The 2010 Highway Capacity Manual (HCM) defines Level of Service (LOS) for signalized intersections as a function of the average vehicle control delay. LOS may be calculated per movement or per approach for any intersection configuration, but LOS for the intersection as a whole is only defined for signalized and all-way stop configurations. The HCM recommends using delay LOS when determining a systems-based analysis for signalized intersections.
This "delay-LOS" method is used as one of the primary performance measures for identifying areas of congestion within the region. Intersection LOS is collected through a variety of sources. All measures are done for a 2-3 hour period, covering the most common peak period for weekday traffic (6-9am or 7-9am) for the AM period and (3-6pm or 4-7pm) for the PM period.
This method focuses on "raw" intersection capacity, that is, the ability for an intersection to process a given traffic demand (volume) with a given lane use configuration and given phase sequence. It is a more simple, hands-on approach to get right to the point of an intersection's ability to handle traffic demands. The critical movement summation method looks at each of the "critical" movements at an intersection. It is a volume-based measure.
At signalized intersections, capacity for a particular movement is defined by two elements: the maximum rate at which vehicles can pass through a given point in an hour under prevailing conditions, and the ratio of time during which vehicles may enter the intersection.