Assessment of Roadway Capacity Estimation Methods

Abstract

Capacity is a central concept in roadway design and traffic control. Estimation of empirical capacity values in practical circumstances is not a trivial problem; it is very difficult to define capacity in an unambiguous manner. Empirical capacity estimation for uninterrupted roadway sections has been studied. 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. The principles of the different methods and the mathematical derivation of roadway capacity are studied and discussed. Among the methods studied are the headway distribution approaches, the bimodal distribution method, the selected maxima, and the direct probability method. Of the methods based on traffic volume counts, the product limit method is recommended for practical application because of sound underlying theory. An example of the application of this promising method is presented. Attempts to determine the validity of existing roadway capacity estimation methods were disappointing because of the many ambiguities related to the derived capacity values and distributions. A reliable and meaningful estimation of capacity is 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 corrected, promising methods for practical use in traffic engineering are the product limit method, the empirical distribution method, and the well-known fundamental diagram method, in that order. The choice of a particular method strongly depends on the available data.

Design and analysis of Full Range Adaptive Cruise Control with integrated collision a voidance strategy

“…Using the concept of equilibrium state detailed in (7) and the relationship between local density  (number of Table 1, we obtain a triangular fundamental diagram with a capacity of 2344 vehicles/hour/lane and critical density of 23 vehicle/km under a time gap setting of 1.2 s. Notice that these values are comparable to the capacity and critical density observed on highways [21].…”

Review of Studies on Mixed Traffic Flow: Perspective of Developing Economies

“…Knowledge of capacity is essential for a traffic engineer for the purposes of design, planning and operational analyses of the road sections. Realizing its importance, many researchers have proposed various direct and indirect empirical methods for its estimation [59]. While direct empirical methods require the observation of various traffic characteristics like headways, volumes, speeds and densities from real traffic data, indirect empirical methods require the same but from a simulation model.…”

An analysis of four methodologies for estimating highway capacity from ITS data

“…A number of authors [7][8][9] have shown that even under ''constant'' conditions, both the maximum traffic flow and the capacity can vary over time and space. Other authors [10] have argued that the actual maximum observed flow rate of the roadway is the best measure of capacity because it: (1) is closer to the definition of capacity listed in the HCM and (2) does not assume a prior speed-flow-density relationship. Referring to the example in Fig.…”

“…Minderhoud et al [10] compared several capacity estimation methods including the headway method, bimodal distribution method, selected maxima method, the fundamental diagram method, and the on-line procedure estimation method. The comparison was done based on theoretical characteristics and no field data was used.…”