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4., Level of Service D - Approaches unstable flow, with <br />tolerable operating speeds being maintained though <br />considerably affected by changes in operating <br />conditions. Fluctuations in volumes and temporary <br />restrictions to flow may cause substantial drops in <br />operating speeds. Drivers have little freedom to <br />maneuver, and comfort and convenience are low, but <br />conditions can be tolerated for short periods of <br />time. <br />5. Level of Service E - Represents operations at even <br />lower operating speeds than in Level D, with <br />volumes at or near the capacity of the Highway. At <br />capacity, speeds are typically, but not always in <br />the neighborhood of 30 mph. Flow is unstable, and <br />there may be stoppages of momentary duration. <br />6. Level of Service F - Forced flow operations at low <br />speeds, where volumes are below capacity. These <br />conditions usually result from lines of vehicles <br />backing up from a restriction downstream. The <br />section under study will be serving as a storage <br />area during parts or all of the peak hour. Speeds <br />are reduced substantially and stoppages may occur <br />for short or long periods of time because of <br />downstream congestion. In the extreme, both speed <br />and volume can be dropped to zero. <br />1. Highway Capacity Manual, Highway Research Board <br />Special Report 209, 1985 <br />56 <br />