FCC to use frequencies in almost any way that they desire, and what I mean by this is on a normal repeater set up your mobiles and portables will transmit high (the repeater RECEIVES HIGH) and repeats usually 5 mhz lower and if the dispatcher talks this goes out 5 mhz Lower so therefore the radios receive 5 mhz Lower, however they may be doing just the opposite, Transmitting Low and receiving high.
Also with new radios the splits can be 15khz, 12.5 khz, 6.2 khz, or 5 khz, the most common is 12.5 khz split but again these agencies can basically set up any way they want, also keep in mind anything that is remotely sensitive is most definitly going to be encrypted, so either you will hear what sounds like an open carrier (Completely Digitally Encrypted) or if it sounds like "White noise" the voice is digitally encrypted, in either case don't waste your time trying to listen to it.
I hope that this basic information was helpful to you.