## 4 Results and Descriptive Analysis
### Driving Workload
#### 4.2.2 Influence of the Median Opening Length on the Driving Workload  
When the participants drove the vehicle to the middle of the buffer area, which was about 100 m from the median opening, they noticed the change in road alignment conditions in front of the median opening, and their driving workload was affected and showed a rising trend, reaching the maximum value until driving to the middle and rear sections of the median opening, and then declining with the influence of the activity area. Statistically, out of a total of 192 tests in this experiment, the driving workload in about 93.75% of the tests was in line with the above trend at the median opening. The U-test results indicated that there were significant differences in driving workload under different median opening lengths and vehicles at a 95% confidence level; the mean value and volatility of driving workload were higher for trucks than for cars. The opening length had a significant positive correlation with the driving workload when the width of the median and the cross slope were constant, and the longer the opening length, the faster the driving workload increased. The track of the vehicles crossing through the opening is S-shaped, forming two circular curves with the same radius connected in reverse. The participants need to complete multiple driving tasks, such as road information perception, judgment, deceleration, left turn and right turn in a short time, which can lead to tension, worry and panic, resulting in an increase in the driving workload. When the median opening length increases, the road alignment conditions become better, and the vehicle speed increases, but the turning radius is small when the vehicle crosses through the opening; the fast running speed of the vehicle will cause the sideway force coefficient between the vehicle and the road to decrease significantly while the centrifugal force increases significantly. Moreover, the direction of the centrifugal force will change in a short time due to the opposite turning direction; this phenomenon will also be exacerbated due to the existence of a crown in the opposite direction on the road. In order to maintain a uniform driving state, participants need to exert considerable mental and physical effort beyond their experience or ability; therefore, their driving workload increases significantly, leading to them easily omit important information and make judgment errors, with further driving safety risks. At this point, some participants will reduce their running speed to decrease the amount of information input per unit time to match their driving ability. The average vehicle speed, vehicle running speed and average driving workload under different median opening lengths were fitted using a cubic function, which can sufficiently explain the changing trend of vehicle speed and driving workload under different opening lengths compared with other function forms. The correlation model was established, as shown in Table 7 and Figure 10, with R2 > 0.9999, indicating a good fitting effect. When the opening length increased, the vehicle running speed, speed standard deviation and driving workload increased. However, the longer the opening length, the slower the running speed growth and the more rapidly the driving workload grows, which will greatly aggravate the psychological impact on the participants, thereby affecting their driving behavior, and leading to an increase in road safety risks. When the opening length was 40 m, the average driving workload was only 0.00921, which is consistent with the downstream median opening. The proportion of higher risk was low, due to the slight driving fatigue; however, a lower driving workload can likely cause a greater driving fatigue in the activity area, which indicates that this length selection was conservative under the experimental road and traffic conditions.