Capacity is a central concept in roadway design and traffic control. The estimation of empirical capacity values in practical circumstances is not a trivial problem, not in the last place because it is very difficult to define capacity in an unambiguous manner. This report deals with empirical capacity estimation for uninterrupted roadway sections. Headways, traffic volumes, speed and density are traffic data types used to identify four groups of capacity estimation methods. Aspects such as data requirement, location choice and observation period were investigated for each method. Also, the principles of the distinguished methods and their mathematical derivation of roadway capacity are studied and discussed. Among others, the Headway Distribution approaches, the Bimodal Distribution Method, the Selected Maxima and Direct Probability Method are examined. Among the methods based on traffic volume counts, the Product Limit Method can be recommended for practical application because of the sound underlying theory. An example of the application of this promising method is shown. Our attempts to determine the validity of existing roadway capacity estimation methods resulted in a disappointing conclusion of the many ambiguities related to the derived capacity values and distributions. Estimating a reliable and meaningful capacity value seems not yet possible. Lack of a clear definition of the notion of capacity is the main hindrance in understanding what exactly represents the estimated capacity value or distribution in the various methods. If this deficiency is removed promising methods for practical employment in traffic engineering seem to be the Product Limit Method, the Empirical Distribution Method and the well-known Fundamental Diagram Method, in this order. The choice for a particular method strongly depends on the available data. (A)
Abstract