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DESCRIPTION OF SIGNALIZED INTERSECTION CAPACITY ANALYSIS _. 1985 HCM OPERATIONS METHOD'S Background The operations method of intersection capacity analysis found in Chapter 9, "Signalized Intersections," of the Highway Capacity Manual, Special Report No. 209, Transportation Research Board, 1985, was used for this study. This method is used in most analyses of existing concliiions or of future situations in which traffic, geometric, and control parameters were well established by projections and trial designs. This method addresses the capacity and level of service of intersection approaches, and the level of service of the intersection as a whole. In this method, capacity and level of service are evaluated separately, and are not related. to each other in a simple one-to-one fashion. Capacity is evaluated in terms of the ratio of demand flow rate to capacity (volume-to-capacity ratio), while level of service is evaluated on the basis of average stopped delay per vehicle (sec/veh). The capacity of the intersection as a whole is not addressed by this method; the design and signalization of intersections focuses on the accommodation of the major movements and approaches comprising the intersection. Capacity is, therefore, only meaningful as applied to these major movements and approaches. Capacity analysis results in the computation of volume-to- capacity ratios for individual movements and a composite volume-to-capacity ratio for the sum of critical movements or lane groups within the intersection. The volume-to-capacity ratio is the actual or projected rate of flow on an approach or designated group of lanes during a peak 15- minute interval divided by the capacity of the approach or designated group of lanes. Level of Service Level of service is based on the average stopped delay per vehicle for various movements within the intersection. While volume-to-capacity affects delay, there are other parameters that more strongly affect it, such as the quality of progression, length of green phases, cycle lengths, and others. Thus for any given volume-to-capacity ratio, a range of delay values may result, and vice- versa. See the table "Level of Service Criteria for Signalized Intersections" for the relationship between the level of service and stopped delay per vehicle. Because delay is a complex measure, its relationship to capacity is also complex. It is possible, for example, to have delays in the range of Level of Service F while the volume-to-capacity ratios is below 1.00, perhaps as low as 0.75-0.85. Very high delays can occur at such volume-to- capacity ratios when some combination of the following conditions exists: the cycle length is long; the lane group in question has a long red time; and/or the signal progression for the subject movement is poor. The reverse is also possible. A saturated approach or lane group with avolume-to-capacity equal to 1.00 may have low delays if the cycle length is short, and/or the signal progression is favorable for the subject movement. Acceptable delays levels do not automatically ensure that capacity is sufficient. The analysis must consider the results of the capacity analysis module and- the level of service module to obtain a complete picture of existing or projected intersection operations. ~' w~~1 ~nq-daEed,. ~5