## 3 Results and Analysis
### Speed and Car following Characteristics
#### 3.2.1 Speed Distribution  
Vehicle speed fluctuations provide valuable insight into driving safety, as erratic speed patterns can significantly increase the likelihood of accidents [45; 46]. According to the detailed speed profiles, tunnel-interchange sections can be differentiated into distinct areas based on vehicle speed attributes: steady car-following area, speed variation area, and diversion exit area, as depicted in Figure 4. A notable observation across most driving samples is that peak speed fluctuations typically occur within 350 m before the transition point, which coincides with the interval of maximum speed reduction. After implementing noise reduction with a three-point moving average window and enhancing curve smoothness using third-order B-spline methods, Figure 5 demonstrates the speed profiles in this zone under different CD and IV conditions.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**CD**} & \multirow{2}{*}{**IV1**} & \multirow{2}{*}{**IV2**} & \multirow{2}{*}{**RV (km/h)**} & \multirow{2}{*}{**V mean (km/h)**} & \multirow{2}{*}{**DCC max (m/s2)**} & \multirow{2}{*}{**DH min (m)**} & \multirow{2}{*}{**TH min (s)**} \\ \cline{4-4} \cline{6-11}  & & & & **Diff.** & \(p\) & **Diff.** & \(p\) & **Diff.** & \(p\) & **Diff.**  
Figure 5a demonstrates the significant impact of CD on speed control within the diversion area. Shorter CDs are associated with a decrease in average speed and an uptick in speed fluctuations. Specifically, larger spacing sections (\(\geq\)700 m) show a speed distribution akin to the GDS group, maintaining a stable range of 85-95 km/h, with an average deviation of around 2.33 km/h. In contrast, smaller spacing sections (\(\leq\)500 m) lead to a reduction in average speed by approximately 10.16%, alongside an increase in standard deviation to 4.58 km/h, thereby adopting a distinct \"accelerate-decelerate-steady\" pattern. The data also suggest an earlier onset of deceleration in tighter spaces, transitioning from 200 m to 100 m before the signage.  
In relation to signage information density, Figure 5b shows that denser signage information increased speed fluctuation and velocity reduction. This trend is particularly evident as drivers encounter more signage information, necessitating a reduction in speed to process the additional data. The average deceleration range widens from 2.58 km/h for 5 units of information to 7.53 km/h for 13 units, with a marked difference emerging beyond 7 units.  
Providing a holistic view, Figure 6 compares driving speeds under various CD and IV combinations. It is evident that in shorter spaces, drivers respond with greater deceleration to the same informational stimuli compared to more spacious sections, likely due to constrained decision-making time. According to Table 6, all IV groups show significant speed reduction differences in 100 m and 300 m sections. However, this distinction fades in 500 m sections between five and seven information units and becomes prominent again only when the spacing extends beyond 700 m and information units increase past nine. This pattern underscores the need for strategic signage placement and design adjustments based on the specific characteristics of each tunnel-interchange connection section.  
Figure 4: Zonal classification of diverging area based on the vehicle speed attributes. The sign displayed feature Chinese characters for specific geographical locations (Jingyang, Xianyang, yanan, etc.) pertinent to the study’s context in China and presented in their native script for accuracy.  
Figure 5: Speed profiles under different CDs and IVs.