Introduction

Capacity, in its broadest sense refers to the maximum vehicle carrying ability of a transportation facility. Two terms in the above definition needs further clarification; they are "vehicle carrying ability" and "transportation facility."

Vehicle carrying ability typically implies the number of vehicles that can be "processed" or "transported." In other words it is the flow , q . Hence, capacity of a facility is the maximum flow at the facility. A transportation facility is any engineering structure created for the purpose of transporting people or goods from one place to another. Examples, of such facilities include: (i) multi-lane expressways, (ii) two-lane roads, (iii) interchanges, (iv) signalized intersections, (v) unsignalized intersections, and others. Typically, one transportation facility differs from another in the way vehicular streams behave at these facilities. From an engineering standpoint it is imperative that one knows what the capacity of a particular facility is; only then will one be able to design a facility which is adequate for a given demand (which typically would be in units of flow ).

In the rest of this module, the concepts of capacity and level-of-service are presented under three sub-headings: (i) capacity for facilities with uninterrupted traffic flow, (ii) capacity at intersections, and (iii) level of service. Further, the intention of these lectures is not to provide details of how capacity is computed but to bring forward the factors that impact capacity and the principles on which capacity computations should be based. The reason for not going into specific details of how capacity is computed is that Indian provisions for capacity computations are still in its infancy and codes in this regard are still in their formative stages