ROUNDABOUT INTERSECTIONS: EVALUATION OF GEOMETRIC AND BEHAVIOURAL FEATURES WITH VISSIM

In the literature, there are many methodologies that allow the evaluation of roundabout performances (Capacity, Levels Of Service, etc): analytical models (HCM, HBS etc.), statistical models (TRRL, SETRA) etc. Each technique considers some aspects of the roundabout in comparison to others (geometric elements, vehicular flow and behavioural parameters). Obtained results are often not comparable among themselves because of distinctive peculiarities of each method. Today, the best way to solve this problem is by using a refined simulation software of vehicular circulation. In this paper the authors introduce the results of a wide survey conducted on an ample range of roundabout scenarios by the use of the simulation software VISSIM. Each scenario describes a fixed roundabout phenomenon using the following variables: geometric elements (inscribed circle radius, circulatory roadway, central and splitter islands etc.); characteristics of the traffic flow (dynamic traffic assignment, approach speed, circulatory speed and reduced speed zones, etc.); behavioral features (priority rules, minimum gap, minimum headway, etc.). The results are presented from the evaluation of stop-line delays. INTRODUCTION Many analytical techniques allow the study of the performances (Capacity, Levels of Service, etc) of roundabout intersections: probabilistic methods (HCM (1), HBS (2), etc.), statistic methods (TRRL (3), SETRA (4), etc.). Each method, when formulated, has to consider some aspects of roundabout circulation in comparison to others (geometric elements, vehicular flow and consumer behaviour). An approach that allows a global vision of the problem today is through the use of refined simulation analysis software of vehicular circulation. PERFORMANCES OF ROUNDABOUT INTERSECTIONS Fundamental Capacity Methods The capacity of each entry is the maximum rate at which vehicles can sensibly be expected to enter the roundabout during a given time under prevailing traffic and geometric features (5). Many methods applicable to two-way stop-controlled and two-way yield controlled intersection capacity are used as the foundation for the evaluation of roundabout performances. Roundabout analysis models are generally divided into two categories: statistical (empirical) models based on the regression of field data; analytical (semi-probabilistic) models based instead on the gap-acceptance theory. Empirical models correlate geometric features and performance measures, such as capacity, average delay and queue length, through the regression of field data. In this way they generate a relationship (generally linear or exponential) between the entering flow of National Roundabout Conference 2008 2 Transportation Research Board Vincenzo Gallelli and Rosolino Vaiana 3 an approach and the circulating flow in front of it (6). These models are better than analytical ones but require a great number of congested (oversaturated conditions) roundabouts for calibration and may have poor transferability to other countries (7)(8). Gap-acceptance models can be developed instead from uncongested sites: the driver on the approach (entering flow) needs to select an acceptable gap in the circulating stream, to carry out the entering manoeuvre. The gap is the headway between two consecutive vehicles on the circulating flow: so the “critical gap” (tc) is the minimum headway accepted by a driver in the entering stream. If the gap accepted is larger than minimum, then more than one driver can enter the roundabout: the time required for an additional vehicle to utilize the same gap in traffic, is defined as “follow-up time” (tf). So these analytical models calculate the roundabout capacity as a function of the critical gap, the follow-up time and the circulating flow. However, for capacity evaluation there are some assumptions: 1. constant values for “tc” and “tf”; 2. exponential distribution for the gaps into the circulating flow; 3. constant traffic volumes for each traffic flow. These specific assumptions make the use of these models difficult in practice. Furthermore, there are other limitations, such as: 1. the estimation of the critical gap is not easy; 2. the geometric factors are not directly taken into account; 3. the inconsistent gaps are not accounted for in theory (forced right of way when traffic is congested, circulating drivers give up right of way, different gap accepted by different vehicles, the rejection of large gap before accepting a smaller one, etc.). A summary of the majority of international methods for the evaluation of roundabout capacity is represented in (6). Micro-Simulation Software Packages The ever increasing use of roundabouts to solve traffic problems has produced a great number of models which are able to predict operational performances. Each of these methods allows many important roundabout features to be estimated such as capacity, average delay and queue length, by the use of empirical