Traffic Simulation Modeling: VISSIM. Koh S.Y Doina 1 and Chin H.C 2

Traffic Simulation Modeling: VISSIM Koh S.Y Doina 1 and Chin H.C 2 Faculty of Engineering, Civil Engineering Department, National University of Singapore ABSTRACT This Undergraduate Research Opportunity Project (UROP) aims to evaluate the convenience and disadvantages of VISSIM, in the various steps involved, when setting up the network, for simulations, evaluations and presentations. The work involves the setting up of a study network in Queenstown, Singapore to test the merits and demerits various functions in VISSIM. The research methodology is first presented, followed by the analysis of VISSIM with regards to its various processes such as data collection, building of the road network, calibration, validation and more. Finally, concluding is the assessment of the merits and demerits when using VISSIM for modeling. INTRODUCTION Transportation today plays an important role in the economic and physical development of any modern city. Today, many micro-simulation software have been made available on the market and used as tools for the evaluation of traffic management and control. Released in 1992, VISSIM is a microscopic, time step and behaviour based simulation model developed to model urban traffic and public transit operations. It is regarded today as a leader in the arena of micro-simulation software. This report aims to evaluate the merits and demerits of VISSIM in the various steps involved when setting up the network, simulations, evaluation and presentation. METHODOLOGY The research consists of a test network. This paper reports on the merits and demerits arising from the use of VISSIM during network preparation, simulation, evaluation and presentation. Study Network Modeled In this report, the Queenstown network in Singapore has been selected to be the test network. This network consists of three major arterial roads, one secondary access road, two signalized junctions and five unsignalized junctions. The network is coded in VISSIM. A screenshot of the study network is shown in Figure 1. 1 Undergraduate, Department of Civil Engeineering, Faculty of Engineering, National University of Singapore 2 Associate Professor CHIN Hoong Chor, Director (Degree Education),The Logistics Institute-Asia Pacific, National University of Singapore

Figure 1. Layout of Study Area Network Evaluation Method In order to evaluate the merits and demerits of using VISSIM, the different functions and capabilities of VISSIM were tested. The road network is first coded in VISSIM. It is then simulated using the Static Routing function with traffic counts at the various junctions of the study network based on data collected on a typical weekday between 0700 hrs to 0900 hrs. Finally, it is simulated using the Dynamic Assignment function with hypothetical Origin-Destination Table created using VISUM. In the above tests performed, the VISSIM is evaluated based on seven processes identified during the research process. The seven categories are data collections, preparation of base data, network coding, calibration and validation, static routing and dynamic assignment function, evaluation and lastly presentation. The merits and demerits when completing each process is evaluated and scored based on the criteria as shown in Table 1. The criteria of each process will be scored on a scale consisting of one to five stars, with five stars being the best except for the experience criteria. A summary provided towards the end of the report will score each process according to Table 1.

Table 1. Criteria Time taken to complete the task Sufficient information from the provided manual Easy in understanding Usefulness Sensitivity Experience requirement RESULTS Data Collection VISSIM provides a microscopic simulation model which is heavily dependent on the parameters and data input used during the network coding. Thus, this section will evaluate the merits and demerits of using VISSIM based on the data needed and data collection process. The basic data set needed for a basic VISSIM network is as shown in Table 2. Table 2. Basic Data Set General Data Network Data Traffic Flow Data Signal Control Data Simulation Time Digital images of plan showing the entire study area Detailed plans for each junction showing lane markings, signal heads and detectors Location of bus stops For static routing: Turn movements for each junction Input flow in vehicles per hour For Dynamic Assignment: OD Matrix Location of zones and parking lots Traffic Composition Vehicle speed at free flow (Speed limit of the road) Travel time Saturation Flows Cycle Length Green, Amber and Red times for each signal group General Data The simulation time can be easily decided according to the needs of the modeler and the purpose of the simulation. The manual provides a step by step approach to alter the simulation time. Network and Signal Data Network and signal control data today can be obtained easily with the advancement of technology. The digital images, geometrical designs of the roads

