I my travels along mostly the I-5 corridor, this is what I hear for WSP Traffic and the Systems they use.
All Traffic can be heard on the VHF Conventional Channels.
King County uses the their 700MHz system, hear car to car chat on system
Pierce County uses the South Sound 911 System, hear car to car chat on system and sometimes the WSP 700Mhz system.
Thurston County uses the WSP 700MHz system, also hear car to car chat there.
Lewis and Cowlitz County can be heard on the JIWN system
Clack County is heard on the CRESA 911 system and their car to car chat
Snohomish County can be heard on the JIWN system north of 128th street in Everett
Skagit County heard on JIWN system sometimes and sometimes not.
Whatcom County heard on the JIWN system.
Kitsap/Mason County heard on the JIWN system.
Please feel free to post any other information that you might have to help all.