Channel 19 is know as the "Universal Trucker or Highway Channel" but on certain interstates they are different. I do believe (it's been a while) that the north-south route of I-5 channel 17 is used. I'll try to dig through my old papers and see if I can come up with my cheat sheet I had.
Ok, found it. Channel 17 is used on N-S routes and 19 is on E-W routes. Back in the day, 17 was used from the Grapevine to Sacramento and 19 was to Reno and Tahoe. 21 was used through out Orange County.
Channel 4 seems to be used by quite a few off-road clubs. NW FJ Cruiser Club (WA-ID-OR) uses it.
http://www.wattora.com/cb4.htm
Hope this helps. You'll also find a lot of companies using channels. The company I work for uses channel 4 on the big road and channel 5 when we hit the yard.
I have a Hi/Lo switch that lets me split the channels so I get 80 channels. It's good when you're with someone and they can do the same. A secret channel eliminates the eaves-droppers.
Happy trails