Prior to the formation of the RTA and Pace, the private suburban bus routes usually were not identified by route number. Most bus companies mentioned route numbers in the printed schedules, but the Evanston Bus Company was possibly the only company to actually use route numbers on its destination signs. But the Evanston Bus Company closed down in 1973, and the CTA began operating 4 bus routes there prior to the RTA's creation.

Shortly after its creation, the RTA adopted a universal numbering system for all suburban bus routes, which eventually became the Pace system. All route numbers would have 3 digits, above the range of numbers already in use by the CTA. Usually, the last digit or two would be the same as the private companies' route numbers.

Groups of numbers were originally assigned to each private company, which continued to operate the routes under the RTA name. And groups of numbers were assigned to routes established in the mid 1970's, originally sponsored by various individual suburbs and mainly operating during rush hours to serve commuter rail stations.

The RTA issued its first system map in November, 1976, showing all bus and train routes in the 6 county area. The second RTA system map was issued in June, 1978. And by then, the RTA had expanded its bus system into new areas, usually contracting with charter or school bus operators to provide service.

The following is the basic original bus route numbering system described in the first RTA system map, along with the service expansions included in the second system map. Many of the original groups of route numbers still remain intact. For those route number groups still intact, the present ranges of route numbers are indicated.

1-199 CTA
200-204 CTA-Evanston
204-291 Nortran (formerly United Motor Coach, now Pace Northwest Division)
301-336 West Towns (now Pace West Division)
347-372 South Suburban Safeway (now Pace South Division)
379-387 Suburban Transit System (now Pace Southwest Division)
400 Bensenville (dial-a-ride)
401-402 Park Forest South (now Pace South Division route 367-University Park)
411-413 Niles
421-426 Wilmette (now Pace North Shore Division)
430-434 Wheeling (service since discontinued)
440-442 Melrose Park (service since discontinued)
451-460 Homewood (service since discontinued)
461-466 Downers Grove (for many years operated independently of Pace)
470-476 Highland Park
480 Elmwood Park (dial-a-ride)
490 Maywood (dial-a-ride)
501-512 Joliet (now Pace Heritage Division)
521-540 Aurora (now Pace Fox Valley Division)
541-558 Elgin (now Pace River Division)
561-582 Waukegan/North Chicago (now Pace North Division)
601-604 Northwestern Transit (only 602 remains, operated under contract)
611 Great Lakes Bus Lines Inc. (Libertyville-Mundelein, discontinued)
621-622 Northbrook (service since discontinued)
631 Elk Grove Village (dial-a-ride)
641-646 Elmhurst (service since discontinued)
651-658 Glen Ellyn (service since discontinued)
661-665 Westmont (now operated under contract)
671-676 Lombard (only 674 remains, operated under contract)
672-689 Naperville (newer routes/now operated under contract)
680-689 Naperville (now operated under contract)
690 Buffalo Grove (service since discontinued)
691-699 Various Northwest Cook County, created after 1978
700-704 Villa Park, created 1977 (service since discontinued)
705-716 Wheaton, created 1977-1979 (now operated under contract)
720-724 Palatine, created 1978 (service since discontinued)
725-729 Barrington, created 1978 (service since discontinued)
750-753 Various south suburban feeder routes, created 1978-1979 (service since discontinued)
801-802 Fox River Valley Service (now operated by Pace)
803-806 Various far west/northwest suburban routes, created 1978
807-808 Worts Transit (McHenry County, created 1978, now operated by First Group)
811-812 Country Bus (experimental routes in Kane County)
819-830 Lisle/Bolingbrook (now operated under contract)
831-832 S.W.I.F.T./Valley Transit (service discontinued)