SCRTD inherited routes that came through the first MTA from Los Angeles Transit Lines (successor to Los Angeles Railway) and Metropolitan Coach Lines (successor to Pacific Electric). Many of the lines carried the same line numbers with them since the days that they were first operated as streetcars and interurban rail lines, and those prior privately owned and operated systems did not coordinate their route numbers (and in some cases alpha letters as routes) with each other.
The planners of SCRTD created a new numbering system to finalize the movement of the entire bus system to a grid based plan. The grid layout changes began around 1974 and took years to complete. The prior routes was a maze of u-turns, circles, right angles and other alignments. After the grid was finalized (it was done it parts, San Fernando Valley, South Los Angeles, etc), a new numbering system was put in place to indicate to passengers the direction and destination of the lines. This was especially important to complete in advance of the 1984 Olympic Games and we still use it today.
Routes 1-99 indicate that the line travels to and from downtown
Routes 100-199 travel east/west in other areas
Routes 200-299 travel north/south in other areas
Routes 300-399 are Limited stop lines
Routes 400-499 are Express lines to and from downtown
Routes 500-599 are Express lines in other areas
Routes 600-699 are neighborhood shuttle lines
Routes 700-799 are Rapid lines
Routes 800-899 are Rail lines