# Study on the Driver Visual Workload of Bridge-Tunnel Groups on Mountains Expressways
## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Number**} & \multicolumn{6}{c}{_MTPA_/**(mm2/s)/Evaluation Grade of Visual Load**} \\ \cline{2-9}  & **Entrance** & **Entrance** & **Entrance** & **Basic** & **Exit Change-Over** & **Exit** & **Exit Access** \\  & **Access Zone** & **Section** & **Change-Over Portion** & **Segment** & **Portion** & **Section** & **Zone** \\ \hline A1 & 4.11/A & 11.1/A & 17.78/A & 45.34/C & 11.83/A & 5.8/A & 3.81/A \\ A2 & 5.2/A & 6.51/A & 25.89/B & 24.36/B & 7.39/A & 2.93/A & 6.02/A \\ A3 & 3.31/A & 5.38/A & 11.27/A & 14.29/A & 7/A & 5.26/A & 3.13/A \\ A4 & - & 23.83/B & 16.04/A & 24.59/B & 11.06/A & 21.07/A & 4.49/A \\ A5 & - & 8.08/A & 8.66/A & 47.44/C & 42/C & 20.83/A & 6.44/A \\ A6 & 57.77/C & 6.01/A & 37.88/C & 13.02/A & 8.57/A & 4.9/A & 8.48/A \\ A7 & 6.19/A & 7.84/A & 8.07/A & 34.51/C & 51.81/C & 10.97/A & 15.1/A \\ A8 & 8.48/A & 21.42/B & 51.35/C & 40.51/C & 36.23/B & 31.7/B & 3.94/A \\ A9 & - & 42.4/C & 67.62/C & 50.44/C & 20.28/B & 3.69/A & 4.68/A \\ B1 & 35.1/C & 16.75/A & 19.91/A & 48.28/C & 23.17/A & 30.83/B & 10.95/A \\ B2 & 7.56/A & 7.88/A & 4.59/A & 47.85/C & 9.11/A & 23/A & 14.33/A \\ B3 & 3.97/A & 9.74/A & 11.31/A & 21.8/B & 5.39/A & 4.78/A & 10.07/A \\ B4 & - & 4.56/A & 4.05/A & 10.64/A & 9.6/A & 3.28/A & 3.55/A \\ B5 & 2.8/A & 14.59/A & 3.09/A & 5.79/A & 16.13/A & 3.33/A & 5.61/A \\ B6 & 7.96/A & 3.86/A & 5.3/A & 12.76/A & 9.08/A & 2.55/A & 32.66/B \\ B7 & 6.66/A & 5.23/A & 3.91/A & 16.89/A & 8.9/A & 30.96/B & 8.22/A \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The driver visual load evaluation results. tunnels is 590 m, as the drivers move close to the entrance section, the pupil diameter remains stable, and after entering the tunnel, the value increases rapidly and peaks. The pupil diameter of the driver in the exit section first decreases slowly and then fluctuates greatly. The above findings show that when the distance between adjacent tunnels is too small, the black hole effect and the white hole effect frequently alternate, exerting a great psychological load on the driver at the tunnel entrance and exit. If the distance between adjacent tunnels is excessively large, it is difficult for drivers to adjust to the environmental changes in time, resulting in a rapid increase in pupil diameter and poor dark adaptation upon entering the tunnel. Therefore, when the distance between adjacent tunnels is less than 200 m, it is recommended to set a full-coverage shading shed between tunnels. When the distance between the adjacent tunnels is greater than 200 m, the shading shed can be set up in the exit section of the upstream tunnel, and the lighting can be strengthened in the entrance section of the downstream tunnel to improve the visual adaptability and comfort of the drivers. _MTPA_ was used to evaluate drivers' visual load in bridge-tunnel groups A and B (Table 4). It can be found that in the entrance access zone, the visual load evaluation grade in the B1 and A6 tunnels is C. The main reason for the discomfort is as follows. Before entering the tunnel, there are long-distance sections with low visual load. While driving through these sections, the drivers have adapted to the light conditions. However, when they enter the tunnel, the environmental conditions suddenly change, and the drivers need to accommodate these variations. In the entrance section, the visual load evaluation grade of the A9 tunnel is C, and that of the A4 and A8 tunnel is B. At the entrance change-over portion, the visual load evaluation grade of the A6, A8, and A9 tunnels is C, and the grade of the A2 tunnel is B. At the exit change-over portion, the A5 and A7 tunnels are rated C in terms of visual load, and the A8 and A9 tunnels are rated B. In the exit section, the B1, B7, and A8 tunnels are rated B for visual load assessment. In the exit access zone, only the visual load evaluation grade of the B6 tunnel is B.