Assessment of Impact of Roadside Frictions on Passenger Car Unit Values and Capacity of Urban Roads in Disordered Traffic Using Microscopic Simulation Model

Abstract

Representation of traffic in terms of its car equivalences (Passenger Car Unit) is more appropriate to estimate capacity in disordered traffic due to the presence of several vehicle types with varying static and dynamic characteristics following poor lane discipline. Many attempts have been made to overcome the complexities involved in accurate estimation of Passenger Car Unit (PCU) in disordered traffic. The widely used method for PCU estimation considers the relative speed and projected area (length × width) of vehicles. However, as a vehicle will be influenced by a larger area than its projected area, which is proportionate to the surrounding vehicle types (effective area of a vehicle); this study aims to deal with the influence of surrounding vehicles while estimating its PCU value under disordered traffic condition. PCU values are influenced by various factors such as traffic conditions, geometric conditions, road side frictions, etc. Among these factors, roadside frictions (e.g., curbside bus stops, undesignated pedestrian crossings, roadside parking) cause significant deterioration in the quality of urban traffic flow and thus, considerably influence road capacity which in turn affects the PCU values. Many research works have been carried out to investigate the impacts of some of the influencing factors (e.g., vehicular composition, traffic volume, road width) on traffic characteristics as well as PCU values. However, the sensitivity of PCU values and capacity due to the presence of roadside frictions are not adequately studied. Furthermore, PCU values and capacity for urban roads recommended by the existing manuals (e.g., IRC 106 1990) are applicable only for the ideal/base sections i.e., the section devoid of any side frictions. The recently published highway capacity manual (Indo-HCM 2017) suggests adjustment factors for capacity estimation of roads with the presence of a few side frictions (e.g., parking, access points, bus stops), however, the adjustment factors for estimating PCU values are not suggested. To address these gaps, this research study aims to estimate PCU values for vehicles under the influence of curbside bus stop and undesignated pedestrian crossings which are the most common roadside frictions being observed in developing countries. As the change in PCU values will have an influence on capacity as well, it is essential to study the impact of curbside bus stop and undesignated pedestrian crossings on capacity. Lack of space for providing ii exclusive bus bays and higher demand for public transport buses in urban roads justify the need for investigating the influence of curbside bus stop. The non-compliant behaviour of pedestrians at undesignated pedestrian crossings creates complex vehicle-pedestrian interactions which affect the capacity of roads and thus, this justifies the need for studying the influence of pedestrians. Furthermore, the influence of side frictions on PCU values and capacity are not considered in the existing studies which used simulation tools. Hence, this study mainly aims at development of a simulation model to examine the influence of these side frictions on PCU values as well as capacity. Methodology of this study involves development and validation of a microscopic simulation model for ideal section (base model) using the data collected from Bangalore city, India. To study the impact of side frictions on PCU values and capacity, two different urban divided arterials are selected; one with the presence of curbside bus stop and an ideal section near it (Ideal_bus), and another with undesignated pedestrian crossings and an ideal section near it (Ideal_ped). Logics involved in base model development are formulated and implemented in MATLAB using object-oriented programming concepts. To estimate the PCU values for different vehicles, a new methodology considering the influence of surrounding vehicles is proposed and incorporated in the validated model. The base validated model is then modified to simulate the traffic manoeuvers on urban roads in the presence of curbside bus stop (bus stop model). To study the vehicle-pedestrian interactions in disordered traffic, the base model is modified in such a way that the movement logics of vehicles and pedestrians consider vehicle-pedestrian interactions (vehicle-pedestrian interaction model). The relative influences of various parameters such as traffic volume, vehicular composition, bus proportion (applicable only for bus stop section), proportion of stopping buses (applicable only for bus stop section), and pedestrian volume (applicable only for pedestrian section) in the presence of side frictions are investigated by carrying out sensitivity analysis. With simulated results from sensitivity analysis, regression models are developed to predict PCU values for different types of vehicles and capacity of road sections, with and without curbside bus stop, and with and without undesignated pedestrian crossings. iii The simulated results of sensitivity analysis indicate the significant differences in PCU values due to the presence of curbside bus stops when vehicular composition, bus proportion, proportion of stopping buses, and traffic volume are varied. For observed vehicular composition and traffic volume, simulated PCU values for bus showed a drastic increment of 28% in bus stop section when compared to that of ideal section. As curbside bus stops create temporal pseudo bottlenecks, capacity of bus stop section significantly gets reduced by 18% from that of ideal section for observed vehicular composition. Due to the impact of undesignated pedestrian crossings, the capacity reduction in pedestrian section when compared to that of ideal section is found to be 19.1% for observed vehicular composition and pedestrian volume. PCU values for vehicles are also found to have significant variations with change in vehicular composition, traffic volume and pedestrian volume. The study findings and results can be used by traffic engineers and planners to predict realistic PCU and capacity values for planning and designing of new facilities with side frictions instead of directly adopting the values available in the existing manuals. The study results find interesting implications in updating standards related to PCU and capacity estimation considering the influence of curbside bus stops and undesignated pedestrian crossings. In future, this research can be extended to study the impact of curbside bus stops, undesignated pedestrian crossings and other roadside frictions (e.g., parking, encroachments) on PCU values and capacity of different facility types (e.g., urban undivided roads, rural roads, intersections) by modifying the logics in the simulation model. The bus stop model can be further modified to simulate the traffic manoeuvers in sections with bus bays and exclusive bus lanes. The model describing the vehicle-pedestrian interactions can be modified to determine surrogate safety measures (e.g., time-to-collision between vehicles and pedestrians) that reflect the safety of urban roads (where the presence of pedestrians is significant).