A: Free flow, with low volumes and high speeds.
B: Stable flow, speeds beginning to be restricted by traffic conditions.
C: In stable flow zone, but most drivers are restricted in freedom to
select own speed.
D: Approaching unstable flow; drivers have little freedom to maneuver.
E: Unstable flow; short stoppages may occur.
F: Forced or breakdown flow.
Source: Location and Design Manual, Vol. 1, Roadway Design, Ohio
Department of Transportation, with permission.