and locations of bus stops can be easily obtained from relevant transportation planning departments. The manual explains the specific uses of the data collected making understanding much easier. Traffic Flow Data Traffic flow data needed for Static Routing can be easily obtained from the installed detectors loops and traffic monitoring systems. On the other hand, O-D Matrix needed for Dynamic Assignment is often difficult to obtain. The input of the O-D Matrix not only involves collected data obtained from traffic counts or license plate matching but also a manual process of balancing the O-D matrix which is often long and tedious. Other traffic flow data needed such as traffic composition, travel time and saturation flows are by nature difficult to obtain. However, default values provided by the system can be used in place. The data needed for the preparation of VISSIM, is generally easy to obtain with the help of technology. Time spent on the data collection process for a VISSIM project is shortened with sophisticated equipment such as detectors loops. The manual provides easy and sufficient explanation for the types and uses of data collected. Preparation of Base Data The preparation of base data include accelerations/decelerations, speed and length distribution of the vehicles, different vehicle types, driving behaviours and different existing link types on the road network. These base data would provide as the base information for the microscopic simulation. This section would focus on the ease of preparing the base data for use as base information for the microscopic simulation. The accelerations/decelerations, average speed and length distribution of the vehicles, different vehicle types and existing link types can be easily obtained. The preparation of data base can be completed within a short period of time with VISSIM s user friendly interface. The manual provides clear explanations for the need and reasons for the set ups. However, it may be time consuming to set up the data base as it is important to understand the various values which are to be changed and how it might affect the simulation. Prior understanding of various transportation concepts is encouraged. Users can choose between the urban, right-side rule, freeway, footpath and cycle path driving behaviours provided in the VISSIM. The manual provides explanations to the step-by-step set up for the choice of different driving behaviours. Modification to the parameters can be easily made by changing the look ahead distance, observed vehicles and others to reflect the appropriate conditions at the driving behaviour window. However, key characteristics of individual driving behaviour makes it difficult in the decision making process when choosing the appropriate driving behaviour of the network modeled. Although many default values and set ups have been provided, they cannot be taken for granted and adjustments have to be made to meet the conditions of the environment. This process needs to be done with care as minor changes in the parameters can affect the simulation. Experience in traffic modeling gives an advantage during the set up of the base data as the correctness of the parameters more than often depends on the judgment of the modeler, which makes it difficult for verifications. Inexperienced traffic modelers may require more time to understand the various parameters and changes needed. The

uncertainties, difficulty in understanding some areas and incompleteness of the manual can be considered as a hindrance to the use of VISSIM. Network Coding Networking Coding includes the building of road network, the placement of priority rules and signal heads. This section would evaluate the convenience of network coding in VISSIM. Road Network The allowance for scaled overlays in various compatible file formats in VISSIM is definitely an advantage on its own. Buttons provided on the left hand side of the user interface allows for convenience when switching between different functions needed during network coding. Setting the overlay to the correct scale allows cumbersome data input such as link lengths for different links of the study area network to be omitted. Links, connectors and their properties can be adjusted and changed according to study network environment and the user s needs. The ease of changing the network s link properties allows the user to modify the network within a short period of time and compare between different transportation planning alternatives easily. For example, number of lanes on the link can be changed within the link properties window without having to re-code the network. VISSIM allows for easy editing of the road network and link properties according to changes in the environment and needs of the modeler. Priority Rules and Signal Heads The placement of priority rules and signal controls can attribute to the large amount of time taken to build the network. The process of priority rules placement cannot be automated, replaced or done away with in the VISSIM network. The setting up of priority rules are needed at almost every junction. They are more than often determined solely by the user, thus a user s experience and judgment will play a part in the accuracy of the model. The manual explains the placement of priority rules; however, it is not in depth. Inaccurate placement of the conflict marker and stop line cannot be detected during simulation. Detection of error is solely based on experience of the user and observations made by them during simulation. Data collection for different signal controls can be easily obtained with the advancement of technology. However, cumbersome data input of different sets of signal controls is needed for the simulation. The manual provides for little explanation for the use of other types of signal controls such as the Vehicle Actuated Controls. Calibration and Validation of Network In traffic simulation and modeling, a user may often neglect this calibration and validation phase. Default values and assumptions given or used in the program are made to represent particular situations and conditions in Germany where the program is written, which may be inappropriate or unsuitable for use on road networks in other countries. Adjustment and verification are needed in order for the model to reflect reality as closely as possible. Thus, the following sections will evaluate VISSIM on its ease and difficulties during the calibration and validation of the VISSIM network. Many parameters are involved in the network building process that need adjustment and verification. These parameters include values used in the priority rules such as the

