Beginning in 1921, intrastate bus service was regulated by the Washington Department of Public Works. Information from annual reports is available through 1928.
Over many routes, more than one operator was initially certified in 1921. But many duplicate operations were discontinued. Most notably from Seattle south to Tacoma and north to Everett. Initially, those routes had many individual operators attempting to operate over coordinated schedules. But in December 1921, Seattle-Tacoma Union Stage Line, and Park Auto Transportation Co. operating north to Everett, emerged as the only operators over those routes.
Certificates Granted 1921/First Half
Certificates Granted 1921/Second Half
Certificates Granted 1922-1924
Some operators initially carried freight only, later the certificates were amended to permit passengers. These annual reports contained various errors and omissions, every effort was made to resolve these inconsistencies.
Certificates Granted 1924 Summary
Recapping the above three links, accounting for mergers and elimination of multiple operators.
Certificates Granted 1925-1928