Excerpt from the Highway Capacity Manual 2000. By the Transportation Reasearch Board, National Academy of Sciences. Use your browser's back button or click here to return to the table.
	SERVICE FLOW RATES       The analytical methods in this manual attempt to establish or predict the maximum flow rate for various facilities at each level of service—except for LOS F, for which the flows are unstable or the vehicle delay is high. Thus, each facility has five service flow rates, one for each level of service (A through E). For LOS F, it is difficult to predict flow due to stop-and-start conditions.       The service flow rate is the maximum hourly rate at which persons or vehicles reasonably can be expected to traverse a point or uniform segment of a lane or roadway during a given period under prevailing roadway, traffic, and control conditions while maintaining a designated level of service. The service flow rates generally are based on a 15-min period. Typically, the hourly service flow rate is defined as four times the peak 15-min volume.       Note that service flow rates are discrete values, whereas levels of service represent a range of conditions. Because the service flow rates are the maximums for each level of service, they effectively define the flow boundaries between levels of service.       Most design or planning efforts typically use service flow rates at LOS C or D, to ensure an acceptable operating service for facility users.