Proceedings of the Eastern Asia Society for Transportation Studies, Vol.8, 2011 Evaluation of Roundabout and Signalized Intersection Designs using Microsimulation Minh Le KIEU Clas RYDERGREN Master Student Assistant Professor Linkoping University Linkoping University Institute of Technology Department of Science and Technology SE-601 74 Norrkoping SE-601 74 Norrkoping Sweden Sweden E-mail: [email protected] E-mail: [email protected] Abstract: With increasing urban traffic flows and increased congestion, the question of design modifications and intersection reconstruction is a common problem for urban traffic managers. In this paper we compare and evaluate the design of a roundabout and intersection in the Norrkoping city, Sweden. The intersection has been simulated with the traffic microsimulation program Aimsun, with observed data from afternoon rush-hours traffic. A roundabout, a signalized intersection design and a “bowtie” intersection design have been proposed and evaluated using results from the simulation. The results from the simulation study indicate that a roundabout design performs better in terms of the travel time compared to a traditional signalized intersection design. A hybrid version of the roundabout and the signalized intersection, a bowtie intersection, shows better performance in terms of travel time passing the junction than both the roundabout and the signalized intersection. Key Words: Roundabout, Signalized intersection, Microsimulation, Sweden 1. BACKGROUND Traffic jams due to congestion in intersections seems to be an almost chronic problem in many metropolitan areas. This stresses the importance of choosing a good design for intersections, either at the time of construction, or when making modifications. When the funding for reconstruction work does not allow for grade-separated designs, the two dominant choices are a roundabout or a signalized intersection. Traffic managers and scientist are discussing on which is the better intersection design in central city area. According to Younes (2000), signalized intersections have better performance in increased traffic demands and better traffic control in high traffic flow urban roads. The Engineering Division of Kansas, U.S estimated that a roundabout project would cost $735,855 in constructing compared to $707,492 for a signalized intersection (Alisoglu, 2010). However, some others studies believe that roundabouts offer continuous and faster traffic flow and less traffic jam than signalized intersection (NCHRP Report 572, 2007) The official guideline FHWA (2000) from the U.S Federal Highway Administration mentions that roundabouts are also believed to be safer for vehicles (Younes, 2000). Shoki et al. (2010) used the software aaSIDRA to compare the design of roundabout and signalized intersection in two intersections in Selangor, Malaysia and showed that the level of service had increased from F to B and C (Highway Capacity Manual, 2000) by replacing a signalized intersection by a roundabout. Especially in intersections where the left turning proportions are relatively high, roundabouts can handle left turn traffic better than the signalized intersection (FHWA, 2000). Proceedings of the Eastern Asia Society for Transportation Studies, Vol.8, 2011 The purpose of this paper is to present results from intersection design analyzes for an intersection between Norra Promenaden and Packhusgatan in Norrkoping city, Sweden. Microsimulation models are constructed. After obtaining a calibrated and validated simulation model of the current traffic situation, a signalized intersection, a roundabout and a “bowtie” design is proposed and evaluated in terms of travel time passing the intersection. The area studied, an intersection between Norra Promenaden/Packhusgatan in Norrkoping, Sweden, is a 4-leg intersection with 2 lanes in each arriving approach. See Figure 1 for an overview of the intersection. The current design is an unsignalized roundabout with give way signs at the approach of each leg. The intersection is congested, especially during the afternoon rush hour. Figure 1 The Norra Promenaden/Packhusgatan intersection In this project, the traffic flows in each direction approaching the intersection is measured and used as input to the simulation. In order to perform the calibration and validation with different types of data, travel time in legs and queue lengths are chosen to be the data for calibration and travel time passing roundabout is chosen to be the data for validation. The structure of the paper is as follows. The data collection procedure is described in Section 2. In Section 3, the calibration of the model is discussed and the calibration results are shown. In Section 4 the results from a model validation is presented, and in Section 5 models for alternative designs are presented. The paper ends with the conclusions in Section 6. 2. DATA COLLECTION To build the simulation model, data have to be collected. The quality of the data is important since it influences the quality of the Aimsun model as well as the calibration, validation and improvement in the later part. In this section, we describe how traffic flows, travel times, and queue lengths are measured. 2.1 Traffic Flow During the collection of traffic flows, the flow was measured every 15 minutes in weekdays. After observing the traffic flow for two days, the data shows that from 15:45 to 17:00, the traffic flows for each leg in each 15 minutes were very high compared to other time period. Hence, the peak hours have been identified to be from 15:45 to 17:00 in weekdays. Proceedings of the Eastern Asia Society for Transportation Studies, Vol.8, 2011 The method used to measure the traffic flow is the following. First of all, the type of vehicle was separated into four categories: car, bus, truck and lorry. Secondly, the turning direction of each leg was divided into three directions: go straight, turning right and turning left. The turning proportion of one leg was defined in the formula (1). / / = (1) . To measure the traffic flow from one leg to the other three legs, one person stands at the corner of the roundabout and counts number of each type of vehicles passing through the yield line of the intersection from 15:45 to 17:00. Each leg was measured five times and the mean value of traffic flows were calculated for the OD matrix creation as the input data for the Aimsun model. The mean values of the traffic flows for the different vehicle types, during the one hour and fifteen minutes time period, are shown in Figure 2. The real values in the figure are the mean values of 5 times counted traffic flow from each leg of the intersection. For instance, the value 1147.2 in the low middle of the Figure 2 shows that there were, on average, 1147.2 cars traveling from South leg to the north leg of the intersection during the studied time period. Figure 2 Mean traffic flow from 15:45 to 17:00 in number of vehicles. At each turn, the first row represents the number of cars, the second trucks, third buses, and the fourth row number of lorries. 2.2 Travel Time on Legs The travel time in one leg is the time for a vehicle passing through this leg. One of the legs measured, the travel time from point A to point B on the west leg, is shown in Figure 3. Proceedings of the Eastern Asia Society for Transportation Studies, Vol.8, 2011 End point Start point B A Figure 3 Measurement of traffic time in legs The travel times have been measured over a stretch of 100 meters for each leg, starting from the give way sign, which was located at the border of the leg and the roundabout. The north leg is an exception with the length of only 96.3 meters since there is a flyover limits the length to 96.3 meters (see Figure 5). Every 1.5 minutes, a car was chosen randomly at the start point with its front end passing the line, and group member A raised his/her hand, group member B at the end point saw the sign and started the timepiece. When the front end of that car passing the end point measured by group member B, the timepiece was stopped and the time was measured. Each leg was measured 3 times and totally 6 days spent on measuring the travel time in legs. For each time, the time period was from 15:45 to 17:00, with 50 samples every collection day, there were 150 travel time samples from each leg. Since the travel time data collections were carried out at the same period as the traffic flow counts, the number of samples was independent with the traffic flow from each leg. The collected values from the 3 observation days have been averaged to get 50 samples for use in the calibration procedure. The mean value for each leg is shown in Table 1. Table 1 Measurement result of travel time in legs (secs) South West East North 19.23 30.17 41.85 29.77 2.3 Queue Lengths The queue length is the number of cars stopped before the yield line waiting to travel into the roundabout. The queue lengths was measured at the same time as the measuring of the travel times. The length is measured in the unit number of cars. The group member A (see Figure 3) at the start point of travel time measurement also measured the queue length. Every 1 minute, the number of cars in the queue for the two lanes was counted, with the start point to be the give way sign. Each leg was measured 3 times and totally 6 days spent on measuring the queue length. For each time, the time period was from 15:45 to 17:00, with 75 samples have been taken every collection day, there were 225 queue length samples from each leg.