## 1 Introduction
established a vehicle track model for a median opening based on the turning driving characteristics of the vehicle and provided a recommended median opening length under the conditions of different speed limits, median widths and super elevations; when the speed limit is 40-60 km/h, the recommended length is 60-125m [24]. Through calculation and simulation, [PERSON] found that, when the speed limit is 60 km/h, the opening length should be 80-120 m [25]; [PERSON] obtained the length of 94-154 m for when the speed limit is 40-60 km/h through similar methods [26]. Through simulation, [PERSON] found that, when the speed limit is 40 km/h and the opening length is 80-120 m, the impact on traffic is small [27]. [PERSON] et al. simulated the traffic organization of a freeway work zone and concluded that the vehicle running speed and capacity were higher when the median opening length was 80-100 m, but the traffic safety was not considered [28]. [PERSON] et al. combined model calculations with traffic simulation, analyzed the relationship between the capacity, operating speed, speed limit compliance rate and opening length, evaluated the safety of the opening using TTC (time to collision) and proposed the recommended values of the median opening length at different design speeds [29]. However, the traffic simulation methods used in the relevant studies lack validation means, and the results obtained vary widely. Most of the studies used default simulation parameters (e.g., following and lane change models), which made it difficult to reflect the influences of different opening lengths on drivers' psychological indicators and driving behaviors. Driving simulation is one of the common methods for studying driving behavior [30]. [PERSON] et al. used driving simulation to analyze the driving behaviors under nine different layouts in freeway crossover work zones. They found that there was a significant difference in the vehicle running speed. When the median opening length was 40-80 m, the vehicle running speed was 53.5-70.2 km/h; when the opening length increased, the driver could safely complete driving at a higher speed without behaviors such as sudden deceleration [31]. Through field investigation and driving simulation, [PERSON] et al. found that, when the vehicle passes through the median opening, its speed will drop significantly, and its running speed will not be lower than the speed limit until the opening length is less than 30 m [32]. [PERSON] et al. employed driving simulation to investigate driving performance using metrics (speed, acceleration, maximum steering wheel speed and lane-changing track) with respect to five median opening lengths, and the results indicated that, when the opening length was too large or too small, it would impact the traffic safety in the work zone; their recommended opening length is 90 m when the speed limit is 60 km/h [33]. However, driving simulators have a limited simulation degree of real vehicles (only 70-80%). The vehicle speed is usually high while the acceleration is low. Their relative validity is only applicable to the study of driving behavior [34]. Compared with traffic simulation and driving simulation, \"naturalistic driving\" means installing data acquisition equipment on vehicles and drivers to monitor and record the actual driving process of drivers at all times. The data from naturalistic driving are true and reliable, as shown in Table 1. However, due to the complexity of the experiment, only a few safety studies in work zones use the natural driving experiment method. The road and traffic conditions change dramatically in the median opening of the work zone, and drivers need to analyze, process and judge the road and traffic information, make decisions and operate the vehicle; the mental stress formed during the whole process is the driving workload [35], and different opening lengths affect drivers differently. However, there is a lack of methods and specifications for determining the median length.