Effect of vehicle composition and delay on roundabout capacity under mixed traffic conditions (original) (raw)

Capacity Estimation Approaches for Roundabouts: A Review

The vehicle ownership is increasing day-by-day with the population growth. The traffic scenario is also changing along with this change. Roundabout has been widely adopted alternative as a type of intersection in many parts of the world due to its convenience for operation with lesser conflict points than the other unsignalized intersections. The capacity of roundabout intersection is the important or we can say key parameter to check the performance of roundabout intersection. The capacity of the roadway roundabout depends on the flow at different legs approaching the roundabout. The performance of roundabouts in terms of queues, delays and saturation index typically depends on entry capacity. This paper discusses and reviews the capacity estimation of roundabout which have done in past by various methods by different research scholars from various parts of the world. This study can contribute to an improved understanding of one of the parameter of performance of roundabout i.e. capacity of roundabout.

Comparative evaluation of roundabout capacities under heterogeneous traffic conditions

Journal of Modern Transportation, 2015

In heterogeneous traffic conditions, roundabout capacity is described by vehicle and driver characteristics which are different from traffic conditions in homogeneous conditions. In the present study, the capacity of the roundabout is determined using various capacity formulas such as gap acceptance models given by Highway Capacity Manual 2010 (US), German model (2001); empirical regression models given by TRRL (UK) and weaving models given by IRC: 65-1976 (India). In addition, microscopic simulation model like VISSIM (PTV Germany) is also used to derive capacity values. Unlike the other capacity estimation models, VISSIM is helpful in estimating capacity values using geometric and driver characteristics and it can also simulate heterogeneous traffic condition accurately. Capacity is estimated after calibrating the simulation model (VISSIM) developed for the roundabout. This is achieved by incorporating different vehicle classes to represent the heterogeneous traffic environment, driver gap acceptance, and lane change parameters. All the required inputs were extracted from the video using semi-automatic data collection methods. Data are used for the estimation of capacity values from different methods mentioned above and for the calibration and validation of simulation model. The capacity values estimated form various formulas except German model are distinctly different from the field values and they are either overestimating or underestimating. Analysis of these observations reveals that the capacity values from VISSIM and German models are nearly matching with the field capacity.

Investigating Entry Capacity Models of Roundabouts under Heterogeneous Traffic Conditions

Transportation Research Record, TRB, SAGE Publication, 2018

The primary objectives of this study are to develop two roundabout entry capacity models using a regression-based multiple non-linear regression model (MNLR) and artificial intelligence (AI)-based ANFIS (adaptive neuro-fuzzy inference system) model under heterogeneous traffic conditions. ANFIS is the latest technique in the field of AI that integrates both neural networks and fuzzy logic principles in a single framework. Required data have been collected from 27 roundabouts in eight states of India. To assess the significance of these models and select the best model among them, modified rank index is applied in this study. The coefficient of determination (R 2) and Nash-Sutcliffe model efficiency coefficient 'E' values are found to be 0.92, 0.91 and 0.98, 0.98 for the MNLR and ANFIS model, respectively. The ANFIS model is found to be the best model in this study. However, from a practical point of view, the MNLR model is recommended for determining roundabout entry capacity under heterogeneous traffic conditions. Sensitivity analysis reports that critical gap is the prime variable and shares 18.43% for the development of roundabout entry capacity. As compared with the Girabase formula (France), Brilon wu formula (Germany), and HCM 2010 models, the proposed MNLR model is quite reliable under low to medium ranges of traffic volumes. Roundabouts are a specialized form of an un-signalized intersection in which traffic moves in one direction around a central island. Direction of traffic, according to left-hand or right-hand driving rules, depends on the specific country. Roundabouts have been used as an effective intersection control measure to improve safety and operational performance under low to high circulating traffic flow conditions. As compared with other, signa-lized, intersections, roundabouts have several basic advantages, such as speed reduction of vehicles, less delay, pedestrian safety, aesthetics, and reduction of conflict points. A signalized intersection has 32 conflict points, whereas a roundabout with two circulating and entry lanes has 8 and 16 conflict points, respectively. Because of the increase in the use of roundabouts, an assessment regarding operational performance needs to be undertaken. Evaluation of capacity at roundabouts is an important aspect of operational performance that describes the present traffic scenario, level of service, delay, accidents, operational cost, and environmental issues. Under heterogeneous traffic conditions, vehicles of various kinds have varying physical and operational characteristics. In developing countries, driver behavior, vehicular characteristics (size, engine efficiency, and years of usage), and intersection geometric design vary significantly. The vehicle fleet is composed of heavy vehicles (HV), light motor vehicles (LMV), motor cycles/ scooters (MC/S), bicycles (BC) and human-drawn vehicles (HDV). These vehicles share the main carriageway without any physical or marked segregation. When there are vehicles of different types with varying static and dynamic characteristics, follow-the-lane discipline and the follow-the-leader concept become irrelevant. Under constraints of flow and space, small-sized vehicles try to move into the circulating area without following lane discipline. Also, HV forcibly merge into the circulating stream of traffic. These points reflect that the rule of priority is not respected under such a heterogeneous traffic condition. In these circumstances, the study of

