These levels of service, designated A through F, from best to worst, cover the entire range of traffic operations that may occur. The factors that may be considered in evaluating level of service include the following. However, in a practical approach to identifying the level of service, travel time and the ratio of demand volume to capacity are commonly used. Traffic density is low, with speeds controlled by driver desires, speed limits, and physical roadway conditions. There is little or no restriction in maneuverability due to presence of other vehicles, and drivers can maintain their desired speeds with little or no delay.
Drivers still have reasonable freedom to select their speed and lane of operation.
Reductions in speed are not unreasonable, with a low probability of traffic flow being restricted. Most of the drivers are restricted in their freedom to select their own speed, change lanes, or pass. A relatively satisfactory operating speed is still obtained, with service volumes perhaps suitable for urban design practice. Fluctuations in volume and temporary restrictions to flow may cause substantial drops in operating speeds. Drivers have little freedom to maneuver, and comfort and convenience are low, but conditions can be tolerated for short periods of time.