or analytical formulations. In particular the theory of gap-acceptance leads to complex assumptions regarding driver behaviour and often it is not easy to obtain good results for atypical roundabout geometries (8). In order to solve this problem there are various software packages that provide roundabout analysis, using several theoretical methods and requiring a variety of input parameters. However, not many software packages allow the user to model roundabouts exactly. These packages can be divided into two categories: deterministic (empirical or analytical) and stochastic simulation models (9). The first ones, such as SIDRA, Rodel, Arcady, Kreisel, etc., analyze roundabout performance with a series of equations, correlating these features (e.g. delay, queues, capacity) with a set of variables. The second ones instead (e.g. Vissim, Corsim, Integration), use an interval-based simulation to describe traffic operations. A summary of the principal international software packages for roundabout feature simulation is shown in Table 1. National Roundabout Conference 2008 3 Transportation Research Board Vincenzo Gallelli and Rosolino Vaiana 4 TABLE 1 Summary of The Principal Softwares for Roundabouts Simulation COUNTRY NAME MODEL NEWLY REFERENCE U.K. RODEL Deterministic Empirical (10) U.K. ARCADY Deterministic Empirical (11) U.K. PARAMICS Stochastic Simulation (12) Australia SIDRA Deterministic Analytical (3) Germany KREISEL Deterministic All methods (11) Germany VISSIM Stochastic Simulation (13) ; (14) U.S.A. HCS/SYNCHRO Deterministic Analytical (14) U.S.A. CORSIM Stochastic Simulation (15) U.S.A. INTEGRATION Stochastic Simulation (16) U.S.A. SIMTRAFFIC Stochastic Simulation (16) France GIRABASE Deterministic Empirical (17) Spain GETRAM Stochastic Simulation (18) A Microscopic Simulation Model: Vissim The simulation of roundabout traffic operations often presents many complexities, because it is not easy to define all the geometric and user-behavioural features. Vissim gives a flexible platform that allows the user to more realistically model a roundabout. It is based on a link-connector instead of a link-node structure which is easily able to build a complete network or, specifically, a single intersection. In addition, Vissim is able to import CAD layout (dxf or jpg) and to set it as a background on which links can be drawn. An appropriate scale is assigned, so that all the measurements are in the same units. In this way it allows, for example, all the geometric elements of a roundabout (splitter islands, lane width, number of lanes, entry width, etc.) to be precisely drawn. Anyway, there are three principal features which are very important to set in order for a correct simulation: 1) approach speed, reduced speed zones and circulatory speed; 2) priority rules; and finally, 3) traffic assignment. Furthermore the driver behavior is also important: Vissim uses a psycho-physical car following model and a rule-based algorithm for lateral movements realized by Wiedemann (’74). Approach Speed, Circulatory Speed And Reduced Speed Zones An accurate definition of the vehicle speeds is very important to achieve a good simulation of a roundabout. National Roundabout Conference 2008 4 Transportation Research Board Vincenzo Gallelli and Rosolino Vaiana 5 FIGURE 1 Description of the principal parameters used in Vissim for circulation rules. Vissim allows the definition of the desired speed of every type of vehicle when the said vehicle enters the network. The approach speed of every leg of the roundabout is taken in a range defined by an empirical speed curve which is created by the user: this curve usually presents an S-form (normal distribution). The vehicles maintain the desired speed until traffic conditions or geometric features require them to change it (19). Vissim uses reduced speed zones in order to change the desired speed: these have been used to set the influence of roundabout entry geometry on the approach speed. The reduced speed zones assign a new speed distribution to the vehicles which begin to decelerate before the start of the same areas (see “deceleration zone” in Figure 1). After the end of these zones the vehicles begin to accelerate in order to reach the previous desired speed if the user does not set a new one. Specifically, for roundabouts, after the reduced speed area of the entry, a Circulatory Speed distribution is set which is derived from vehicle radial dynamics equilibrium: ) ( 127 t f q R V + ⋅ ⋅ = (1) With these assumptions: q=0; ft=0.23; R=Ri-(Ci/2). This equation allows the average speed (Vm) to be obtained of the circulating vehicles into the roundabout and the range of the circulatory speed distribution to be set. In fact, considering this as a normal distribution and considering standard deviation σ=5Km/h (this is derived from field data), it is therefore possible to define the extreme values of the range as Vm±(1,96·σ) in order to consider the 95 percentile of the circulatory speed. Priority Rules The most important aspect to