Performance Analysis of Roundabouts under Mixed Traffic Flow Conditions

The capacity of the roundabout mainly depends on the flow at different entry legs approaching the roundabout. The critical gap is a crucial input parameter which mainly gives the idea about the gap acceptability behavior of vehicle's driver. The performance of roundabout or rotary intersection in terms of queue lengths, avg. control delays and degree of saturation typically depends on entry capacity. Two roundabout intersections with different geometric characteristics are taken up for performance analysis. Different modes of transportation pass through these junctions. These roundabout intersections are working under mixed traffic flow conditions. All approach entry capacity has been analyzed for both roundabouts at their entry legs. Entry capacity verses circulating flow relationship have been developed & entry capacities evaluated by different models are compared in this paper which shows the behavior and variations of each entry capacity models in our condition. This paper deals with the performance of roundabouts which is based on various parameters such as critical headway, follow-up headway, entry (approach) capacity, circulating traffic, and average control delay. Capacity analysis results indicate that most of the approaches of Medical Square roundabout has greater than 0.85 degree of saturation and these approaches are critical for traffic flow & all entry approaches of Krida Square roundabout has degree of saturation under 0.85.

Comparative Evaluation of Roundabout Capacities Methods for Single-lane and Multi-lane Roundabout

Journal of Engineering

A roundabout is a highway engineering concept meant to calm traffic, increase safety, reduce stop-and-go travel, reduce accidents and congestion, and decrease traffic delays. It is circular and facilitates one-way traffic flow around a central point. The first part of this study evaluated the principles and methods used to compare the capacity methods of roundabouts with different traffic conditions and geometric configurations. These methods include gap acceptance, empirical, and simulation software methods. Previous studies mentioned in this research used various methods and other new models developed by several researchers. However, this paper's main aim is to compare different roundabout capacity models for acceptable capacity predictions for single-lane and multi-lane roundabouts. In conclusion, previous studies such as RODEL, SIDRA, Swiss, HCM6, and IRC overestimate capacity, while the GHCM method underestimates it. Each VISSIM and SIDRA predicted higher capacity than HCM2...

Analysis of Non-Conventional Roundabout Performance in Mixed Traffic Conditions

2020

Traffic congestion is the most critical issue faced by those in the transportation profession today. Over the past few years, roundabouts have been recognized as a measure to promote efficiency at intersections globally. In developing countries like India, this type of intersection still faces a lot of issues, such as bottleneck situations, long queues and increased waiting times, due to increasing traffic which in turn affect the performance of the entire urban network. This research is a case study of a non-conventional roundabout, in terms of geometric design, in a small town in India. These types of roundabouts should be analyzed for their functionality in mixed traffic conditions, prevalent in many developing countries. Microscopic traffic simulation is an effective tool to analyze traffic conditions and estimate various measures of operational performance of intersections such as capacity, vehicle delay, queue length and Level of Service (LOS) of urban roadway network. This study involves analyzation of an unsymmetrical non-circular 6-legged roundabout known as "Kala Aam Chauraha" in a small town Bulandshahr in Uttar Pradesh, India using VISSIM simulation package which is the most widely used software for microscopic traffic simulation. For coding in VISSIM, data are collected from the site during morning and evening peak hours of a weekday and then analyzed for base model building. The model is calibrated on driving behavior and vehicle parameters and an optimal set of calibrated parameters is obtained followed by validation of the model to obtain the base model which can replicate the real field conditions. This calibrated and validated model is then used to analyze the prevailing operational traffic performance of the roundabout which is then compared with a proposed alternative to improve efficiency of roundabout network and to accommodate pedestrians in the geometry. The study results show that the alternative proposed is an advantage over the present roundabout as it considerably reduces congestion, vehicle delay and queue length and hence, successfully improves roundabout performance without compromising on pedestrian safety. The study proposes similar designs for modification of existing non-conventional roundabouts experiencing excessive delays and queues in order to improve their efficiency especially in the case of developing countries. From this study, it can be concluded that there is a need to improve the current geometry of such roundabouts to ensure better traffic performance and safety of drivers and pedestrians negotiating the intersection and hence this proposal may be considered as a best fit.

