Here is some info if you are driving on the West Coast.
In California, Oregon and Washington we do it a little different and you wont find us on 19 in some areas.
Here is the breakdown as I know it. Truckers use channel 15 from LA to Lost Hills Hwy 46 on the I-5.
They then go to channel 17 from lost hills HWY 46 all the way through Oregon and Washington State.
HWY 99, and 97 use channel 17 also, all the way up through Washington State.
LA to San Diego used to use channel 21, don't know if they do anymore.
In all other areas in the US they use channel 19. Including I-15 throughout California.