# Analyzing Delay and CO Emissions: A Simulation Study of the Median U-Turn Method at Intersections
## 2 Materials and Methods  
### Setting of Simulation Model and Parameters  
VISSIM is a microscopic traffic simulation software based on time and driving behavior developed by PTV, a company in Germany. It is widely used for evaluating traffic planning and design schemes [33]. This paper uses real traffic conditions to verify the feasibility of using the VISSIM software to study U-turns. This paper used the VISSIM software (Figure 2b) to simulate the real traffic movements at the intersection of Dongguan Road and Hongwei Road in Dongguan city in Guangdong province (Figure 2a). The average delay in real-world intersection is 5.2 s, and the simulated average delay in the VISSIM model is 5.24 s. The difference between the actual average delay and the average delay obtained by the VISSIM software is small, and the simulated traffic movements are similar to the real conditions, so it is feasible to use the VISSIM software to study this U-turn method.  
The simulation scheme here in VISSIM is as follows: the object is a plane cross intersection, with three lanes for each entrance and exit. Each entrance includes three lanes, that is, a left-turn lane, a straight lane, and a right-turn lane. The width of the central median is 4 m, the median U-turn opening is located 400 m away from the intersection, and the length of the median U-turn opening is 8 m. The schematic diagrams of the median U-turn method and direct left-turn method at an intersection are shown in Figure 1. When the median U-turn method is used as shown in Figure 3a, the signal timing plan at the intersection includes no left-turn phase and left-turning traffic is prohibited within the intersection. Left-turning traffic needs to move across the intersection first, then make a U-turn at the downstream opening crossover, and finally turn right at the intersection to complete the left turn. When the direct left-turn method is used as shown in Figure 3b, the signal timing plan at the intersection includes a left-turn phase and left-turning traffic directly make a left turn within the intersection. At the same time, there is a signal at each downstream opening to control the U-turn behavior of vehicles. Specifically, when the median U-turn method is used, the U-turn signal at the downstream opening should display a green light. When the direct left-turn method is used, the U-turn signal at the downstream opening displays a red light. In addition, signs prohibiting pedestrians and non-motorized vehicles access are set at each downstream opening to prevent pedestrians and non-motorized vehicles from crossing the road through the opening.  
The VISSIM software is used to model the traffic delay and pollutant emissions at the intersection using an median U-turn method or a direct left-turn method with different through traffic volume \(\mathrm{Q_{s}}\) and left-turn ratio \(\mathrm{R_{ls}}\), and then the suitable through traffic volumes and left-turn ratios of the intersection using a median U-turn method can be determined with the goal of smaller traffic delay and pollutant emissions. The simulation models for the median U-turn method and the direct left-turn method are shown in Figure 4, respectively.  
Figure 3: The left-turn diagram at an intersection. (**a**) U-turn; (**b**) direct left turn.  
Figure 2: The real-world intersection and the intersection model in VISSIM. (**a**) The real traffic movements at intersection; (**b**) the simulated traffic movements at intersection in VISSIM.