Investigation of passenger car equivalent (PCE) for left and right turn flows at signalized intersection in Istanbul

Existence of various vehicles with different characteristics affects the traffic condition. It is necessary to consider these differences in the traffic management and design of new traffic facilities. Knowing the fact that an intersection is the most sensitive part of the road networks, presence of heavy vehicle in an intersection affects it adversely. Particularly this issue is intense in the signalized intersections where heavy vehicles stop and start motion that increases headway and causes a delay. Passenger car equivalency value for any vehicle shows equivalent number of cars that have the same effect at the same traffic conditions. By using PCE value, it is practical to demonstrate different types of vehicles in the current traffic conditions with the same unit. At the same time, the different modes of vehicle's deceleration, acceleration, maneuverability and occupied area can be taken into account in any relevant calculation to get much more realistic results. In this study, in Istanbul for right and left turn traffıc flows in the 4 signalized intersections, PCE values are determined for 6 different vehicle modes such as minibus, light truck, midibus, lorry, bus, semi-trailer truck and thereby difference between heavy vehicle's maneuverability has been taken into account in the calculated PCE values. Two different approaches exist to calculate the PCE value for continuous and discontinuous flow. To calculate PCE in intersections as discontinuous flow two methods based on headway and delay are used. In this study, the headway method that is more practical in terms of data acquisition is used. Here, studies by Miller, Molina- Cruz-Casas and Prtha saha was basis of our obseravation and analysis. Each of these studies based on the headway method has a special approach to calculate the PCE value. The results obtained are much more realistic from Miller's study to PerthaSaha in a condition that amount of data, needed for calculation increases at the same order. In headway method, the headway values must reach to saturation point and they should not be affected by initial lost time. In order to achieve this, only the headway values after the fourth vehicle in queue are considered. In this study, headway values gathered from 4 intersections (for 2 right turns and 2 left turns). We used field observation instead of computer simulation in data collection. All the data gathered by one person that measures headway values by stopwatch and simultaneously recording type of vehicles in the field. Later headway values were matched with vehicle combinations. As mentioned before, there are seven classified vehicle types; therefore, there will be 49 vehicle combinations. Then averaged headway values of any combination are calculated. These values will be used in PCE calculation. First intersection was located in Ataşehir district, Kozyatağı neighborhood and the headway values were gathered from separated lanes, which were assigned, to left turn vehicles at E 80 access road. The second road where left turn headway values were measured from the separate lanes were assigned for left turn incoming vehicles in Fulya street access road of MKB Meslek Lisesi intersection which is located at Küçükçekmece district and Halkalı neighborhood in Istanbul. First intersection where right turns are investigated was located in Beşiktaş district, Beşiktaş square and the headway values were gathered from seperate lanes which were assigned to right turn vehicles at Barbaros boulevard access road. The second road where right turn headway values were measured from separate lanes assigned for right turn incoming traffic flows in the Oğuzhan street access road of Fındıkzade intersection which was located at Fatih district and Molla Gurani neighborhood in Istanbul. Through the measurement of headway values between vehicles, the mode of vehicles were enlisted simultaneously and it resulted in a classifying list of headway values according to heavy vehicle combinations. Measured headway values, considered to have dissimilar characteristics regarding to left turn or right turn movement, thus evaluated individually. There are two intersections in each of these conditions and in any of them; there are two lanes that were assigned to left turn or right turn movements separately. Data which belongs to left turn or right turn lanes of the same intersection and in addition, data belonging to left turn or right turn lanes of different intersections were examined in terms of whether coming from the same characteristic groups and consequently the possibility to consider as a united bigger data group. Headway values which were measured for the mentioned lanes were examined in order to determine whether the data came from the same population or not and if they are conformed to the t-test. In other words, we examined the gathered data whether they represent the same statistical characteristic or not. As it is known there are two kinds of intersections with right turning flow and left turning flow. The suggested t-test not only was performed between right and left turns in different intersections but also was performed between the intersections that have the same turning condition based on right and left lanes. Even though two sets of data statistically did not represent the same characteristics, aggregating them and getting a bigger data set seemed a more practical way which lead to more realistic PCE values. Acquired saturated headway values were evaluated with 3 different methods of headway method separately. The first method, PCE value was computed with calculating the heavy vehicle average headway value ratio to the automobile. In the second method, compared to the first method, the delay effects of heavy vehicles on the posterior vehicles were taken into account as well. In the third method, the headway values were adjusted with a correction coefficient; depending on the number of each heavy vehicle combination, and PCEs calculation was done with the new data. At the end of this study, PCE values which were acquired with these three different methods and divided to right turning and left turning volumes are given. The PCE values which calculated using Miller approach for various heavy vehicles were assessed in this study and was analyzed regarding to right turning and left turning flows. According to the results obtained, the PCE values for right turning and left turning flows that have been achieved with Miller approach approximately are the same and there is no meaningful connection between them. Result using Molina- Cruz-Casas method shows that there is not any significant difference between the PCE values for right and left turning flows. The values calculated using Partha Saha method for left turning vehicles is more than the corresponding values for right turning vehicles. This comparison applied only to three of vehicles out of seven which are automobile, truck and minibus. This reduction in the studied number makes interpretation of the results unreliable. Comparing the values calculated with three different methods which were used in this study shows that the values have a decreasing trend from Molina to Miller method and there is no meaningful relation between Partha Saha and Miller approaches.