Transportation Research Board - Level of Service Definitions. Use your browser's back button or click here to return to the table.
	Level of Service Definitions - Level of Service Rating - Definition A - Free flow; insignificant delays. B - Stable operations; minimal delays. C - Stable operations; acceptable delays. D - Approaching unstable; queues develop rapidly but no excessive delays. E - Unstable flow; significant delays. F - Forced flow; low operating speeds. Source: Transportation Research Board 1994