I have a GRE PSR-600 installed in our "out of town" car. There are 20 files of 1800 frequencies or talkgroups in each file. We have relatives in the Bay Area, southern California, Reno, and the Phoenix. I have written files for each area with enroute frequencies/talkgroups as well. I have other files for less frequented areas. I have scan lists set up with the highway patrol and DOT's near the top, then each county in the order I travel them. I make the highway patrol and DOT's separate scan lists (banks) because I want to make sure I hear everything on them and can turn off the local county banks when the highway radio traffic gets busy. This is because the highway traffic is most likely to affect our drive.
I live in Mammoth Lakes, CA and have an eastern Sierra file, followed by a California Desert, file, then a Los Angeles file for trips that way, and a southern Arizona file if we head to Phoenix. When I travel to the Bay Area I have one file for the western slope Sierra Nevada and central valley south counties, then a Bay Area file when I reach that point. I have the CHP and Caltrans in separate banks near the top.
I've written each file with geographical trigger points that are normally a county or state line, with some overlap in the highway lists so I can receive traffic from the next area ahead. An example is in my California Desert file in the highway list (bank) I've included the western most district of the Arizona DOT, as I can start receiving them starting at the summit between Indio and Blythe.
I hope this helps.