# Extraction of Catastrophe Boundary and Evolution of Expressway Traffic Flow State
## 2 Analysis of Catastrophe Characteristics of Traffic Flow
### Analysis of Catastrophe Characteristics of Traffic Flow  
The expressway weaving section is an area where vehicle lane-changing occurs frequently. Many instances of vehicle weaving result in a frequently congested section; thus, it can be considered a critical research object. Because field investigations have a huge workload and it is challenging to extract representative data for targeted problems, a combination of field investigation and simulation was used for data collection. The weaving section of Hang'an Expressway at Anshan No. 2 Road in Qingdao was selected as the investigation site. Through video detection and the GPS (Global Positioning System) floating car method [23; 24], the length, number of lanes, traffic volume, and other parameters of the weaving section could be effectively investigated. The investigation period was from 22 to 26 November 2021, taking this real scenario as the research object. Using the traffic simulation platform Vissim, the field investigation results were used as input parameters to build a scenario for data simulation. The specific scheme of traffic simulation was as follows:  
1. Simulation scenario: According to the survey results, the mainline of the expressway was a two-way six-lane straight section, the length of the weaving section was 210 m, the lane width was 3.75 m, and the simulation interval was 1000 m.
2. Traffic volume: According to the actual investigation, it was found that the traffic volume in this weaving section was 1200-3700 pcu/h in 1 day; thus, the calibration simulation traffic volume was 1000-4000 pcu/h, the step length was 500 pcu/h, and the simulation time of each interval step length was 3600 s.
3. Weaving volume ratio: In order to study the congestion characteristics of the weaving section, according to the \"Highway Capacity Manual\", the maximum weaving volume ratio for a weaving section with four lanes was selected as 0.35. 4. Traffic composition: According to the actual survey results, there were few large vehicles on expressways in the city, with small vehicles accounting for 0.98 and large vehicles accounting for 0.02. 5. Expected speed: According to the actual investigation results, the expected speed of the mainline of the expressway was set as 80 km/h, and the expected speed of the ramp was set as 40 km/h. The vehicle speed conformed to a Weibull distribution. 6. Driver behavior: The action point model of the Vissim simulation platform was used to describe the drivers' behavior. Data detection: According to the \"Highway Capacity Manual\", the weaving section ranged from 0.6 m upstream of the convergence triangle to 3.7 m downstream of the separation triangle. The weaving section was divided into inner, middle, and outer lanes. The equidistant segmentation method is adopted. Starting from the separation triangle, data detection points were set every 5 m to the upstream point, with a total of 129 detectors. The test sections were numbered in sequence. The data detection interval was 30 s.
* Data output: The traffic volume, time occupancy, and average speed were taken as the output in intervals. In order to verify whether the simulation platform has high reliability, under the same simulation scenario, detectors were set at the exit, middle, and entrance sections of the weaving area to extract the measured data and simulation data. Then, the measured data of free and congested flow, which are easy to observe, were compared with the simulation results. The validation results of the model are shown in Tables 1 and 2. From the verification results shown in Tables 1 and 2, it can be seen that there was little difference between the measured results of speed and traffic volume under different sections and traffic flow conditions and the simulation data. The relative error of all comparison results was no more than 8%. Therefore, using the Vissim simulation platform for traffic simulation is reasonable and reliable.