minimum gap time, the minimum headway and the maximum speed of the vehicle and more. Fine tuning these large amounts of parameters can be difficult, especially during the calibration and validation process, as they are modified and tested based on the judgment and experience of the user. The calibration process of the VISSIM project is on its own a difficult task. While VISSIM provides default values and the manual provides for simple explanations to the meaning and function of each parameter, it fails to explain the calibration and validation process of the VISSIM project. Simulations There are two types of simulation in VISSIM, Static Routing and Dynamic Assignment Simulation. The following section will evaluate the VISSIM on the usefulness of Static Routing and Dynamic Assignment. Static Routing Function VISSIM static routing is used to model the current transportation road network via routes created based on a fix sequence of links and connectors. Static routing involves input data such as vehicles counts for each turn movements and the corresponding traffic signal cycles. As mentioned in the above section, the help of technology has helped to make easy the data collection process for Static Routing required data. It is also easy for inexperienced modelers to understand and apply the function. The manual provides sufficient information and techniques required to create the routes. In the case of a small network, the use of this function may seem easy; however a different situation is seen when a larger network is being modeled. A larger network may involve the manual creation of large number of routes which complicates matters. This process becomes cumbersome and time consuming. When flow is involved in the creation of routing decision, this function seems to lose its usefulness as the future traffic flow by route is difficult to predict accurately. Static Routing function may not be as useful when planning for the future and its usefulness is only limited to the evaluation of the present situation. Dynamic Assignment Function Dynamic Assignment is designed to model the route choice behaviour of drivers, omitting the creation of static routes and instead using the O- D Matrix as flow input. Creation of zones using the parking lot function in VISSIM is a cumbersome process and slight alterations in the network may result invalid zones. Invalid zones will then have to be recreated in order to generate results from the evaluation files. VISSIM s close affiliation to VISUM can affect its user friendliness as an independent software when dynamic assignment function comes into play. The O-D Matrix used in VISSIM has to be created using VISUM. Without the program and the knowledge of using VISUM, one is unable to use the dynamic assignment function for route choice dependent variables such as the impact of variable message signs or traffic diversion. Edges of the network also have to be created properly. In a case where edges and nodes are not drawn or defined properly, the simulation will fail. The manual fails to provide an in-depth explanation to the definition of edges. This function also allows for the evaluation of forecasted traffic conditions due to new development and redevelopment by defining the origin and destination zones in the road

network. Future traffic flow can then be modeled according to the forecasted traffic conditions and evaluated with VISSIM s evaluation function. Evaluation The above sections of this report have discussed the conveniences and difficulties of building a VISSIM network using the Static Routing and Dynamic Assignment functions. The following section will evaluate the advantages provided by VISSIM and limitations for the evaluation of networks. VISSIM itself has various evaluation tools such as travel time, density, delay and queue lengths, vehicle counts and detectors to measure the effectiveness of the link or network. Evaluations can be done for any time period and interval within the time frame of the simulation. Evaluation of point locations for intersections, paths or the entire networks can be easily obtained via the evaluation tools in VISSIM and recorded in the designated evaluation files. The management of different types of evaluation files can be organized using Microsoft Assess and then be evaluated in an organized manner. However, the user might want to take note of the VISSIM limitation during the simulation. Turn movements during simulations may not reflect realistic situations. For example, a particular junction with two right turning lanes may not be used simultaneously unless one of the right turning lanes has reached lane saturation. These situations can result in inaccurate predictions on the network and alternatives chosen. Presentation This section concentrates on the benefits and disadvantages of using VISSIM during presentation. The link numbers which decides the overlap order of the links cannot be changed. Connectors will always be in front of links which makes it aesthetically unpleasant during presentation. Hence, proper planning of the link and connector drawings has to be done for an aesthetically pleasant project. Also, besides the technical component of the program, a two or three dimensional modeling of the network and traffic flow provides for great visual presentations. Not only will the current transportation system be evaluated at a network level but people who are outside of the profession will be able to see the simulation using the three dimension function.

SUMMARY OF RESULTS The convenience of its application has been evaluated and a summary of the score has been tabulated in Table 3. Table 3. Summary Time taken to complete the task Sufficient information from the provided manual Data Collections Preparation of Base Data Network Coding Calibration & Validation Static Routing Dynamic Assignment Evaluation Presentation Easy understanding Usefulness Sensitivity Experience based CONCLUSION VISSIM is a program with abilities to display and visualize complex traffic flow in a graphical way. It is able to cope with the analyses of various traffic and transit operations under various conditions and aid the assessment of traffic impacts of physical and operational alternatives in transportation planning. VISSIM itself can be considered as a difficult program to handle due to its complexity and brief explanation in the manual. However, users cannot deny that some functions and interface provided by VISSIM has made the modeling of road networks more user friendly.

REFERENCES Faber Maunsell, Transportaion, VISSIM Microsimulation Software. Retrieved 9 December 2007 from http://www.fabermaunsell.com/marketsandservices/47/17/index.jsp Internet PTV AG, VISSIM. Retrieved 9 December 2007 from http://www.ptvvision.com/cgi-bin/traffic/traf_vissim.pl PTV, VISSIM 4.10 User Manual ACKNOWLEDGEMENTS Surbana International Consultants Pte Ltd Mr Chaing Kok Sing Mr Li Shoujie