## 5 Discussion  
This paper aimed to investigate the influence of different median opening lengths on the vehicle speed and driving workload in crossover work zones through a naturalistic driving experiment. This study illustrated that better road traffic conditions in the work zone will lead to an increase in vehicle speed. Since a lower speed limit is often used in work zones, some drivers think that the speed limit is too low and adopt speeding behaviors, causing other following drivers to accelerate, resulting in a low speed limit compliance rate in some areas of the work zone. In addition, drivers with different driving styles will form a heterogeneous speed preference, resulting in increased speed variability. In different areas of work zones, due to the differences in road and traffic conditions, the driving workload has a linear positive correlation with vehicle speed, but has no fixed relationship with acceleration. A lower median opening length will lead to a sharp deceleration of vehicles and increase the speed difference between adjacent areas. When the opening length is large, aggressive drivers will cross at a high speed, while conservative drivers will feel nervous due to the high driving workload and tend to slow down, resulting in an increased standard deviation of acceleration and speed; when the median opening length is moderate, it corresponds to the lowest standard deviation of acceleration and speed, and the lowest collision probability [29]. Higher vehicle running speeds and speed variability can increase the risk of accidents such as rear-end collisions in work zones [12]; a good speed limit strategy will reduce the risk of accidents [39], and variable speed limits in work zones are more effective than fixed speed limits [40; 41]. Therefore, during the layout and control processes of work zones, the mutual coordination of the areas such as the median opening should be considered, and an appropriate speed limit strategy should be adopted to improve the speed limit compliance rate, reduce the speed variability and avoid the impacts of bad driving behaviors on the road safety in work zones. Factors such as the median width, lane width, vehicle operating speed, sideway force coefficient and cross slope determine the median opening length, and all these road traffic conditions will have different influences on the driving workload. Relevant research shows that a large cross slope, high vehicle running speed and small lane width and turning radius will cause driver tension [35], which should be taken into account when arranging the median opening. In practical application, operation efficiency and safety factors should be considered when determining the opening length; safety is mainly related to vehicle speed characteristics, while operational efficiency is mainly related to capacity. Instead of using the traditional method of calculating the maximum flow in uncongested conditions, [PERSON] et al. estimated the work zone capacity using the mean queue-discharge flow rate from the resulting bottleneck at the end of a bottleneck area [42]. An increasing number of studies have shown that it is more reasonable to use a queuing flow to calculate the work zone capacity [43]. The capacity at the median opening can be obtained by combining experimental observation with simulation. It has been shown that the capacity has a positive correlation with the opening length, and the capacity increases quickly at low speeds while increasing slowly at high speeds [44; 45]. When the opening length is 40, 70, 100 and 130 m, the capacity is 1280, 1380, 1460 and 1520 pcu/h, respectively; when the opening length increases to 200 m, the capacity becomes 1610 pcu/h. Herein, the capacity tends to be constant, and the capacity at the opening is consistent with the upstream transitional area and buffer area of the work zone. The opening length is an important influencing factor for the capacity of work zones.