The terms used for the different degrees of highway capacity are indicated in Section E 120, Definitions. The “Highway Capacity Manual” relates the ability of a highway to carry traffic to two levels: practical capacity and possible capacity. AASHTO accepted this terminology and introduced an additional term, design capacity. The "Highway Capacity Manual” has since eliminated the terms practical capacity and design capacity and substituted the single word capacity for what had been referred to as possible capacity. The “Highway Capacity Manual” also introduced the level of service concept. The six levels of service which are shown in Section E 654, Signalized Intersection Capacity, are a qualitative measure of operating conditions from excellent to intolerable, including capacity. AASHTO’s “A Policy on Geometric Design of Rural Highways,” 1965 Edition, refers to the “Highway Capacity Manual,” 1965 Edition, for basic values but continues to use of the term design capacity and possible capacity.
Although the terminology is different, the overall concepts in each publication are compatible. For example, the AASHTO design capacity, Section E 020F(1b), is the same in essence as the ‘Highway Capacity Manual,” Section E maximum service volume for a selected level of service. Also, numerically, the AASHTO term possible capacity is identical to the “Highway Capacity Manual” term capacity. Therefore, the relations presented on the figures in Section E 650, Intersections at Grade, are equally applicable to the "Highway Capacity Manual,” Section E 020F(3a), the “Highway Capacity Manual,” Section E 020F(4a), and the AASHO procedure, Section E 020F(1b).
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