Geometric factors influencing entry capacity of roundabouts under heterogeneous traffic conditions

Archives of Transport

Roundabout entry capacity is influenced by geometric features of the roundabout, traffic flow characteristics, vehicle and driver characteristics, as well as, environmental conditions. The major methods for estimating roundabout entry capacity are based on either gap acceptance theory or on empirical relations. Roundabout geometry is the sole aspect which can be entirely manipulated by the designers to improve the entry capacity. Limited studies have been conducted to analyze the influence of geometric elements of a roundabout on its capacity, for heterogeneous traffic conditions. Many developing countries like India, Malaysia, Indonesia etc., have heterogeneous traffic conditions on their roads. Data is collected from twenty-one entries of six roundabouts, where heterogeneity in traffic is observed. Seven different vehicle categories are considered such as motorised two wheeler, three wheeler, car, mini bus, light commercial vehicle (LCV), heavy commercial vehicle (HCV) and bus. A ...

An Assessment of the Highway Capacity Manual 2010 Roundabout Capacity Model

This paper presents an assessment of the roundabout capacity model given in the new Highway Capacity Manual 2010 (HCM 2010) with a focus in its use in the SIDRA INTERSECTION software. The model which is based on US research on roundabouts (NCHRP 572) can be viewed as a non-linear empirical (regression) model with a theoretical basis in gap-acceptance methodology. The general importance of some fundamental aspects of the model is discussed. Various shortcomings of the HCM 2010 roundabout capacity model listed in HCM 2010, Chapter 21, and some related model extensions provided by the SIDRA INTERSECTION software, one of the alternative tools referred to in HCM 2010, are discussed with a view to future development of the HCM roundabout capacity model. Using a multilane roundabout example given in HCM 2010, capacity and the resulting degree of saturation (v/c ratio), delay, level of service and queue length estimates from the HCM 2010 model are compared with those from the SIDRA Standard...

Comparison of Capacity Models for Two-Lane Roundabouts

Transportation Research Record: Journal of the Transportation Research Board, 2003

Multilane roundabout capacity models are presented and contrasted in the context of a case study. A two-lane roundabout in Copenhagen, Denmark, was investigated, and data were collected that enable the estimation of critical gaps, follow-on times, entry capacity, and delay. The primary objective of this research was to evaluate the need for more complex capacity models than currently exist in order to properly represent driver gap-acceptance behavior at multilane roundabouts. The complexity arises when drivers are assumed to simultaneously accept pairs of critical gaps in the outer and inner circulating lanes before they enter the roundabout. This approach requires that circulating-lane headways and critical gaps be evaluated independently and not be superposed in a single traffic stream as most current capacity guides assume. The approach also implies that circulating-lane volume allocation can have a considerable impact on entry capacity. The field results indicated that indeed th...

A COMPARISON OF TWO ROUNDABOUT CAPACITY MODELS

This paper presents the results of a detailed comparison of two roundabout capacity models, namely a model developed by the first author of this paper based on her research conducted in Poland (referred to as the Macioszek model) and the model used in the SIDRA INTERSECTION software based on roundabout research conducted in Australia (referred to as the SIDRA Standard model due to other model options in the software). Both models are lane-based analytical models based on gap acceptance theory with an empirical basis, and both models use Cowan's bunched exponential distribution of circulating road headways. On the other hand, the two models use different capacity and headway distribution equations, and they are calibrated for different traffic conditions in two different countries. Comparisons of entry lane capacity estimates from the two models for one-lane and for two-lane roundabouts are presented. Analyses indicated that the entry lane capacities at one-lane roundabouts in Poland are lower but close to those in Australia whereas the entry lane capacities at two-lane roundabouts in Poland are much lower than those in Australia. Calibrating the SIDRA Standard model using data for 21 one-lane roundabouts and 12 two-lane roundabouts resulted in capacity estimates with very good match to driving conditions in Poland.