# Study on the Driver Visual Workload of Bridge-Tunnel Groups on Mountains Expressways
## 3 Results
### Evaluation of Driver Visual Load  
ISO 2631-1-1997 \"Mechanical vibration and shock\" [44] uses the \"weighted root mean square acceleration\" as a basic indicator for evaluating whole-body vibration in humans. The pupil size variation induced by the abrupt illumination changes at tunnel entrances and exits is a transient process. In this respect, this phenomenon is similar to the differential settlement-induced vehicle vibration at bridgeheads. The weighted root mean square of the pupil dilation velocity can be used to evaluate the visual workload of drivers [45]. The formula is as follows:  
\[V_{\omega}(t_{0})=\left[\frac{1}{\tau}\int_{t_{0}-\tau}^{t_{0}}V_{\tau}^{2}(t )dt\right]^{\frac{1}{2}} \tag{1}\]  
where \(V_{\omega}(t_{0})\) represents the instantaneous pupil area changing frequency-weighted velocity amplitude; \(\tau\) denotes the integration time constant; \(t\) represents time (integration variable);  
Figure 16: Drivers’ pupil diameter at natural light environment on tunnel exit. Figure 15: Drivers’ pupil diameter at natural light environment on tunnel entrance. \(t_{0}\) stands for the observation time (instantaneous). A small \(\tau\) indicates transient vibration or transient shock. \(MTPA\) refers to the maximum value of \(V_{\omega}(t_{0})\). When measuring the \(MTPA\) at tunnel entrances and exits, the recommended value for \(\tau\) is 1 s. The expression for \(MTPA\) is as follows:  
\[MTPA=\max\{V_{\omega}(t_{0})\} \tag{2}\]  
Relevant studies have shown that \(MTPA\) correlates with the duration of the visual oscillation. The duration of visual oscillation is defined as the period between the starting point when the pupil area increases by more than 50% compared to the previous moment, and the ending point when the pupil area decreases by more than 50% compared to the preceding moment. Due to the short duration of the visual oscillation, to achieve a quantitative assessment of the visual psychological and physiological load on the drivers, the indicator is multiplied by a conversion coefficient to obtain the converted visual oscillation duration, denoted as \"\(t_{c}\)\". Research suggested that visual stimuli shorter than 0.1 s do not cause adverse effects on drivers' visual perception, and 0.2 s is the typical minimum duration of visual stimuli in psychological experiments. [PERSON] et al. found that at tunnel entrances and exits, when \(t_{c}<0.2\) s, the visual oscillation may cause slight discomfort to drivers but does not affect their driving behavior. However, when \(t_{c}>1\) s, which indicates a severe visual oscillation, a significant psychological and physiological load will be exerted on the driver [46]. This impairs drivers' ability to perceive the road and relevant traffic information, such as traffic signs and the presence of vehicles ahead and behind, significantly increasing the risk of traffic accidents. With the two indicators, namely, \(MTPA\) and the converted visual oscillation duration, a visual comfort evaluation index system for tunnel-bridge groups is established (Table 3), and the evaluation results of drivers' visual load are divided into five levels: A to E.  
Based on the lighting conditions in different sections of the tunnel, this study categorizes the tunnel into seven segments: the entrance access zone, the entrance section, the entrance change-over portion, the basic segment, the exit change-over portion, the exit section and the exit access zone, as shown in Figure 17[47]. The visual load was calculated, and the results are shown in Table 4.