Design capacity is the maximum volume of traffic that a proposed intersection would be able to serve without congestion rising above a preselected level, generally the level of service (LOS).
LOS can be calculated for roadway segments or intersections. LOS calculations for a roadway segment is appropriate when a roadway lacks stop signs and traffic signals that stop the flow of traffic. This generally includes freeways and arterials. Where a roadway has traffic signals or stop signs, LOS for intersections governs.
There are six levels of service classifications, "A" through "F." "A" represents the best, "F" represents the worst. LOS "A" and "B" are considered to be good. LOS "C" and "D" are considered to be acceptable. LOS "E" and "F" are considered to be unacceptable.